CA1337082C - Image processing apparatus - Google Patents
Image processing apparatusInfo
- Publication number
- CA1337082C CA1337082C CA000616718A CA616718A CA1337082C CA 1337082 C CA1337082 C CA 1337082C CA 000616718 A CA000616718 A CA 000616718A CA 616718 A CA616718 A CA 616718A CA 1337082 C CA1337082 C CA 1337082C
- Authority
- CA
- Canada
- Prior art keywords
- signal
- video signal
- digital video
- pulse
- generating
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Landscapes
- Facsimile Image Signal Circuits (AREA)
- Fax Reproducing Arrangements (AREA)
Abstract
An apparatus for generating a high quality image from a digital video signal includes a system for gamma correcting the digital video signal with a digital look up table and for converting the resultant digital signal to an analog video signal. Another circuit generates a triangular wave reference pattern signal and a comparator compares the analog video signal with the triangular wave reference pattern signal to form a pulse-width-modulated signal. A raster scanning print engine producing, for example, a laser beam, scans over a recording medium in accordance with the pulse-width-modulated signal, thereby forming an image of high quality on the recording medium of a print engine. This apparatus can also be used with an analog video signal by first converting the analog video signal to a digital video signal with an analog to digital converter.
Description
-This application is a division of Applicat1on Serial No. 515,897, filed August 13, 1986.
BACKGROU~JD OF T~E I~E2~TION
Field of the Invention Tne present inve-ltion relates to an apparatus for generating an image from a dig1tal video input signal.
The apparatus is improved so as to reproduce an image with high quality.
Descript1on of the Prior Art In the past, meenods generally rererred to as tne dither method and tne density pattern method have been proposed for reproduc1ng lmages of half tone~. These known methods, nowever, cannot provlde satlsfactory gradation of dot size when the slze o~f the thresnold dot matrix is small ana, therefOre~ require the use of a threshold matrix navin~ a larger size. Thls is turn reduces the resolution and undesirably allows t~e texture of the image to appear too dlstinct1ve due tO the periodic structure of the maerix. Therefore, deterloratlon of the quality of the output lmage results.
~ 2 - ~ 337082 In order to mltigate the above described problems, it has Deen proposed to moalfy the dlther metnoa so as to allow finer control of the dot size by the use of a plurality of dither matrices. This metnod, however, requires a 5 complicated circuit arrangement for obtaining syncnronism or operation Detween the ditner matrlces so tnat the system as a whole is large in size, complicated in constructlon, ana slow. Thus, tnere is a practical limit in the incremental increase of dot size and the resultant increment of density available by the use of a plurality of dither matrices. In ~.S. Patent No. ~,916,096, a method of improving the conventional screening process lS
describea. As set fortn in this U.S. Patent No.
3,916,096, at column 8, lines 19 througn 31:
The conventional screening process when applied to a scanned image can be regarded as a form of pulse-widtn-modulation whereDy a line of length X is laid down and repeated at intervals of Y. The percentage transmlscion (or reflection) of the reproduced image is then Y - XtY [sic.
should read (Y-X)/Y~. To De a linear process (Y - X) must be directly proportional to the amplltuàe or the scanned vldeo signal where the signal amplitude represents the percentage opt1cal transmission of the recorded original image. A way of achieving thls lS by comparing the amplitude of the video signal w1th a sawtooth wave form and laying a line formlng a portion of a dot whenever the sawtooth is larger than the viàeo ~ignal.
See also U.S. Patent No. 4,040,094, which relates to slmilar suo~ect matter.
~owever, even if the method descrioed in thls patent is used ln an apparatus for reproduction of an lmage, the preclslon of gradation reproduction deteriorates due to tne delay of response of the apparatus.
The conventional method described in U.S. Patent No.
3,916,U~6, produces a linear mapping from the analog video slgnal to the pulse-width-modulated signal. As is known in the art of prlnting, this linear mapping does not produce acceptable results because of tne non-linear distortions lntroduced ln tne nalf-tone prlnting process, in particular when used with a laser beam print engine.
Thererore, tO obtaln high quality half-tone printing, a method of non-linear mapping must be found. And, the method disclosea ln the noted U.S. Patent, as ~uoted above, uses a com~lex arrangement to allow the use of aifferent sawtooth waveforms on successive scans.
S~IMARY OF THE INVE~TION
Accordingly, an ob~ect of tne present invention is to provide an image processlng apparatu~-, for generating an image from a digital video signal, that can overcome the prooleJls of the prlor art descrloed aDove.
Anotner ob~ect of the presen~ invention is to provlde an image processing apparatus, for generatlng an image from a digital video signal, that permits reproauctlon of images with high ~uality.
Still anotner ob~ect of the present inventlon is tO
provide an image processing apparatus, for generating an image ~rom a diqital video signal, that can provide, with a very simple arrangement, a superlor quality half-tone image.
_ 4 _ 1 3 3 7 0 8 2 Another object of the present invention is to provide an ima~e processlng apparatus, for senerating an image from a digital vldeo signal, that permlts reproduction of images with high quality at high speed.
A further object of the present invention is to provide an image proce~sing apparatus, for generating an image from a digital video signal, that can reproduce tone information with a high gradation and without impairing resolution.
Still another object of tne present invention is to provlde an image processlng apparatus that can correct the tonal properties of the vldeo image ~y providing a non~ ear mapping of the video signal onto a pulse-width-modulated signal with a very flexiDle arrangement.
In accordance with a preferred emDodiment, the image processing apparatus of the present invention processes a digltal image inyut signal and includes a raster scanning print engine for generating a series of successive ~cannlng lines. ~ pulse-wiatn-moaulated sisnal generator generates a pulse-width-modulated signal from a digital image input slgnal input to the apparatus, A circuit tnen applies tAe pulse-width-moàulated signal to the print englne to cause it to generate each line as a succession of line segments. Tne lengths of the line segments are controlled to produce a variaDle density line screen from the line segments with the line screen comprising a plurality of columns of the line segments, In accordance with another aspect of a preferred emDodiment of the present invention, the image processing apparatus includeS a pattern slgnal generator for generating a pattern signal of prede~ermined period, pulse- widtn-moaulated signal generator then generates a pu~se-wiatn-modulated signal ln accoraaslce with tile video `` - 1 337082 signal and the pattern signal that can be utilized by a raster scanning prlnt enSine or lmage forming device to form an image.
More spec1fically, the print engine scans lines on a recording medium with a beam in accordance with the pulse-width-modulated signal, and a synchronizing signal generator generates a synchronizing signal for each line scanned on the recording medium. The pattern signal generator generates the pattern signal of predetermined period in accordance with the synchronizing signal.
In accordance with still another aspect of the invention, the digital input signal has a characteri~tic, and a cnaracteristic converting device converts the cnaracteristic ln ~rder to produce a converted digital video signal. This signal is converted to an analog video signal Dy a digital to analog converter. A
pulse-width-modulated signal is thereafter generated from tnii analog video signal and the pattern signal.
Other aspects, features, and advantages of the present invention will Decome apparent from the followlng detailed description of the preferred emDodiments taken in con~unction witn the acco~panying drawing, as well as from the concluding claims.
BRIEF DESCRIP~ION OF THE DRA~IING
Fig. 1 lS a simplified schematlc illustratlon of a preferred embodiment of the apparatus for generatlng an lmage from a digital video slgnal ln accoraance with the present inven~ion;
Fig. 2 shows waveforms of s1gnal~ o~talned at different portions of the apparatus for generatlng an imace from a 1 33708.~
digltal video signal snown ln Fig. 1.
Fig. 3 shows how Figs. 3A and 3B are ~ssembled together to illustrate details of the embodiment of the apparatus for generating an image from a digital video slgnal shown in Fig. l;
Fig. 4 is a scheMatic illustration of an optical scannlng system in a laser Deam printer to which the invention is appiicable:
Flg. 5 ~nows waveforms of signals oDtalned at different portlons of the clrcult shown in Figs. 3A and 3B:
Fig. 6 lS an illustra~ion of trlangular wave signals formeà in the circ~it shown in Figs. 3A and 3B;
Figs. 7(a) to 7(c) are illustrations or how triangular wave signals ~ay be adjusted in the emDodiment of the invention:
Fig. 8 is an illustration or a look-up table of a gamma converting ROM 12;
Fig. 9 is a diagram showing the relationsnlp between input video signals and converted video signals;
Figs. 10(a) and 10(b~ illustrate the relationship between the scanning lines and the conversion ta~le as useà;
Flg. 11 lS a clrcuit diagram of a circuit for causlng phase shift of triangular wave slgnals between lines:
Fig. 12 lS an illustration of trlangular wave signals appearing ln respectlve lines at different phases; and ~ 3370~
Fig. 13 is an illustratio~ of another embodiment of the inventlon.
DEscRIpTIo~ OF T~E PREFERRED E~BODIMENTS
A preferrea emooaLment of the invent ~ wlll De described in detail hereln witA reference to the accompanying drawlng .
Referr~ng first to Fig. 1 scnematlcally showing an embodiment of the invention, a digital data output device 1 is adapted tO recelve an analog video data from a CCD
sensor or a video camera (neither of whicn is shown~ and to perform an A/D (analog-to-digital) conversion of the analog video signal so as tO convert that signal into a digital video signal, where each picture element (pixel) is represented by a predetermined number of bits carrying tone lnforma~ion. The di~ital video signal may be temporarily stored ~n a memory or, alternatively, may be suppLied from an external device b~, for example, telecommunication. The signal from the digital data output device 1 is used as the address for a digital look-up table for aam~a correct1on 9. The resultant output, whlch in tne prererred emDodiment is an eLgnt (d) bit digital numDer rangln~ from OOH to FFH representing 256 poss1Dle tonal gradatlon levels as àescr~bed further below, lS converted back into an analog signal by means of a D~A (digital to analog) converter 2 so as to form an analog signal whicA is updated for each picture element.
Tne analog video signal representing the picture elements is fed to one of the input terminals of a comparator circult 4. simultaneously~ analog reference pattern signals having a triangular waveform are produced by a pattern signal generator 3 at a perlod correspondina to the deslred pitcA of the half-tone screen. The pattern - a -slgnals (a trlans'e wave) are fed to the other input termlnal of tne comparator c1rcult 4. Meanwhile a horizontal synchronizing signal generatlng circuit 5 generates horlzontal synchronizing signals for respective lines, while an oscillator (reference clocK generating clrcuit) 6 generates reference clocks. In synchronism with the horizontal synchronizing signal, a timing signal generating circuit 7 counts down the reference clocKs, to, for example, 1/4 perloà. The signal derived from the timi-lg signal generating circuit 7 is used as the clocK
for the transfer of the àigital vldeo signal and also as the latcn timlng signal for the D/A converter 2.
In ~ne emDoaimenc aescrLbeà, since the apparatus is intended for use in a laser beam printer, the horizontal synchronizing signal correspollàs to a beam detect tBD) signal which lS ~nown per se. The comparator circu1t 4 co~pares tne level or the analog video signal with the level of the pattern signal of triangular waveform ana produces a pulse-wiath-modulated signal. The pulse-widtn-mcàulatea s~gnal is su~plied to, for example, the laser modulator circui; of a raster scanning print englne 8 for moaulating tne laser Deam. As a result, the lase{ beam is turnea on and O~c in accordance w~th the pulse wldtA tnereDy formlllg a nalf-cone lmace on the recording medium of the raster scannlns print engine 8.
Fig. 2 shows the wavefor~s of signals oDtained in certain cornponents of the apparatus snown in ~ig. 1. More specifically, the portion (a) of Fig. 2 diagra~matically shows the reference clocKs generated by the oscillator 6, while the portion ~b) shous the l~orizontal synchronizing signal mentioned above. The portlon (c~ shows the pixel clocKs which are producea by countlng down the reference clocKs with the timing signal generating circuit 7. More specifically, the pixel clock shown in the portion (c~ of ~ ! 3370~
g Fig. 2 1S the signal which lS obtained ~y counting down the reference clocKs into 1/4 period by tne operat~on of the timing signal generating circuit 7 in synchronism with the norlzontal syncnroniziny signal. The pixel cloc~ thus o~tained is delivered to the D/A converter 2 to be used as the aigital vldeo signal transfer cloc~. The portion (d) of Fig. 2 shows the pattern signal synchronizing clock (screen cloc~) which is obtained by countinq down the reference c~ock into 1/12 period by operation of the timing signal generating circuit 7 in synchronism with the horizontal syncnronizing signal. In the illustrated case, one pattern signal syncl~ronizing cLock is generate~ for every three pixel clocks. The pattern signal syncnr-onlzing or screen ClOCK tnus oDtained is deliYered to the pattern signal generator 3 to be used as the syncnronl~ing signal in the generation of the pattern signal. The portion ~e) of Fig. 2 shows the digital video signaL whicn is OUtpUt from the digital data output device 1. And the portion ~) snows the analog video signal after the D/A conversion conducted by tne D~A converter 2. It will be seen fro~ the pOL tions of Fig. 2 that picture element data or analog level are produced in synchronism with tne pixel clocks. It will also De seen tnat the denslty or lmage Decomes higher, i.e., approaches blac~, as the level of the analog video slgnal rises.
As shown oy a solid line curve in the portion (g) of Fig.
2, the output from tne pattern generator 3 is obtalned in synchronism with the clocks shown in the portion (d) and is input to the comparator circu1t 4. The broken line curve in the portioll (g) of Fig. 2 shows the analog video signal shown in tne portion (f). This video signal is comparea by the comparator circuit 4 with tne pattern signal of triangular waveform derived from the pattern siqnal generator 3 so that the analog video ~ignal is converted into a pulse-width-modulated signal as shown in the portion (h) of ~ig. 2.
The descriDed emoodiment of the invention permits a substantlally contlnuous or linear pulse modulation and, hence, ensures a high gradation of the image output by vlrtue of the fact that the digital video slgnal is converted into an analog video signal which is then compared with the triangular wave signal of a predetermined period.
It is to be noted also that, in the descriDed embodiment of the invention, the pattern signal synchronizing cloc~
(screen clock) for generation of the pattern signal, e.g., the triangular wave slgnal, is generated in synchronism witn tne l~orl~ontal syncnron~z1ng signal by making use of reference cloc~s having a frequency much nigner than tnat or the pattern slcnai synchronlzing slgnal. Therefore, tne ~itter of the pattern signal derived from the pattern s~gnal generator ~, e.g., the offset of the pattern signal from one scan line to the next, is reducea to less than 1/12 of the period of the ~attern signal. This precision is required to insure a hign quality half-tone reproducrion in whlch the line screen is formed un1formly and smoothly from one scan line to the next. Therefore, the àenslty informatlon can ~e accurately pulse-width-modulated by making use of this pattern signal which has a small fluctuation, so that the image can be reproduced with high quality.
Fig. 4 is a scnematic perspectlve view of the optical scanning system incorporated in the laser beam printer (a raster scanning print engine~ to which the present invention is applied. The scannlng system has a semlconductor laser adapted tO emlt a laser beam moaulace~
in accordance with the pulse-width-modulated signal ment1oneQ a~ove.
The optlcal laser beam modulated by the semiconductor laser 21 is collimated by a collinlator lens ~0 and is optically deflected by a polygonal mirror (applying means) 22 having a plurality of reflecting surfaces. The deflected Deam is focused to form an image on a photosensitiVe drum 12 by an image forming lens 23 referred to an fe lens, so as to scan the drum 12. Durina tne scannlng Dy the beam, and when the beam reaches the end of each scanning line, it is reflected by a mirror 24 and is directed to a Deam detector 25. The beam detection (BD) signal produced by tne Deam detector 25 is used as the n~rizontal syncnronizing slgnal as ls known. ~rhus, in tne descr~Ded emDodiment, the horizontal synchronizing s~gnal lS COnStltUted Dy tne BD signal.
It will be seen that tAe BD signal is detected for each of the lines of scanning by tne laser beam and is used as the timing signal for the transmission of tne pul-~e-width-moaulateu signal to the semiconductor laser.
As used in the subject specification in description of the preferred emDodinlent~ and as used in the concluding claims, tne term ~line-segment~ means a dot wnich is formed on a recording meàium, tne length (size) of which is variable in accordance witn the width of the pulse widtn in the supplied pulse-width-moaulatea signal.
The apparatus for generating an image from a digital viaeo signal of the invention will be descrloed more fully witn specific referenCe to Fig~. 3A and 3B wnich snow detail~
of tne apparatus shown in Fig. 1.
As stated before, the preferred emDodiment described herein maKes use of tne BD signal as the norlzontal synchronizing slgnal. The BD signal, however, is ~asically asynchronous witn tne plxel clock and, - 12 - ~ 3~ 7~8 2 therefore, would normally cause jitter in the horizontal direction. In the àescribed em~odiment, therefore, jitter is reduced to less than lJ4 of the width of a pixel, by making use of an oscillator 100 tnae can produce reference cloc~s (72~-CLK) (72 megahertz clock) of a fre~uency which is 4 times hlgner than that of tne pixel clocks. A BD
synchronizing circuit 200 is used for this purpose. The reference clock (72M-CL~) from the oscillator 100 is supplied to D latches 201, 202, and 203 through a buffer 101, while the BD slgnal is input to the aata terminal D
of the D latch 201 through a terminal 200a so as to be syncnronized with tne reference cloc~s. In adàition, tne BD slgnal is delayed by the D latches 202 and 203 by an a,~ount corresponaing to 2 (two~ reference cloc~ pulses.
The BD signal thus delayed is delivered to one of the input termlnals of a ~OR gate 103, while tne other input terminal of the r~G~ gaee 103 receives the inverted output of the D latc~l Z01. The output from the ~OR gate 103 is input to one of the input termlnals of a NOR gate 104, while the other input terminal of the NOR gate 104 receives the output of a flip-flop circuit 102.
~ith this arrangement, the flip-flop circuit 102 produces cLocks (36~l-CLK) (36 meaahert~ cloc~) whicn are oDtained Dy dividing the freauency of the reference cloc~ by 2 (two~. Thus, the output (36r~-CLK) from the flip-flop circuit lD2 is syrlcnronous wltn the ~D signal to within one period of the cloc~ 72~-CL~.
The output of the D latch 203 i~ aelayed by tne D latches 204, 205, and 206 by an amounc corresponding to 3 (three) pulses of the output (36r~-CLK) of tne flip-flop clrcuit 102.
The inverteà output from the D latcn 201 and tne outpue from the D latcn 206 are delivered tO a NOR gate 207, so that an internal horizontal synchroniz~g signal (BD-Pulse) is formeà in synchronism (within one period) with the reference clock.
Fig. 5 shows the timlns of the signals obtained at varLous portions of tne BD synchronizing circuit 200. More specifically, A-l sno~ the BD signal, A-2 shows the reference clock (72M-CLX) produced Dy the oscillator 100, and ~-3 snows the lnverted out~ut from the D latch 201, obtained by synchronizing the BD signal with the reference clock (72M-CLK). A-~ shows the output from the D latch 203, obtained by delaying the signal A-3 by an amount corresponding tO 2 (two) reference clocK pulses. A-5 shows the clock (36M-CLK) output from the flip-flop circult 102, A-6 shows the output from the D latch 206, obtained by delaying the signal A-4 by an amount correspon~ g to 3 (three) pulses of the clocks (36M-CLK), and A-7 shows tAe internal horizontal synchronizing (BD-Pulse). It wlll be seen that the internal horizontal synchronizin~ signal tB~-Pulse) rises in synchronism with the rise of the first reference clock t72M-CLX) after the rise of tne BD signal, an~ is held at level ~1~ for a period correspond1ng to 8 (eignt) pulses of the reference ClOCK. This internal horizon~al synchronizing signal (B~-Pulse) constitutes the reference for the horizontal driving of the circuit of this em~odiment.
An expianation of the video slgnals will now be mabe aaain with reference to Figs. 3A and 3B. The pixel clocks tPIXEL-CLKJ are formed by dividlns the freauency of the signai (36M-CLX) by 2 (two) by means of the J-K flip-flop circuit 105. A 6-Dit digltal video signal is latched in the D latch 10 by the pixel clock (PIXEL-CLX), anà the output is delivered to a ROM 12 for gamma converslon. The 8-bit video signal produced through the conversion by the ROi~ 12 is further converted into an analog signal by the - 14 _ 1 3 3 7 08 2 D/A converter 13 and is delivered to one of the input terminals ~f the comparator 15 in order to be compared with the triangular wave signal explained below. The pulse-widtn-modulated signal obtained as a result of the comparison is delivered to the laser driver of a raster scanning print engine.
Still referring to Figs. 3A and 3B, reference numeral 300 designates a screen clock generating circuit which generates the screen clock, i.e., the analog reference pattern signal synchronizing clock, which is used as the reference for the generation of tne triangular wave signal. A counter 301 is used as a frequency divider for dividing the freyuency of the signal (36M-CL~) output from the flip-flop circuit 102. The counter 301 has input terminals D, C, B, and A which are preset with predetermined data by means of a switch 303. The ratio of the frequency aivision is de,ermlnea by the values set at t~ese input terminals D, C, B, and A. For instance, when tne values ~ , and ~1~ are set in the terminals D, C, B, alld A, respectivel~, tne frequency of the signal (36M-CL~) is diviaed into 1/3.
Meanwhile, horizontal synchronization is attained by the NOR gate 302 and tAe (BD-~ulse) signal. The frequency of tne divided signal is further divided into 1~2 by a J-K
flip-flop circuit 304, so that a screen clock having a duty ratio of 50% is formed. A triangular wave generating circuit 500 generates triangular waves by using this screen clock as the reference.
Fig. 6 shows waveforms or signals appearing at various components of the screen clock generating circuit 300.
(It is noteà, nowever, that the scaies of ~igs. 5 and 6 are different). More specifically, B-l shows the internal synchroniz~ng signal (BD-PULS~`), B-2 shows the signal (36M-CLK) and B-3 snows the screen clock (SCREEN CLK~.as obtained wnen values ~ 0~ are set in the.
termlnals D, C, B, A o tAe counter 301, respectively..
B-4 represents tne triangular wave signal as obtained when the screen clock B-3 is used as the reference. on the other hand, 3-5 shows the screen cloc~ (SCREEN CL~) as obtained when values ~ 0~ are set in the input ter~1nals U, C, B, A of the counter 301. B-6 shows the triangular wave signal as obtained when the screen cloc~ (SCRcEN CLK) shown in B-S is used as tne reference obtained. It will be seen that the period of the tr1angular wave signal shown by ~-4 corresponds to 2 (two) picture elements, wnile the period of the triangular wave slgnal snow. by B-6 corresponds to 4 (four~ picture elements. 1~huS, the period of the triangular wave signal can De varlea as deslred oy appropr1ately setting the switch 303. In the emDodiment described, the period of tne triangular wave is chanyea~le between a duration corresponding to 1 (one) picture element and a duratio corresponding to 16 (sixteen) picture elements.
The trianqular wave signal generating clrcuit 500 will now be descriDed, again with reference to Figs. 3A ana 33.
The screen clock (SCREEN CL~) is received by the buffer SUl, and the triangular wave is generated by an integrator comprising by a variable resistor 502 and a capacitor 503. The triangular wave signal is then delivered to one of the input terminals of the comparator 15 through a capacitor 504, a protective resistor 506, ana a buffer amplifier 507. The triangular wave signal generatlng c~rcu~t 500 na~ two variable reslstors, namely, varlabie resistor 502 for adjusting the amplitude of the triangular wave slgnal, and a varlable resls~or 505 ror adjustlng the bias or ofrset of the triangular wave signal. The a~ustment of tne amylltude an~ the offset of the triangular wave signal by the variable resistors 502 and -505 is conducted in a manner wAlcn will be explained witn reference to Figs. 7(a) tO 7(c3. In Fig. 7(a), a triangular wave signal lri-l ~efore adjustment is shown by a solid line curve. By adjustlng the variable reslstor 502, the signal Trl-l is changed into an amplified triangular wave signal Tri-2 shown by a broken llne curve. Then, the variable resistor 505 can be adjusted to s~lft or ad~ust the offset of the wave so as to form a triangular wave signal Tri-3 shown ~y a one-dot-ana-one-dasn line curve. It i~ thus possible to o~taln a trlangular wave signal Aaving the desired aMplituae ana orr~et.
AS statea oerore, tne trlangular wave si~nal tnu~ ~ormed is compared ~y the comoarator 15 with the output of the D/A converter l~, i.e., witn tne analog Vlaeo signal. The relationship Detween the triangular wave signal and the analog VlaeO signal is pref~r~o~y sucn tnat the maxi.mum level of tne triangular wave equals the level of the output of tne ~/A converter 13 as obtainea wnen the lnput to the converter 13 Aas the maximum level (FF~, where H
indicates a hexidecimal numDer), wnile tne minimum value of the triangular wave signal eauals the level of the output of the ~/A converter 13 as obtained when the input to this converter has tne minimum level (00H). Since the amplitude and tAe orfset of tne trlangular wave can be controllea as desired, it is possible to obtain this preferred condltion without difficulty.
More particularly, according to the invention, the amplicuae ana the offset of the triangular wave slgllaL are ad~usted in tne following manner. In general, a laser drlver ror emltting a laser Dea-n ha~ a certain delay time in its operation. The delay time until the laser beam is actually emltted is further increased due to the beam emitting cnaracteristics of the laser. ~rherefore~ the ` - 17 - 1337~2 laser does not start emitting the laser beam until the wldth of the pulse 1nput to the drlver exceeds a predetermined value. This means that, in the case wnere tne input signal is a series of periodic pulses as in the case of the described embodiment, the laser does not emit a Deam unless the lnput signal pulse has a duty ratio greater than a predetermined value. Collversely, when the duty ratio of tne pulse is increased beyond a certain level, i.e., when the period of low level of the pulse is shortenea, tne la-er tends to stay on, that is, the bea~
is continuously emitted. For these reasons, if the adjustment of t~e trlangular wave signal is conàucted in the manner shown in Fig. 7(b), the gradation levels around the rninimum ieveL (0~) and near the maximum level (FFH) are omitted from the 2S6 gradation levels of the input data which m.ay ~e lnput tO tne D/~ converter 13, so that the gradation deteriorates undesirably. In the emDodiment describea, t~erefore, the variable resistor~ 502 and 5U5 are adjusted so that the pulse width just below that which will cause the laser to besln emission is obtained at the OOH level of the data input to tne D/A converter 13, and so that the pulse widtn whicn will render the laser continuously on is ootained at tne FF~ level of the data input to the D/A converter 13. This manner of ad~ustment of the variable resistors 502 and 505 is shown in Fig.
7~c).
As wiil be unaerstooa from Fis. 7(c), tnis preferred embodiment is designed so that the comparator lS produces an output pulse of a certain pulse width (a pul~e Wldth 7USt ~elaw that which will cause the laser to beg~n emission) wnen the minimum input data 00~ is supplied to the D/A converter 13. ~he preferred emDodiment is also designed so tnat, wnen the rnaximuln input data F~ff is supplied to the D/A converter 13, the comparator produces output pulses tne duty ratlo of whlch is not 100% but ~ ~37082 which lS large enough to allow the laser to emit the beam contlnuously. Thls arrangement ~erm~ts the emission time of the laser to vary nearly over the entire range of the 256 graaation levels of the lnput data, thus ensuring high gradation of the reproduced image.
It snoula-~e under~to~c that the method descriDed above is not limited to a laser printer but may also be utilized in to an inK jet printer, a thermal printer or otner raster scanning devices.
rrhe ~OM 12 for gamma conversion will now De explained in detall with reference to Fig. 8. The ROM 12 is provided to allow a nigh gradation of density in the reproduced image. Although the descriDed emDodiment employs a ROr~
having a capacity of 256 bytes as ROM 12, a capacity of 64 bytes is basically enough because the input digital video signal is a 6-bit signal. Fig. 8 snows the memory map of the ~OM 12 for gamma conversion. Since this RO~ has a capacity of 25c bytes, it can contain 4 (four) separate correction tables, namely TABLE-l including addresses U0}~
to 3F~, TA~LE-2 including aadresses 4U~ to 7FH, TABLE-3 including addresses ~0~ to BFH, and TABLE-4 including the aaaresses C~H to FFH .
Fig. 9 shows a practlcal exampie of tne input-output characteristics of each of the conversion taoles, i.e., the relationship between the input video slgnals and the converted output video signal. As will be seen from tnis Figure, the 64 (sixty-four) levels of the lnput video signal are converted into levels 0 to 255 (00H to FFH ) in accordance witn the respective conversion taDles. The change-over between tne converslon tables can be made Dy varylng the signal applied to uppec terminals A6 anà A7 of the ROM 12 as shown in Figs. 3A and 3B. The descrlDed emDodiment is deslgned to allow thls switchlng for eacn 1 3370a2 1~ --line, Dy the operatlon of a circuit 400 shown in Fig. 3A.
In operation, the internal norlzorltal syncnronlzing signal (BD-2ulse) is input to a counter 401 the output of Which is celiverea throu~n termlnals ~A and ~ to the terminals A6 and A7 of the ROM 12. The counter 401, in cooperation with an RCO ~nverter 4~2 anà a switcn 403 constitutes a ring counter, so tnat the period of ~-ching of tne converslon taDle can De varled in accoraance witn the state of the switch 403. For instance, when the switch 403 has the state ~l~ (at terminal B), ~1~ (at terminal A), TABLE-4 is always selected, whereas, when the state of tne switcn 403 is ~1~ (at term~nal B), 0~ (at terminal A), TABLE-4 and TABLE-3 are sel~cted alternately. When the swltcn 403 has the s~ate of U~ (at termlnal B), ~0 (at terminal A), T~R~-l, TABLE-2, TABLE-3, and TABLL-4 are successlvely seLected for successiYe lines, as shown in Fig. 10a. MoreoYer, it is possible to improve the gradation by cnangins tne conversion table for successive line~.
In general, ln tne electropnotograpnic reproductlon of an image, the gradation lS more difficult to obtain in the lignt por~ion of the image tnan ln the dark por~ion of the image. Therefore, as in the example shown in Fig. 9, the converslon taDles are suostantially duplicated in tne aark portions of tne imaae and differ in the lignt po~tion so as to provide opt1ma} gradation.
In the preferred emDodiment, the switching of the table can also be made in tne directlon of the main scan by the laser beam.
More speclfically as shown in Figs. 3A ana 3B, a sianal can be formed by dividing the frequency of the screen clock (SCRLEN-CL~) Dy 2 (two) by means of a J-K flip-flop circuit 404, inputting the resulting signal to one input - 2~ - ~ 337~2 termlnal or an exclusive OR clrcult 406, the other input or whicn is connected to terminal ~B of tne counter 401 and the output termlnal of which is then connected to ROM
12 tnrough a latch ll. Witn th1s arrangement, it is S possible to chan~e the conversion table in a stagqered manner as shown ln Fig. l0(b), thus attaining a further improvement in the gradation. A reference numeral 405 denotes a switcn for selectlng either switching of the table in tne staggered manner described above or not so switching. The staggered switching of the table is selected when this switch has the ~l~ level and is not selecte~ when the swltch has the ~ level. The numerals appearing in frames of the table snown in ~ig. 10(b) repre~en~ the numDers of tne selected convers1on ta~les l to 4. Thus, the period of the screen clock in the em~o~iment correspollas to ~he period of ~ (tnree~ pixel clocks.
It wlll oe understooa from tAe descriptio~l provided aDove that the scanning lines produced by the laser in accordance ~ith data from the conversion tables of tne ROM
12 are each generated as a succession of line segments.
The line segments of successlve scann1ng lines collectively form a plurality of columns that define a line screen.
More particularly, when tne video signal processed by the circuit shown in cigs. 3A and 3B is directly delivered to a reproducing means such as a laser bea~;l printer, the reproduced image has a structure with vertical columns (in the descrlDed emDoalment ~ the line screen lS com~oseà of vertical columns of line seg~ents of successive scanning llnes wnich form in the reproduc~d image) due to the fact that the phase of the triangular wave signal is the same a~ tnat of tne lnternal horlzontal syncllronlzlng signal (~D-~ulse) for each line. 1he circuit in the present ` ` - 1 337082 embodiment lS one in which the triangular wave is formed arter the reference clocks are counted by 12 (twelve) from the rise of the BD-Pulse signal. The timing for the generation of triangular waves is the same for each line, and so each pAase of the triangular waves on each line is the same. The image aata is output from the digital data output device 1 as stated above. The digital data output device 1 outputs image data with a predetermined timing in synchronism with a signal e~uivalent to the BD-Pulse lD signal. More particularly, the aata output device 1 is adapted to receive the ~D signal. This device 1 starts to count tne rererence clocx after receiving the BD signal, and be~ins transmission of the image data after counting tne rererence cloc~s u~ to a predetermined nur.lber. As a conse~uence, the timing of transfer of the image data necessary ror image reproductlon is the same on each line, and a high quality reproduced image with no image jitter can be proauced. ~a tne tim.~ng of tne generation of the triangular waves and the timing of transfer of the image data necessary for image reproduction have the same relation on all of the lines, the reproduced image has its vertical column structure with no image jitter, which is effective, for example, in reducing a particular Moire pattern. Again th~s vertical column structure comprises a line screen having a vertical columnar axis extending at an angle, that is perpendicular to the raster scanning lines.
It is also possi~le to obtaln a reproduced image having a structure compriSing oDlique line screen columns, if the pna~e of the ~riangular wave slgnals is made to De offset slightly for successive lines. This is effective in reàucin~ tne l1oire pattern wnlcn appears undeslraDiy w~en an ori~inal dot imaae is read and processed. The angle of lnclination of tne o~liyue columns can be determined as desired by suitaoly selectlng the amount of shift of the phase of the screen clocks for successive lines. For islstance a reprobuced image comprisin~ scanning lines having oDlique columns inclined at 45 degrees can be oDta~ned Dy shifting tl~e triangular wave signal by an amount corresponding to one p~cture element, i.e. by phase snifting tne triangular wave sigrlal lZ0 degrees for each of the successive columns. Fig. 11 shows a circuit for reproauc1ng an image comprising oDliyue columns. ~lore specifically, a reproduced imagè comprising oblique columns can De ootalned by usinq tnis circuit in place or tne screen clock generating circuit 300 in the circuit sAown in Fis. ~.
R~ferring agaln ~o Fi~. 11, tne internal hor~zontal syncnronizing signal (BD-Pulse) is laccned by the pixel clocks (PIXEL-CL~) oy mean~ of D latches ~56 ana 3~7, so that three internal horizontal synchronizing signals (BD-Pul~e~ ilaving dirferent pnases are pro~uced. Then, one of these three internal horizontal synchronizing signais (BD-~ulse) is selectea for eacn line Dy operation of a counter 35~, inverters 359 and 360, and gate circuits ~ol to 367. The seiected slgnal 1S lnput as a LOAD signal to a counter 351 thereby changing the phase of the screen clocKs for success1ve lines. Tne counter 351 is adapted to divide the frequency of the signal (36M-CLK) into 1/3 w~ile tne J-K fllp-flOp circuLt ~54 furtner civides the frequency of the output from tne counter 351 into 1/2.
Wlth tnis arrangement~ it is posslole to generate one screen clock for every three picture elements.
F-g. 12 ShOWS timing of the screen clock generated ~y the clrcuit of Flg. 11 and the triangular wave signal for successlve llnes. These thre~ trlangulâr wave slgna~s are generated in seauence of eacn see of each 3 lines.
~Ihen the reference pattern signal is generated in syncnronl~m with a group of plcture elements as in the case of the emDodiment described, it iS possible to shift the syncnron~zing slsnal used in tne generation of the pattern signal by an amount corresponding to one half of the reference pattern slsnal perlod for each successive set of scan lines e~ual to the widtn of the pattern signal. ~ucn a metnod allows the positlon of the center of growth of tne pulse width to De shifted in each of ~uccesslve lines, so that the output image can have an appearance resemoiing that proaucea by half-tone dots arranaed along o~ ue lines.
In the clrcuit shown in Figs. 3A and 3B, the RO.'1 12 is u~ed for the purpose of gamma conversion. Thls, nowever, is not tne only element suitable for this purpose and the ROtl 12 may oe re~lacea Dy an S-~li connectea to tne DA~A
8US line of a co~puter. ~ith sucn an arrangement, it is possiDle to rew-lte the gam~la converslon taole as desired in accordance w th, for exampie, a cAange in the kind of the orislnal~ tnus lncreasing tne adaptaDility of the apparatus or tne invent~on.
Fig. 13 snow~ an exam~le of a clrcuit wnich is usable in place of the ROM 12 in the circ~it shown in Figs. 3A anà
3B. This clrcuit has, as wlll De seen from tnis Figure, an S-RAl~ 12a for ga.~ma conversion, a decoder 30, a mlcrocomyuter 31 ior re~rlting tne gamma conversion tables, tri-state buffers 32 and 33, and a bi-directional tri-state ouffer 34.
The mude changlns switcnes 304, 4U3 anà 405 in the circult shown in Figs. 3A ana 3B may be controlled Dy the microcomputer 31 so as tO increase the flexiDility of the system as a wnole.
` - 1 337082 Although the lnvention has been described with reference to speclfic emoodiments and in specific terlns it is to be understood that this description lS only illustrative purpose~ ana that various other cnanges and modifications are posslble without departing from the scope of the invention.
BACKGROU~JD OF T~E I~E2~TION
Field of the Invention Tne present inve-ltion relates to an apparatus for generating an image from a dig1tal video input signal.
The apparatus is improved so as to reproduce an image with high quality.
Descript1on of the Prior Art In the past, meenods generally rererred to as tne dither method and tne density pattern method have been proposed for reproduc1ng lmages of half tone~. These known methods, nowever, cannot provlde satlsfactory gradation of dot size when the slze o~f the thresnold dot matrix is small ana, therefOre~ require the use of a threshold matrix navin~ a larger size. Thls is turn reduces the resolution and undesirably allows t~e texture of the image to appear too dlstinct1ve due tO the periodic structure of the maerix. Therefore, deterloratlon of the quality of the output lmage results.
~ 2 - ~ 337082 In order to mltigate the above described problems, it has Deen proposed to moalfy the dlther metnoa so as to allow finer control of the dot size by the use of a plurality of dither matrices. This metnod, however, requires a 5 complicated circuit arrangement for obtaining syncnronism or operation Detween the ditner matrlces so tnat the system as a whole is large in size, complicated in constructlon, ana slow. Thus, tnere is a practical limit in the incremental increase of dot size and the resultant increment of density available by the use of a plurality of dither matrices. In ~.S. Patent No. ~,916,096, a method of improving the conventional screening process lS
describea. As set fortn in this U.S. Patent No.
3,916,096, at column 8, lines 19 througn 31:
The conventional screening process when applied to a scanned image can be regarded as a form of pulse-widtn-modulation whereDy a line of length X is laid down and repeated at intervals of Y. The percentage transmlscion (or reflection) of the reproduced image is then Y - XtY [sic.
should read (Y-X)/Y~. To De a linear process (Y - X) must be directly proportional to the amplltuàe or the scanned vldeo signal where the signal amplitude represents the percentage opt1cal transmission of the recorded original image. A way of achieving thls lS by comparing the amplitude of the video signal w1th a sawtooth wave form and laying a line formlng a portion of a dot whenever the sawtooth is larger than the viàeo ~ignal.
See also U.S. Patent No. 4,040,094, which relates to slmilar suo~ect matter.
~owever, even if the method descrioed in thls patent is used ln an apparatus for reproduction of an lmage, the preclslon of gradation reproduction deteriorates due to tne delay of response of the apparatus.
The conventional method described in U.S. Patent No.
3,916,U~6, produces a linear mapping from the analog video slgnal to the pulse-width-modulated signal. As is known in the art of prlnting, this linear mapping does not produce acceptable results because of tne non-linear distortions lntroduced ln tne nalf-tone prlnting process, in particular when used with a laser beam print engine.
Thererore, tO obtaln high quality half-tone printing, a method of non-linear mapping must be found. And, the method disclosea ln the noted U.S. Patent, as ~uoted above, uses a com~lex arrangement to allow the use of aifferent sawtooth waveforms on successive scans.
S~IMARY OF THE INVE~TION
Accordingly, an ob~ect of tne present invention is to provide an image processlng apparatu~-, for generating an image from a digital video signal, that can overcome the prooleJls of the prlor art descrloed aDove.
Anotner ob~ect of the presen~ invention is to provlde an image processing apparatus, for generatlng an image from a digital video signal, that permits reproauctlon of images with high ~uality.
Still anotner ob~ect of the present inventlon is tO
provide an image processing apparatus, for generating an image ~rom a diqital video signal, that can provide, with a very simple arrangement, a superlor quality half-tone image.
_ 4 _ 1 3 3 7 0 8 2 Another object of the present invention is to provide an ima~e processlng apparatus, for senerating an image from a digital vldeo signal, that permlts reproduction of images with high quality at high speed.
A further object of the present invention is to provide an image proce~sing apparatus, for generating an image from a digital video signal, that can reproduce tone information with a high gradation and without impairing resolution.
Still another object of tne present invention is to provlde an image processlng apparatus that can correct the tonal properties of the vldeo image ~y providing a non~ ear mapping of the video signal onto a pulse-width-modulated signal with a very flexiDle arrangement.
In accordance with a preferred emDodiment, the image processing apparatus of the present invention processes a digltal image inyut signal and includes a raster scanning print engine for generating a series of successive ~cannlng lines. ~ pulse-wiatn-moaulated sisnal generator generates a pulse-width-modulated signal from a digital image input slgnal input to the apparatus, A circuit tnen applies tAe pulse-width-moàulated signal to the print englne to cause it to generate each line as a succession of line segments. Tne lengths of the line segments are controlled to produce a variaDle density line screen from the line segments with the line screen comprising a plurality of columns of the line segments, In accordance with another aspect of a preferred emDodiment of the present invention, the image processing apparatus includeS a pattern slgnal generator for generating a pattern signal of prede~ermined period, pulse- widtn-moaulated signal generator then generates a pu~se-wiatn-modulated signal ln accoraaslce with tile video `` - 1 337082 signal and the pattern signal that can be utilized by a raster scanning prlnt enSine or lmage forming device to form an image.
More spec1fically, the print engine scans lines on a recording medium with a beam in accordance with the pulse-width-modulated signal, and a synchronizing signal generator generates a synchronizing signal for each line scanned on the recording medium. The pattern signal generator generates the pattern signal of predetermined period in accordance with the synchronizing signal.
In accordance with still another aspect of the invention, the digital input signal has a characteri~tic, and a cnaracteristic converting device converts the cnaracteristic ln ~rder to produce a converted digital video signal. This signal is converted to an analog video signal Dy a digital to analog converter. A
pulse-width-modulated signal is thereafter generated from tnii analog video signal and the pattern signal.
Other aspects, features, and advantages of the present invention will Decome apparent from the followlng detailed description of the preferred emDodiments taken in con~unction witn the acco~panying drawing, as well as from the concluding claims.
BRIEF DESCRIP~ION OF THE DRA~IING
Fig. 1 lS a simplified schematlc illustratlon of a preferred embodiment of the apparatus for generatlng an lmage from a digital video slgnal ln accoraance with the present inven~ion;
Fig. 2 shows waveforms of s1gnal~ o~talned at different portions of the apparatus for generatlng an imace from a 1 33708.~
digltal video signal snown ln Fig. 1.
Fig. 3 shows how Figs. 3A and 3B are ~ssembled together to illustrate details of the embodiment of the apparatus for generating an image from a digital video slgnal shown in Fig. l;
Fig. 4 is a scheMatic illustration of an optical scannlng system in a laser Deam printer to which the invention is appiicable:
Flg. 5 ~nows waveforms of signals oDtalned at different portlons of the clrcult shown in Figs. 3A and 3B:
Fig. 6 lS an illustra~ion of trlangular wave signals formeà in the circ~it shown in Figs. 3A and 3B;
Figs. 7(a) to 7(c) are illustrations or how triangular wave signals ~ay be adjusted in the emDodiment of the invention:
Fig. 8 is an illustration or a look-up table of a gamma converting ROM 12;
Fig. 9 is a diagram showing the relationsnlp between input video signals and converted video signals;
Figs. 10(a) and 10(b~ illustrate the relationship between the scanning lines and the conversion ta~le as useà;
Flg. 11 lS a clrcuit diagram of a circuit for causlng phase shift of triangular wave slgnals between lines:
Fig. 12 lS an illustration of trlangular wave signals appearing ln respectlve lines at different phases; and ~ 3370~
Fig. 13 is an illustratio~ of another embodiment of the inventlon.
DEscRIpTIo~ OF T~E PREFERRED E~BODIMENTS
A preferrea emooaLment of the invent ~ wlll De described in detail hereln witA reference to the accompanying drawlng .
Referr~ng first to Fig. 1 scnematlcally showing an embodiment of the invention, a digital data output device 1 is adapted tO recelve an analog video data from a CCD
sensor or a video camera (neither of whicn is shown~ and to perform an A/D (analog-to-digital) conversion of the analog video signal so as tO convert that signal into a digital video signal, where each picture element (pixel) is represented by a predetermined number of bits carrying tone lnforma~ion. The di~ital video signal may be temporarily stored ~n a memory or, alternatively, may be suppLied from an external device b~, for example, telecommunication. The signal from the digital data output device 1 is used as the address for a digital look-up table for aam~a correct1on 9. The resultant output, whlch in tne prererred emDodiment is an eLgnt (d) bit digital numDer rangln~ from OOH to FFH representing 256 poss1Dle tonal gradatlon levels as àescr~bed further below, lS converted back into an analog signal by means of a D~A (digital to analog) converter 2 so as to form an analog signal whicA is updated for each picture element.
Tne analog video signal representing the picture elements is fed to one of the input terminals of a comparator circult 4. simultaneously~ analog reference pattern signals having a triangular waveform are produced by a pattern signal generator 3 at a perlod correspondina to the deslred pitcA of the half-tone screen. The pattern - a -slgnals (a trlans'e wave) are fed to the other input termlnal of tne comparator c1rcult 4. Meanwhile a horizontal synchronizing signal generatlng circuit 5 generates horlzontal synchronizing signals for respective lines, while an oscillator (reference clocK generating clrcuit) 6 generates reference clocks. In synchronism with the horizontal synchronizing signal, a timing signal generating circuit 7 counts down the reference clocKs, to, for example, 1/4 perloà. The signal derived from the timi-lg signal generating circuit 7 is used as the clocK
for the transfer of the àigital vldeo signal and also as the latcn timlng signal for the D/A converter 2.
In ~ne emDoaimenc aescrLbeà, since the apparatus is intended for use in a laser beam printer, the horizontal synchronizing signal correspollàs to a beam detect tBD) signal which lS ~nown per se. The comparator circu1t 4 co~pares tne level or the analog video signal with the level of the pattern signal of triangular waveform ana produces a pulse-wiath-modulated signal. The pulse-widtn-mcàulatea s~gnal is su~plied to, for example, the laser modulator circui; of a raster scanning print englne 8 for moaulating tne laser Deam. As a result, the lase{ beam is turnea on and O~c in accordance w~th the pulse wldtA tnereDy formlllg a nalf-cone lmace on the recording medium of the raster scannlns print engine 8.
Fig. 2 shows the wavefor~s of signals oDtained in certain cornponents of the apparatus snown in ~ig. 1. More specifically, the portion (a) of Fig. 2 diagra~matically shows the reference clocKs generated by the oscillator 6, while the portion ~b) shous the l~orizontal synchronizing signal mentioned above. The portlon (c~ shows the pixel clocKs which are producea by countlng down the reference clocKs with the timing signal generating circuit 7. More specifically, the pixel clock shown in the portion (c~ of ~ ! 3370~
g Fig. 2 1S the signal which lS obtained ~y counting down the reference clocKs into 1/4 period by tne operat~on of the timing signal generating circuit 7 in synchronism with the norlzontal syncnroniziny signal. The pixel cloc~ thus o~tained is delivered to the D/A converter 2 to be used as the aigital vldeo signal transfer cloc~. The portion (d) of Fig. 2 shows the pattern signal synchronizing clock (screen cloc~) which is obtained by countinq down the reference c~ock into 1/12 period by operation of the timing signal generating circuit 7 in synchronism with the horizontal syncnronizing signal. In the illustrated case, one pattern signal syncl~ronizing cLock is generate~ for every three pixel clocks. The pattern signal syncnr-onlzing or screen ClOCK tnus oDtained is deliYered to the pattern signal generator 3 to be used as the syncnronl~ing signal in the generation of the pattern signal. The portion ~e) of Fig. 2 shows the digital video signaL whicn is OUtpUt from the digital data output device 1. And the portion ~) snows the analog video signal after the D/A conversion conducted by tne D~A converter 2. It will be seen fro~ the pOL tions of Fig. 2 that picture element data or analog level are produced in synchronism with tne pixel clocks. It will also De seen tnat the denslty or lmage Decomes higher, i.e., approaches blac~, as the level of the analog video slgnal rises.
As shown oy a solid line curve in the portion (g) of Fig.
2, the output from tne pattern generator 3 is obtalned in synchronism with the clocks shown in the portion (d) and is input to the comparator circu1t 4. The broken line curve in the portioll (g) of Fig. 2 shows the analog video signal shown in tne portion (f). This video signal is comparea by the comparator circuit 4 with tne pattern signal of triangular waveform derived from the pattern siqnal generator 3 so that the analog video ~ignal is converted into a pulse-width-modulated signal as shown in the portion (h) of ~ig. 2.
The descriDed emoodiment of the invention permits a substantlally contlnuous or linear pulse modulation and, hence, ensures a high gradation of the image output by vlrtue of the fact that the digital video slgnal is converted into an analog video signal which is then compared with the triangular wave signal of a predetermined period.
It is to be noted also that, in the descriDed embodiment of the invention, the pattern signal synchronizing cloc~
(screen clock) for generation of the pattern signal, e.g., the triangular wave slgnal, is generated in synchronism witn tne l~orl~ontal syncnron~z1ng signal by making use of reference cloc~s having a frequency much nigner than tnat or the pattern slcnai synchronlzing slgnal. Therefore, tne ~itter of the pattern signal derived from the pattern s~gnal generator ~, e.g., the offset of the pattern signal from one scan line to the next, is reducea to less than 1/12 of the period of the ~attern signal. This precision is required to insure a hign quality half-tone reproducrion in whlch the line screen is formed un1formly and smoothly from one scan line to the next. Therefore, the àenslty informatlon can ~e accurately pulse-width-modulated by making use of this pattern signal which has a small fluctuation, so that the image can be reproduced with high quality.
Fig. 4 is a scnematic perspectlve view of the optical scanning system incorporated in the laser beam printer (a raster scanning print engine~ to which the present invention is applied. The scannlng system has a semlconductor laser adapted tO emlt a laser beam moaulace~
in accordance with the pulse-width-modulated signal ment1oneQ a~ove.
The optlcal laser beam modulated by the semiconductor laser 21 is collimated by a collinlator lens ~0 and is optically deflected by a polygonal mirror (applying means) 22 having a plurality of reflecting surfaces. The deflected Deam is focused to form an image on a photosensitiVe drum 12 by an image forming lens 23 referred to an fe lens, so as to scan the drum 12. Durina tne scannlng Dy the beam, and when the beam reaches the end of each scanning line, it is reflected by a mirror 24 and is directed to a Deam detector 25. The beam detection (BD) signal produced by tne Deam detector 25 is used as the n~rizontal syncnronizing slgnal as ls known. ~rhus, in tne descr~Ded emDodiment, the horizontal synchronizing s~gnal lS COnStltUted Dy tne BD signal.
It will be seen that tAe BD signal is detected for each of the lines of scanning by tne laser beam and is used as the timing signal for the transmission of tne pul-~e-width-moaulateu signal to the semiconductor laser.
As used in the subject specification in description of the preferred emDodinlent~ and as used in the concluding claims, tne term ~line-segment~ means a dot wnich is formed on a recording meàium, tne length (size) of which is variable in accordance witn the width of the pulse widtn in the supplied pulse-width-moaulatea signal.
The apparatus for generating an image from a digital viaeo signal of the invention will be descrloed more fully witn specific referenCe to Fig~. 3A and 3B wnich snow detail~
of tne apparatus shown in Fig. 1.
As stated before, the preferred emDodiment described herein maKes use of tne BD signal as the norlzontal synchronizing slgnal. The BD signal, however, is ~asically asynchronous witn tne plxel clock and, - 12 - ~ 3~ 7~8 2 therefore, would normally cause jitter in the horizontal direction. In the àescribed em~odiment, therefore, jitter is reduced to less than lJ4 of the width of a pixel, by making use of an oscillator 100 tnae can produce reference cloc~s (72~-CLK) (72 megahertz clock) of a fre~uency which is 4 times hlgner than that of tne pixel clocks. A BD
synchronizing circuit 200 is used for this purpose. The reference clock (72M-CL~) from the oscillator 100 is supplied to D latches 201, 202, and 203 through a buffer 101, while the BD slgnal is input to the aata terminal D
of the D latch 201 through a terminal 200a so as to be syncnronized with tne reference cloc~s. In adàition, tne BD slgnal is delayed by the D latches 202 and 203 by an a,~ount corresponaing to 2 (two~ reference cloc~ pulses.
The BD signal thus delayed is delivered to one of the input termlnals of a ~OR gate 103, while tne other input terminal of the r~G~ gaee 103 receives the inverted output of the D latc~l Z01. The output from the ~OR gate 103 is input to one of the input termlnals of a NOR gate 104, while the other input terminal of the NOR gate 104 receives the output of a flip-flop circuit 102.
~ith this arrangement, the flip-flop circuit 102 produces cLocks (36~l-CLK) (36 meaahert~ cloc~) whicn are oDtained Dy dividing the freauency of the reference cloc~ by 2 (two~. Thus, the output (36r~-CLK) from the flip-flop circuit lD2 is syrlcnronous wltn the ~D signal to within one period of the cloc~ 72~-CL~.
The output of the D latch 203 i~ aelayed by tne D latches 204, 205, and 206 by an amounc corresponding to 3 (three) pulses of the output (36r~-CLK) of tne flip-flop clrcuit 102.
The inverteà output from the D latcn 201 and tne outpue from the D latcn 206 are delivered tO a NOR gate 207, so that an internal horizontal synchroniz~g signal (BD-Pulse) is formeà in synchronism (within one period) with the reference clock.
Fig. 5 shows the timlns of the signals obtained at varLous portions of tne BD synchronizing circuit 200. More specifically, A-l sno~ the BD signal, A-2 shows the reference clock (72M-CLX) produced Dy the oscillator 100, and ~-3 snows the lnverted out~ut from the D latch 201, obtained by synchronizing the BD signal with the reference clock (72M-CLK). A-~ shows the output from the D latch 203, obtained by delaying the signal A-3 by an amount corresponding tO 2 (two) reference clocK pulses. A-5 shows the clock (36M-CLK) output from the flip-flop circult 102, A-6 shows the output from the D latch 206, obtained by delaying the signal A-4 by an amount correspon~ g to 3 (three) pulses of the clocks (36M-CLK), and A-7 shows tAe internal horizontal synchronizing (BD-Pulse). It wlll be seen that the internal horizontal synchronizin~ signal tB~-Pulse) rises in synchronism with the rise of the first reference clock t72M-CLX) after the rise of tne BD signal, an~ is held at level ~1~ for a period correspond1ng to 8 (eignt) pulses of the reference ClOCK. This internal horizon~al synchronizing signal (B~-Pulse) constitutes the reference for the horizontal driving of the circuit of this em~odiment.
An expianation of the video slgnals will now be mabe aaain with reference to Figs. 3A and 3B. The pixel clocks tPIXEL-CLKJ are formed by dividlns the freauency of the signai (36M-CLX) by 2 (two) by means of the J-K flip-flop circuit 105. A 6-Dit digltal video signal is latched in the D latch 10 by the pixel clock (PIXEL-CLX), anà the output is delivered to a ROM 12 for gamma converslon. The 8-bit video signal produced through the conversion by the ROi~ 12 is further converted into an analog signal by the - 14 _ 1 3 3 7 08 2 D/A converter 13 and is delivered to one of the input terminals ~f the comparator 15 in order to be compared with the triangular wave signal explained below. The pulse-widtn-modulated signal obtained as a result of the comparison is delivered to the laser driver of a raster scanning print engine.
Still referring to Figs. 3A and 3B, reference numeral 300 designates a screen clock generating circuit which generates the screen clock, i.e., the analog reference pattern signal synchronizing clock, which is used as the reference for the generation of tne triangular wave signal. A counter 301 is used as a frequency divider for dividing the freyuency of the signal (36M-CL~) output from the flip-flop circuit 102. The counter 301 has input terminals D, C, B, and A which are preset with predetermined data by means of a switch 303. The ratio of the frequency aivision is de,ermlnea by the values set at t~ese input terminals D, C, B, and A. For instance, when tne values ~ , and ~1~ are set in the terminals D, C, B, alld A, respectivel~, tne frequency of the signal (36M-CL~) is diviaed into 1/3.
Meanwhile, horizontal synchronization is attained by the NOR gate 302 and tAe (BD-~ulse) signal. The frequency of tne divided signal is further divided into 1~2 by a J-K
flip-flop circuit 304, so that a screen clock having a duty ratio of 50% is formed. A triangular wave generating circuit 500 generates triangular waves by using this screen clock as the reference.
Fig. 6 shows waveforms or signals appearing at various components of the screen clock generating circuit 300.
(It is noteà, nowever, that the scaies of ~igs. 5 and 6 are different). More specifically, B-l shows the internal synchroniz~ng signal (BD-PULS~`), B-2 shows the signal (36M-CLK) and B-3 snows the screen clock (SCREEN CLK~.as obtained wnen values ~ 0~ are set in the.
termlnals D, C, B, A o tAe counter 301, respectively..
B-4 represents tne triangular wave signal as obtained when the screen clock B-3 is used as the reference. on the other hand, 3-5 shows the screen cloc~ (SCREEN CL~) as obtained when values ~ 0~ are set in the input ter~1nals U, C, B, A of the counter 301. B-6 shows the triangular wave signal as obtained when the screen cloc~ (SCRcEN CLK) shown in B-S is used as tne reference obtained. It will be seen that the period of the tr1angular wave signal shown by ~-4 corresponds to 2 (two) picture elements, wnile the period of the triangular wave slgnal snow. by B-6 corresponds to 4 (four~ picture elements. 1~huS, the period of the triangular wave signal can De varlea as deslred oy appropr1ately setting the switch 303. In the emDodiment described, the period of tne triangular wave is chanyea~le between a duration corresponding to 1 (one) picture element and a duratio corresponding to 16 (sixteen) picture elements.
The trianqular wave signal generating clrcuit 500 will now be descriDed, again with reference to Figs. 3A ana 33.
The screen clock (SCREEN CL~) is received by the buffer SUl, and the triangular wave is generated by an integrator comprising by a variable resistor 502 and a capacitor 503. The triangular wave signal is then delivered to one of the input terminals of the comparator 15 through a capacitor 504, a protective resistor 506, ana a buffer amplifier 507. The triangular wave signal generatlng c~rcu~t 500 na~ two variable reslstors, namely, varlabie resistor 502 for adjusting the amplitude of the triangular wave slgnal, and a varlable resls~or 505 ror adjustlng the bias or ofrset of the triangular wave signal. The a~ustment of tne amylltude an~ the offset of the triangular wave signal by the variable resistors 502 and -505 is conducted in a manner wAlcn will be explained witn reference to Figs. 7(a) tO 7(c3. In Fig. 7(a), a triangular wave signal lri-l ~efore adjustment is shown by a solid line curve. By adjustlng the variable reslstor 502, the signal Trl-l is changed into an amplified triangular wave signal Tri-2 shown by a broken llne curve. Then, the variable resistor 505 can be adjusted to s~lft or ad~ust the offset of the wave so as to form a triangular wave signal Tri-3 shown ~y a one-dot-ana-one-dasn line curve. It i~ thus possible to o~taln a trlangular wave signal Aaving the desired aMplituae ana orr~et.
AS statea oerore, tne trlangular wave si~nal tnu~ ~ormed is compared ~y the comoarator 15 with the output of the D/A converter l~, i.e., witn tne analog Vlaeo signal. The relationship Detween the triangular wave signal and the analog VlaeO signal is pref~r~o~y sucn tnat the maxi.mum level of tne triangular wave equals the level of the output of tne ~/A converter 13 as obtainea wnen the lnput to the converter 13 Aas the maximum level (FF~, where H
indicates a hexidecimal numDer), wnile tne minimum value of the triangular wave signal eauals the level of the output of the ~/A converter 13 as obtained when the input to this converter has tne minimum level (00H). Since the amplitude and tAe orfset of tne trlangular wave can be controllea as desired, it is possible to obtain this preferred condltion without difficulty.
More particularly, according to the invention, the amplicuae ana the offset of the triangular wave slgllaL are ad~usted in tne following manner. In general, a laser drlver ror emltting a laser Dea-n ha~ a certain delay time in its operation. The delay time until the laser beam is actually emltted is further increased due to the beam emitting cnaracteristics of the laser. ~rherefore~ the ` - 17 - 1337~2 laser does not start emitting the laser beam until the wldth of the pulse 1nput to the drlver exceeds a predetermined value. This means that, in the case wnere tne input signal is a series of periodic pulses as in the case of the described embodiment, the laser does not emit a Deam unless the lnput signal pulse has a duty ratio greater than a predetermined value. Collversely, when the duty ratio of tne pulse is increased beyond a certain level, i.e., when the period of low level of the pulse is shortenea, tne la-er tends to stay on, that is, the bea~
is continuously emitted. For these reasons, if the adjustment of t~e trlangular wave signal is conàucted in the manner shown in Fig. 7(b), the gradation levels around the rninimum ieveL (0~) and near the maximum level (FFH) are omitted from the 2S6 gradation levels of the input data which m.ay ~e lnput tO tne D/~ converter 13, so that the gradation deteriorates undesirably. In the emDodiment describea, t~erefore, the variable resistor~ 502 and 5U5 are adjusted so that the pulse width just below that which will cause the laser to besln emission is obtained at the OOH level of the data input to tne D/A converter 13, and so that the pulse widtn whicn will render the laser continuously on is ootained at tne FF~ level of the data input to the D/A converter 13. This manner of ad~ustment of the variable resistors 502 and 505 is shown in Fig.
7~c).
As wiil be unaerstooa from Fis. 7(c), tnis preferred embodiment is designed so that the comparator lS produces an output pulse of a certain pulse width (a pul~e Wldth 7USt ~elaw that which will cause the laser to beg~n emission) wnen the minimum input data 00~ is supplied to the D/A converter 13. ~he preferred emDodiment is also designed so tnat, wnen the rnaximuln input data F~ff is supplied to the D/A converter 13, the comparator produces output pulses tne duty ratlo of whlch is not 100% but ~ ~37082 which lS large enough to allow the laser to emit the beam contlnuously. Thls arrangement ~erm~ts the emission time of the laser to vary nearly over the entire range of the 256 graaation levels of the lnput data, thus ensuring high gradation of the reproduced image.
It snoula-~e under~to~c that the method descriDed above is not limited to a laser printer but may also be utilized in to an inK jet printer, a thermal printer or otner raster scanning devices.
rrhe ~OM 12 for gamma conversion will now De explained in detall with reference to Fig. 8. The ROM 12 is provided to allow a nigh gradation of density in the reproduced image. Although the descriDed emDodiment employs a ROr~
having a capacity of 256 bytes as ROM 12, a capacity of 64 bytes is basically enough because the input digital video signal is a 6-bit signal. Fig. 8 snows the memory map of the ~OM 12 for gamma conversion. Since this RO~ has a capacity of 25c bytes, it can contain 4 (four) separate correction tables, namely TABLE-l including addresses U0}~
to 3F~, TA~LE-2 including aadresses 4U~ to 7FH, TABLE-3 including addresses ~0~ to BFH, and TABLE-4 including the aaaresses C~H to FFH .
Fig. 9 shows a practlcal exampie of tne input-output characteristics of each of the conversion taoles, i.e., the relationship between the input video slgnals and the converted output video signal. As will be seen from tnis Figure, the 64 (sixty-four) levels of the lnput video signal are converted into levels 0 to 255 (00H to FFH ) in accordance witn the respective conversion taDles. The change-over between tne converslon tables can be made Dy varylng the signal applied to uppec terminals A6 anà A7 of the ROM 12 as shown in Figs. 3A and 3B. The descrlDed emDodiment is deslgned to allow thls switchlng for eacn 1 3370a2 1~ --line, Dy the operatlon of a circuit 400 shown in Fig. 3A.
In operation, the internal norlzorltal syncnronlzing signal (BD-2ulse) is input to a counter 401 the output of Which is celiverea throu~n termlnals ~A and ~ to the terminals A6 and A7 of the ROM 12. The counter 401, in cooperation with an RCO ~nverter 4~2 anà a switcn 403 constitutes a ring counter, so tnat the period of ~-ching of tne converslon taDle can De varled in accoraance witn the state of the switch 403. For instance, when the switch 403 has the state ~l~ (at terminal B), ~1~ (at terminal A), TABLE-4 is always selected, whereas, when the state of tne switcn 403 is ~1~ (at term~nal B), 0~ (at terminal A), TABLE-4 and TABLE-3 are sel~cted alternately. When the swltcn 403 has the s~ate of U~ (at termlnal B), ~0 (at terminal A), T~R~-l, TABLE-2, TABLE-3, and TABLL-4 are successlvely seLected for successiYe lines, as shown in Fig. 10a. MoreoYer, it is possible to improve the gradation by cnangins tne conversion table for successive line~.
In general, ln tne electropnotograpnic reproductlon of an image, the gradation lS more difficult to obtain in the lignt por~ion of the image tnan ln the dark por~ion of the image. Therefore, as in the example shown in Fig. 9, the converslon taDles are suostantially duplicated in tne aark portions of tne imaae and differ in the lignt po~tion so as to provide opt1ma} gradation.
In the preferred emDodiment, the switching of the table can also be made in tne directlon of the main scan by the laser beam.
More speclfically as shown in Figs. 3A ana 3B, a sianal can be formed by dividing the frequency of the screen clock (SCRLEN-CL~) Dy 2 (two) by means of a J-K flip-flop circuit 404, inputting the resulting signal to one input - 2~ - ~ 337~2 termlnal or an exclusive OR clrcult 406, the other input or whicn is connected to terminal ~B of tne counter 401 and the output termlnal of which is then connected to ROM
12 tnrough a latch ll. Witn th1s arrangement, it is S possible to chan~e the conversion table in a stagqered manner as shown ln Fig. l0(b), thus attaining a further improvement in the gradation. A reference numeral 405 denotes a switcn for selectlng either switching of the table in tne staggered manner described above or not so switching. The staggered switching of the table is selected when this switch has the ~l~ level and is not selecte~ when the swltch has the ~ level. The numerals appearing in frames of the table snown in ~ig. 10(b) repre~en~ the numDers of tne selected convers1on ta~les l to 4. Thus, the period of the screen clock in the em~o~iment correspollas to ~he period of ~ (tnree~ pixel clocks.
It wlll oe understooa from tAe descriptio~l provided aDove that the scanning lines produced by the laser in accordance ~ith data from the conversion tables of tne ROM
12 are each generated as a succession of line segments.
The line segments of successlve scann1ng lines collectively form a plurality of columns that define a line screen.
More particularly, when tne video signal processed by the circuit shown in cigs. 3A and 3B is directly delivered to a reproducing means such as a laser bea~;l printer, the reproduced image has a structure with vertical columns (in the descrlDed emDoalment ~ the line screen lS com~oseà of vertical columns of line seg~ents of successive scanning llnes wnich form in the reproduc~d image) due to the fact that the phase of the triangular wave signal is the same a~ tnat of tne lnternal horlzontal syncllronlzlng signal (~D-~ulse) for each line. 1he circuit in the present ` ` - 1 337082 embodiment lS one in which the triangular wave is formed arter the reference clocks are counted by 12 (twelve) from the rise of the BD-Pulse signal. The timing for the generation of triangular waves is the same for each line, and so each pAase of the triangular waves on each line is the same. The image aata is output from the digital data output device 1 as stated above. The digital data output device 1 outputs image data with a predetermined timing in synchronism with a signal e~uivalent to the BD-Pulse lD signal. More particularly, the aata output device 1 is adapted to receive the ~D signal. This device 1 starts to count tne rererence clocx after receiving the BD signal, and be~ins transmission of the image data after counting tne rererence cloc~s u~ to a predetermined nur.lber. As a conse~uence, the timing of transfer of the image data necessary ror image reproductlon is the same on each line, and a high quality reproduced image with no image jitter can be proauced. ~a tne tim.~ng of tne generation of the triangular waves and the timing of transfer of the image data necessary for image reproduction have the same relation on all of the lines, the reproduced image has its vertical column structure with no image jitter, which is effective, for example, in reducing a particular Moire pattern. Again th~s vertical column structure comprises a line screen having a vertical columnar axis extending at an angle, that is perpendicular to the raster scanning lines.
It is also possi~le to obtaln a reproduced image having a structure compriSing oDlique line screen columns, if the pna~e of the ~riangular wave slgnals is made to De offset slightly for successive lines. This is effective in reàucin~ tne l1oire pattern wnlcn appears undeslraDiy w~en an ori~inal dot imaae is read and processed. The angle of lnclination of tne o~liyue columns can be determined as desired by suitaoly selectlng the amount of shift of the phase of the screen clocks for successive lines. For islstance a reprobuced image comprisin~ scanning lines having oDlique columns inclined at 45 degrees can be oDta~ned Dy shifting tl~e triangular wave signal by an amount corresponding to one p~cture element, i.e. by phase snifting tne triangular wave sigrlal lZ0 degrees for each of the successive columns. Fig. 11 shows a circuit for reproauc1ng an image comprising oDliyue columns. ~lore specifically, a reproduced imagè comprising oblique columns can De ootalned by usinq tnis circuit in place or tne screen clock generating circuit 300 in the circuit sAown in Fis. ~.
R~ferring agaln ~o Fi~. 11, tne internal hor~zontal syncnronizing signal (BD-Pulse) is laccned by the pixel clocks (PIXEL-CL~) oy mean~ of D latches ~56 ana 3~7, so that three internal horizontal synchronizing signals (BD-Pul~e~ ilaving dirferent pnases are pro~uced. Then, one of these three internal horizontal synchronizing signais (BD-~ulse) is selectea for eacn line Dy operation of a counter 35~, inverters 359 and 360, and gate circuits ~ol to 367. The seiected slgnal 1S lnput as a LOAD signal to a counter 351 thereby changing the phase of the screen clocKs for success1ve lines. Tne counter 351 is adapted to divide the frequency of the signal (36M-CLK) into 1/3 w~ile tne J-K fllp-flOp circuLt ~54 furtner civides the frequency of the output from tne counter 351 into 1/2.
Wlth tnis arrangement~ it is posslole to generate one screen clock for every three picture elements.
F-g. 12 ShOWS timing of the screen clock generated ~y the clrcuit of Flg. 11 and the triangular wave signal for successlve llnes. These thre~ trlangulâr wave slgna~s are generated in seauence of eacn see of each 3 lines.
~Ihen the reference pattern signal is generated in syncnronl~m with a group of plcture elements as in the case of the emDodiment described, it iS possible to shift the syncnron~zing slsnal used in tne generation of the pattern signal by an amount corresponding to one half of the reference pattern slsnal perlod for each successive set of scan lines e~ual to the widtn of the pattern signal. ~ucn a metnod allows the positlon of the center of growth of tne pulse width to De shifted in each of ~uccesslve lines, so that the output image can have an appearance resemoiing that proaucea by half-tone dots arranaed along o~ ue lines.
In the clrcuit shown in Figs. 3A and 3B, the RO.'1 12 is u~ed for the purpose of gamma conversion. Thls, nowever, is not tne only element suitable for this purpose and the ROtl 12 may oe re~lacea Dy an S-~li connectea to tne DA~A
8US line of a co~puter. ~ith sucn an arrangement, it is possiDle to rew-lte the gam~la converslon taole as desired in accordance w th, for exampie, a cAange in the kind of the orislnal~ tnus lncreasing tne adaptaDility of the apparatus or tne invent~on.
Fig. 13 snow~ an exam~le of a clrcuit wnich is usable in place of the ROM 12 in the circ~it shown in Figs. 3A anà
3B. This clrcuit has, as wlll De seen from tnis Figure, an S-RAl~ 12a for ga.~ma conversion, a decoder 30, a mlcrocomyuter 31 ior re~rlting tne gamma conversion tables, tri-state buffers 32 and 33, and a bi-directional tri-state ouffer 34.
The mude changlns switcnes 304, 4U3 anà 405 in the circult shown in Figs. 3A ana 3B may be controlled Dy the microcomputer 31 so as tO increase the flexiDility of the system as a wnole.
` - 1 337082 Although the lnvention has been described with reference to speclfic emoodiments and in specific terlns it is to be understood that this description lS only illustrative purpose~ ana that various other cnanges and modifications are posslble without departing from the scope of the invention.
Claims
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Image processing apparatus for forming an image, said apparatus comprising:
video signal output means for generating an analog video signal;
pattern signal generating means for producing a pattern signal of predetermined period, one period of said pattern signal corresponding to a plurality of pixels of the analog video signal;
a pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with the analog video signal generated by said video signal output means and said pattern signal generated by said pattern signal generating means; and image forming means for forming an image in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means, wherein said image forming means scans lines on the recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means thereby forming an image on said recording medium, said image forming means including means for generating a synchronizing signal for each line scanned on the recording medium, said pattern signal generating means generating the pattern signal of predetermined period in accordance with said synchronizing signal.
2. Image processing apparatus according to claim 1, wherein said pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period.
3. Image processing apparatus according to claim 1, wherein said pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal.
4. Image processing apparatus according to claim 1, wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means.
5. Image processing apparatus according to claim 1, further comprising reference clock generating means for generating a reference clock, said pattern signal generating means producing a clock for generating said pattern signal by dividing the frequency of said reference clock in accordance with said synchronizing signal.
6. Image processing apparatus according to claim 1, further comprising digital video signal generating means for generating a digital video signal, wherein said video signal output means includes digital-to-analog converting means for converting the digital video signal generated by said digital video signal generating means into the analog video signal.
7. Image processing apparatus according to claim 6, wherein said digital video signal generating means includes digital video signal output means for outputting a digital video signal having a characteristic, and characteristic converting means for converting the characteristic of said digital video signal output from said digital video signal output means and for producing a converted digital video signal therefrom and wherein said digital-to-analog converting means converts the converted digital video signal generated by said characteristic converting means into the analog video signal.
8. Image processing apparatus according to claim 6, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the minimum value.
9. Image processing apparatus according to claim 6, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the maximum value.
10. Image processing apparatus according to claim 6, wherein one period of the pattern signal corresponds to a plurality of pixels of the digital video signal.
11. Image processing apparatus according to claim 6, wherein said apparatus further comprises reference clock generating means for generating a reference clock, said pattern signal generating means producing a first clock for generating said pattern signal by dividing the frequency of said reference clock and wherein said apparatus further comprises means for generating a second clock by dividing the frequency of the reference clock, said digital video signal generating means generating the digital video signal in synchronism with the second clock.
12. Image processing apparatus according to claim 1, wherein one period of said pattern signal corresponds to two pixels of the analog video signal.
13. Image processing apparatus according to claim 1, wherein one period of said pattern signal corresponds to three pixels of the analog video signal.
14. Image processing apparatus for forming an image, said apparatus comprising:
video signal output means for generating a video signal;
pattern signal generating means for producing a pattern signal of predetermined period, one period of said pattern signal corresponding to a plurality of pixels of the video signal;
a pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with the video signal generated by said video signal output means and said pattern signal generated by said pattern signal generating means; and image forming means for forming an image in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means, wherein said image forming means scans lines on the recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means thereby forming an image on said recording medium, said image forming means including means for generating a synchronizing signal for each line scanned on the recording medium, said pattern signal generating means generating the pattern signal of predetermined period in accordance with said synchronizing signal.
15. Image processing apparatus according to claim 14, wherein said pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period.
16. Image processing apparatus according to claim 14, wherein said pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal.
17. Image processing apparatus according to claim 14, wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means.
18. Image processing apparatus according to claim 14, further comprising reference clock generating means for generating a reference clock, said pattern signal generating means producing a clock for generating said pattern signal by dividing the frequency of said reference clock in accordance with said synchronizing signal.
19. Image processing apparatus according to claim 14, further comprising digital video signal generating means for generating a digital video signal, wherein said video signal output means includes digital-to-analog converting means for converting the digital video signal generated by said digital video signal generating means into an analog video signal.
20. Image processing apparatus according to claim 19, wherein said digital video signal generating means includes digital video signal output means for outputting a digital video signal having a characteristic, and characteristic converting means for converting the characteristic of said digital video signal output from said digital video signal output means and for producing a converted digital video signal therefrom and wherein said digital-to-analog converting means converts the converted digital video signal generated by said characteristic converting means into an analog video signal.
21. Image processing apparatus according to claim 19, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the minimum value.
22. Image processing apparatus according to claim 19, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the maximum value.
23. Image processing apparatus according to claim 19, wherein one period of the pattern signal corresponds to a plurality of pixels of the digital video signal.
24. Image processing apparatus according to claim 19, wherein said apparatus further comprises reference clock generating means for generating a reference clock, said pattern signal generating means producing a first clock for generating said pattern signal by dividing the frequency of said reference clock and wherein said apparatus further comprises means for generating a second clock by dividing the frequency of the reference clock, said digital video signal generating means generating the digital video signal in synchronism with the second clock.
25. Image processing apparatus according to claim 14, wherein one period of said pattern signal corresponds to two pixels of the video signal.
26. Image processing apparatus according to claim 14, wherein one period of said pattern signal corresponds to three pixels of the video signal.
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Image processing apparatus for forming an image, said apparatus comprising:
video signal output means for generating an analog video signal;
pattern signal generating means for producing a pattern signal of predetermined period, one period of said pattern signal corresponding to a plurality of pixels of the analog video signal;
a pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with the analog video signal generated by said video signal output means and said pattern signal generated by said pattern signal generating means; and image forming means for forming an image in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means, wherein said image forming means scans lines on the recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means thereby forming an image on said recording medium, said image forming means including means for generating a synchronizing signal for each line scanned on the recording medium, said pattern signal generating means generating the pattern signal of predetermined period in accordance with said synchronizing signal.
2. Image processing apparatus according to claim 1, wherein said pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period.
3. Image processing apparatus according to claim 1, wherein said pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal.
4. Image processing apparatus according to claim 1, wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means.
5. Image processing apparatus according to claim 1, further comprising reference clock generating means for generating a reference clock, said pattern signal generating means producing a clock for generating said pattern signal by dividing the frequency of said reference clock in accordance with said synchronizing signal.
6. Image processing apparatus according to claim 1, further comprising digital video signal generating means for generating a digital video signal, wherein said video signal output means includes digital-to-analog converting means for converting the digital video signal generated by said digital video signal generating means into the analog video signal.
7. Image processing apparatus according to claim 6, wherein said digital video signal generating means includes digital video signal output means for outputting a digital video signal having a characteristic, and characteristic converting means for converting the characteristic of said digital video signal output from said digital video signal output means and for producing a converted digital video signal therefrom and wherein said digital-to-analog converting means converts the converted digital video signal generated by said characteristic converting means into the analog video signal.
8. Image processing apparatus according to claim 6, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the minimum value.
9. Image processing apparatus according to claim 6, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the maximum value.
10. Image processing apparatus according to claim 6, wherein one period of the pattern signal corresponds to a plurality of pixels of the digital video signal.
11. Image processing apparatus according to claim 6, wherein said apparatus further comprises reference clock generating means for generating a reference clock, said pattern signal generating means producing a first clock for generating said pattern signal by dividing the frequency of said reference clock and wherein said apparatus further comprises means for generating a second clock by dividing the frequency of the reference clock, said digital video signal generating means generating the digital video signal in synchronism with the second clock.
12. Image processing apparatus according to claim 1, wherein one period of said pattern signal corresponds to two pixels of the analog video signal.
13. Image processing apparatus according to claim 1, wherein one period of said pattern signal corresponds to three pixels of the analog video signal.
14. Image processing apparatus for forming an image, said apparatus comprising:
video signal output means for generating a video signal;
pattern signal generating means for producing a pattern signal of predetermined period, one period of said pattern signal corresponding to a plurality of pixels of the video signal;
a pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with the video signal generated by said video signal output means and said pattern signal generated by said pattern signal generating means; and image forming means for forming an image in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means, wherein said image forming means scans lines on the recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means thereby forming an image on said recording medium, said image forming means including means for generating a synchronizing signal for each line scanned on the recording medium, said pattern signal generating means generating the pattern signal of predetermined period in accordance with said synchronizing signal.
15. Image processing apparatus according to claim 14, wherein said pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period.
16. Image processing apparatus according to claim 14, wherein said pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal.
17. Image processing apparatus according to claim 14, wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means.
18. Image processing apparatus according to claim 14, further comprising reference clock generating means for generating a reference clock, said pattern signal generating means producing a clock for generating said pattern signal by dividing the frequency of said reference clock in accordance with said synchronizing signal.
19. Image processing apparatus according to claim 14, further comprising digital video signal generating means for generating a digital video signal, wherein said video signal output means includes digital-to-analog converting means for converting the digital video signal generated by said digital video signal generating means into an analog video signal.
20. Image processing apparatus according to claim 19, wherein said digital video signal generating means includes digital video signal output means for outputting a digital video signal having a characteristic, and characteristic converting means for converting the characteristic of said digital video signal output from said digital video signal output means and for producing a converted digital video signal therefrom and wherein said digital-to-analog converting means converts the converted digital video signal generated by said characteristic converting means into an analog video signal.
21. Image processing apparatus according to claim 19, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the minimum value.
22. Image processing apparatus according to claim 19, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the maximum value.
23. Image processing apparatus according to claim 19, wherein one period of the pattern signal corresponds to a plurality of pixels of the digital video signal.
24. Image processing apparatus according to claim 19, wherein said apparatus further comprises reference clock generating means for generating a reference clock, said pattern signal generating means producing a first clock for generating said pattern signal by dividing the frequency of said reference clock and wherein said apparatus further comprises means for generating a second clock by dividing the frequency of the reference clock, said digital video signal generating means generating the digital video signal in synchronism with the second clock.
25. Image processing apparatus according to claim 14, wherein one period of said pattern signal corresponds to two pixels of the video signal.
26. Image processing apparatus according to claim 14, wherein one period of said pattern signal corresponds to three pixels of the video signal.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000616718A CA1337082C (en) | 1985-08-15 | 1993-09-15 | Image processing apparatus |
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US76593885A | 1985-08-15 | 1985-08-15 | |
| US765,938 | 1985-08-15 | ||
| CA000515897A CA1313703C (en) | 1985-08-15 | 1986-08-13 | Apparatus for generating an image from a digital video signal |
| CA000616427A CA1326286C (en) | 1985-08-15 | 1992-06-26 | Image processing apparatus |
| CA000616718A CA1337082C (en) | 1985-08-15 | 1993-09-15 | Image processing apparatus |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000616427A Division CA1326286C (en) | 1985-08-15 | 1992-06-26 | Image processing apparatus |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1337082C true CA1337082C (en) | 1995-09-19 |
Family
ID=27167625
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000616718A Expired - Lifetime CA1337082C (en) | 1985-08-15 | 1993-09-15 | Image processing apparatus |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1337082C (en) |
-
1993
- 1993-09-15 CA CA000616718A patent/CA1337082C/en not_active Expired - Lifetime
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA1313703C (en) | Apparatus for generating an image from a digital video signal | |
| US4800442A (en) | Apparatus for generating an image from a digital video signal | |
| EP0717551B1 (en) | Image processing apparatus | |
| US4897734A (en) | Image processing apparatus | |
| EP0212990B1 (en) | Image processing apparatus | |
| EP0213949A2 (en) | Image processing apparatus | |
| CA1337082C (en) | Image processing apparatus | |
| CA1325844C (en) | Image processing apparatus | |
| CA1122699A (en) | Color televison camera | |
| Hallows et al. | Electronic halftones | |
| JPH0831954B2 (en) | Image processing device | |
| JPH0810895B2 (en) | Image processing device |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |