US2472774A - Irregular interlace scanning system - Google Patents

Description

L. F. MAYLE 2,472,774

IRREGULAR INTERLACE SCANNING SYSTEM 5 Sheets-Sheet 1 lJune 7, 1949.

Filed oct. 417.

ATTORNEY June 7 1949 I l.. F. MAYLE 2,472,774-

IRREGULAR INTERLACE SCANNING SYSTEM Filed Oct. 1v, 1945 s sheets-sheet 2 FIG.3

INVENTOR LOUIS F. MAYLE June 7, 1949. L. F. MAYLE 2,472,774

IRREGULAR INTERLACE SCANNING SYSTEM Filed Oct. 17, 1945 3 Sheets-Sheet 3 Patented June 7, 1949 UNITED STATE IRREGULAR INTERLACE SCANNING SYSTEM Louis F. Mayle, Fort Wayne, End., assignor, by

mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application October 17, 1945, Serial No. 622,752

6 Claims.

This invention relates generally to a television system, and more particularly relates to a method of and apparatus for effecting secrecy in television systemsj It is conventional practice to scan the image of an object to be transmitted in interlaced fashion. Thus, each complete frame, representative of the optical image at a certain instant, compri-ses gener-ally a grou-p of fields of consecutively scanned parallel lines arranged so that the lines of one eld fall between the lines of another field in each group. According to the standards adopted by the Radio Manufacturers Association, a frame comprises two fields. Two general methods have been devised for effecting inter- For secrecy purposes it would also be desirable laced scanning. According to Ione conventional te sean the image to be transmitted in such a interlaced scanning system, an odd number of manner that each frame comprises a large numlineS iS used TOI Scanning each frame so that ber of fields which are scanned, transmitted and each field comprises a fractional number of lines. reassembled in predetermined irregular sequenC- Hen-ce, the line scanning frequency is not a mul- A system of this type has obvious advantages for tiple of the field scanning frequency. If a frame retaining secrecy of the transmitted television incomprises two fields, for example, the number of tellgence and may iind application for news lines per eld is one-half plus an integer. Each services and the like where subscribers may relield is scanned beginning at the top of the image `eeive television pictures transmitted by a cenarea. However, the beginning of the scanning of tral station. By usingr a prearranged code, the successive fields is displaced by a fraction of a television pictures can only be received by those line along the top edge of the image area. The who have knowledge of the code.

eld scanning as well as the line scanning fre- It is the principal object of the invention, quenCY repeats f01 each eld Scanning period. therefore, to provide a television secrecy system. The retrace period of successive fields, however, A further object of the invention is to provide begins after 4a different fractional amount of the apparatus for effecting interlaced scanning for last line has been scanned. Interlaced scanning television systems so that a group of lield-s of parsvstems of this type have been devised Where alici lines equivalent to a frame is scanned, each frame comprises two or more elds of contransmitted and received one after the other in secutively scanned parallel lines. predetermined irregular sequence.

In accordance with another interlaced scan- Another object of the invention is to provide ning system, an even number of lines is used for apparatus for displacing each field in a group of each frame COTnPISng WO eldS. Her-e the line fields of parallel consecutively scanned lines in scanning frequency is a multiple of the iield irregular predetermined sequence and changing Scanning frequency. However. the return trace at will the sequence of the displacements of suc- `between successive elds of a group is of unequal cessive elds in any one group. duration and, hence, the first line 0f successive In accordance with the present invention, there fields begins at different levels. The number of is provided a scanning circuit for determining the line-s in each field is an integer. The vertical order of interlace of successive scanning fields deflection -of consecutive fields in each group is of a television tube comprising rst electron deusually effected by means of rectangular pulses fleeting means for defining the horizontal comwhic'h are superimposed upon the conventional ponent of a scanning trace. Second electron desawtooth wave of field scanning frequency. fleeting means are provided which include a It would be desirable vto scan the image area source of waves for defining the vertical comat a rate of more than two fields per frame but ponent of the scanning trace. Means are fursoon as the number of elds in a frame exther provided for generating pulses of irregular ee-eds two, difficulty is experienced due to the amplitude to provide irregular order of interlace, phenomenon known as crawling that is the and, iinally, means for superimposing the pulses appearance of moving waves in the reproduced on the waves. picture when the lines -of the iields of each frame For a better understanding of the invention, follow each other in consecutive order. It has together with other and further objects thereof, been recognized in the art that it is desirable for reference is made to the following description, this reason to scan a plurality of elds forming taken in connection with the accompanying drawa frame in irregular sequence. This, however, ings, and its scope will Abe pointed out in the can not be effected by means of the interlaced 6o appended claims,

(Cl. IHS-7.7)

scanning syst-em where an odd number of lines is used for scanning each frame. In that case the beginning of the scanning of successive field-s is displaced by a fraction of a line and hence the fields follow each other in regular sequence. However, a television system has been suggested where each frame comprises four fields. The first pair of fields is displaced with respect to the succeeding pair of fields in the manner of an even line interlaced scanning system. On the other hand, each pair of fields is scanned in accordance with the odd line interlaced scanning system so that every field comprises a fractional number of lines.

In the accompanying drawings:

Fig. 1 is a schematic circuit diagram, mostly in block form, of a television transmitting system and associated electronic commutator embodying the present invention;

Figs. 2, 3 and 4 are curves representing electric waves developed at various parts of the transmitting system of Fig. 1;

Fig. 5 illustrates diagrammatically the path traveled by the scanning beam over the target of the transmitting system of Fig. 1; and

Fig. 6 is a schematic circuit diagram, mostly in block form, of a television receiving system and associated electronic commutator in accordance with the invention.

Referring now more particularly to Fig. l of the drawings, there is illustrated a television transmitting system including picture signal generating tube I0, which may either be of the storage type, as shown, or an image dissector tube.

Signal generating tube I comprises electron gun I I, shown schematically, mosaic electrode I2 and line scanning elements I3 and field scanning elements I4, which may consist of pairs of scanning coils. For developing scanning currents in accordance with the present invention, there are provided sawtooth line scanning Wave generator I and sawtooth field scanning wave generator Iii connected, respectively, to scanning coils I3 and Ill. there is further provided electronic commutato-r I1 for developing groups of rectangular pulses of predetermined amplitudes. Electronic commutator or cathode ray tube I1 includes electron gun i8 comprising cathode 20, control grid 2l, rst anode 22 and second anode 23. Electron gun I8 is arranged for developing an electron beam and directing it towards a group of twelve targets 24 arranged in three rows 25, 26 and 21, each row comprising four targets. Targets 24 are elec trically insulated from each other and made of conductive material, such as' metal. Pairs of deecting plates 30, 3| and 32, 33 are provided for periodically deecting the electron beam across targets 24.

Operating potentials are supplied for the var ious elements of cathode ray tube I1 from a suitable source illustrated as battery 34 connected across potentiometer 35 and having its positive pole grounded. By means of suitable taps connected to potentiometer 35, control grid 2| is maintained at a potential that is a few volts negative against that of cathode 20, while anodes 22 and 23 are supplied with increasingly positive potentials with respect to cathode 20. Second anode 23 is connected to ground. Deilecting plates and 32 are also connected to ground as illustrated. Electrode 34A, which may consist of a conductive coating on the inner surface of envelope 35A of tube I1, is also grounded.

A group of resistors, equal in number to the number of targets 24 in cathode ray tube I1, is indicated at 36 and connected in series between ground and pulse amplifier 31. Each target 24 has been shown connected between a selected number of resistors 33, indicated individually at lili to H2, and ground to develop groups of rectangular pulses of predetermined different amplitudes. As will be explained in detail hereinafter, the number of targets 24 determines the number of pulses in a group, which in turn determines the number of fields in each frame of the interlaced scanning pattern. For the purpose of changing at will the connections between targets 24 and resistors 36 there may be provided In accordance with the present invention, Y

f field-:nf frame In the present case M, the number of lines per frame, must be divisible by 11., the number of fields per frame, in order to obtain an even line interlaced scanning pattern. Thus, M/n is an integer which equals N, the number of lines per field.

We may now assume that n, the number of fields per frame, is twelve corresponding to the twelve targets 2li of cathode ray tube I1. We may further assume a frame frequency iframe of 30 per second. If we assume that M, the number of lines per frame is 600, then N the number of lines per field equals The line frequency fline=TLNframe=12 X 50 X 30: 18,000

The eld frequency ffieid=nfframe=l2 30=360- Referring again to Fig. 1 of the drawings, there is provided line frequency synchronizing signal generator 40 for developing synchronizing signals of rectangular wave shape at the line frequency, that is at 18,000 cycles per second. Synchronizing signal generator 40 is connected to line scanning wave generator I5 for synchronizing it at the line scanning frequency. Multivibrator 4i, connected to line frequency synchronizing signal generator 40, reduces the line frequency by the factor N, the number of lines per eld. Hence, multivibrator 4I develops rectangular pulses at a frequency of 18,000/ or 360 cycles per second, which is equivalent to the ield frequency. Multivibrator I is connected to eld scanning wave generator I6 for synchronizing it at the eld scanning frequency. Counter 42 is .connected to multivibrator 4I and develops groups of pulses at one-fourth the frequency of that of the rectangular pulses developed by multivibrator 4I, that is at a frequency of cycles per second. The pulses developed by counter 42 are of rectangular stepped wave shape, as indicated at 43. Counter 44 is connected to counter 42 and develops groups of pulses of stepped rectangular wave shape, illustrated at 45, each group having a frequency equal to one-third that of one group of pulses 43 or 30 cycles per second.

Counter 42 is connected to deilecting plate 33, while counter 44 is connected to deecting plate 3 I. Hence. the electron beam developed by electron gun I8 is deflected horizontally across each row 25, 25 and 21 of targets 24 by means of the electric eld developed by deflecting plates 32, 33 connected to counter 42. The wave shape of dellecting pulses i3 is such that the electron beam impacts each target 24 for a predetermined duration which is substantially equal to the eld scanning period. The electron beam traverses one row of targets 2li at a frequency which is three times the frame frequency or 90 cycles per second. Deflecting plates 30 and 3| connected to counter 34 develop an electric field which deflects the electron beam vertically across targets 24, that is from row 25 to 25 and then to 21. The wave shape of deectin-g pulses 45 is such that all three rows 25 to 21 are scanned once during the vframe scanning period and, accordingly, all twelve targets 24 are scanned once in this interval. The deecting pulses developed by counters 42 and 44 are locked in with the line scanning frequency and eld scanning frequency by means of their interconnection with multivibrator 4I and synchronizing signal generator 4D.

When the electron beam developed by electron gun I 3 impacts one of the targets 24, current source 34 is effectively connected to that target through a resistance represented by the resistance of the electron beam. In accordance with the present invention, each target 24 is connected between a selected number of resistors 36 and ground as illustrated. Thus, by way of example target H5, being the rst target in row 25, has

- been shown connected between resistors IUI to i lil, inclusive, that is between ten resistors and ground. When the electron beam impacts target I i5, a current iioWs from ground through resistors lill to IIIi, inclusive, target II and through the electron beam back to battery 34. Target H5, which is the next target in row 25 has been shown connected between resistors IBI to w8, inclusive, that is between eight resistors and ground. Accordingly, when the electron beam impacts target II5, a negative voltage drop is developed across resistors IBI to III) which is impressed upon grid 41 of electron tube 48. Cathode 49 of tube 48 is connected to ground through battery 53 positively biasing cathode 49 whichis equivalent to a negative bias of grid 41. The negative pulses developed across resistors 35 are thus amplified by tube 48. forming part of pulse amplifier 31. Amplifier 31 may either be a direct current amplifier or an alternating current amplier well compensated for low frequencies and arranged to pass the rectangular pulses developed across resistors `36.

It will. now be evidentl that the amplitude of the pulses developed across resistors 36 will depend upon which one of targets 24 is impacted by the electron beam. Since the current of the electron beam is constant, the Voltage drop between ground and resistors 35 depends upon the number of resistors which are connected between the electron beam and ground. Accordingly, when the electron beam impacts target II5 there are ten resistors itil to lill connected between the hea-m and ground. The negative voltage drop developed across resistors II to IIB is, therefore, ten times as large as the voltage drop developed when the beam impacts target I I 'l connected between resistors iill and ground.. In View of the fact that there are twelve targets 24 and twelve resistors lill to IIZ, groups of pulses are developed. each group consisting of twelve pulses having different amplitudes. The sequence of the twelve pulses in each group, that is the sequence of the amplitudes of the pulses, may be chang-ed hy changing the connections between targets 24 and resistors 35.

As will be explained hereinafter, the sequence in which the iields in any frame are scanned depends upon the amplitudes of the pulses developed by electronic commutator I1. Accordingly, there is an almost unlimited number of combinations in which the twelve iields may be scanned and transmitted.

Referring now to Fig. 2, there is illustrated sawtooth -current wave 52 which is developed by eld scanning wave generator I6 at the field frequency, that is at 360 cycles per second. The groups of rectangular pulses developed by electronic commutator I1 and amplified by pulse amplier 31 are superimposed upon sawtoothl current wave 52. This is effected by arranging pulse amplifier 311 in one of the leads connecting field scanning wave Vgenerator I5 to scanning coils I4.

The output wave developed by pulse amplifier 31 is illustrated on enlarged scale in Fig. 3. Fig. 3 shows one group of rectangular pulses 53. The duration of each pulse corresponds to the field scanning period. Each group of pulses 53 cornprises twelve rectangular pulses of different amplitudes, as clearly shown in Fig. 3. Thus, the duration of one group of pulses 53 equals the frame scanning period which corresponds to 30 cycles per second.

It should be understood that the amplitude of sawtooth current wave 52 is much larger than that of rectangular pulses 53. Each sawtooth current wave 52 deflects the electron beam in signal generating tube I ll across substantially the entire height of mosaic electrode I2, while each of the rectangular pulses 53 deflects the electron beam over a few of the 600 lines of a picture frame.

The composite current wave impressed upon deiiecting coils I4 is illustrated schematically in Fig. 4. Sawtooth current wave 54 of Fig. 4 is obtained by superimposing sawtooth current wave 52 upon rectangular pulses 53. As stated above, the duration of each rectangular pulse 53 equals that of one cycle of sawtooth current wave 52. The trace portions 55 of sawtooth wave 54 are equal in length. However, the retrace portions 56 differ in length in accordance with the superimposed rectangular pulses 53.

The interlaced scanning pattern of the electron beam eilected by the electric eld developed by sawtooth wave 54 in scanning coils I4, and the line scanning sawtooth current Wave developed by generator I5 is illustrated in Fig. 5. The scanning of each of the twelve fields of which a frame consists begins at the left hand edge of the picture area. Each eld comprises fifty parallel lines which are scanned so that every twelfth of the 600 lines of the picture area is scanned during one eld scanning period. The beginning of the scanning of each eld is determined by rectangular pulses 53. Thus, the rst iield in a group of fields begins with the third line 51, as indicated in Fig. 5. Then, in accordance with the pattern of rectangular pulses 53. the second eld begins with the fifth line 58, as indicated in Fig.y 5. The iields follow each other in predetermined irregular sequence until the entire picture area/ has been scanned within one frame scanning period equivalent to twelve eld scanning periods or at-a frequency of 30 cycles per second. As

pointed out hereinabove, the sequence in which the fields are scanned may be changed at will by changing the connections between targets 24 of electronic commutator I1 and resistors 35. The fields are transmitted and received according to a predetermined code.

Thus, mosaic electrode I2 is scanned by the electron beam developed by electron gun II in accordance with a predetermined interlaced scanning pattern. When mosaic electrode I2 is scanned, picture signals are developed across grounded resistor 63 connected to the signal plate of mosaic electrode I2. The thus obtained voltage variations are amplified in video amplifier 5 I. Connected in cascade to video amplifier 5I are synchronizing signal mixer 52, modulator 63, power amplier 64 and dipole antenna 65. Synchronizing signal mixer 62 is connected to line synchronizing signal generator 40 and multivibrator 4I to mix the horizontal and vertical synchronizing signals with the amplified video signals obtained from video amplifier 6|. Carrier oscillator 65 connected to modulator 63 develops a carrier wave which is modulated with the composite television signal obtained from mixer 62 in modulator stage 63. The modulated carrier Wave is then amplified by power amplier 6d and radiated into space by dipole antenna 65.

Referring now to Fig. 6 in which like components are designated by the same reference numerals as were used in Fig. 1, there is illustrated a television receiving system including picture signal reproducing tube 10. The modulated carrier signal radiated by dipole antenna 65 is received by dipole antenna 1| and amplified by radio frequency amplifier 12. The amplified carrier wave obtained in the loutput circuit of amplifier 12 is mixed with another carrier wave developed by local oscillator 13 and demodulated by rst detector stage 14 to obtain an intermediate frequency signal. Connected in cascade to rst detector 14 are intermediate frequency amplifier 15 arranged for amplifying the intermediate frequency signal and second detector 18 for developing the demodulated video signal. Synchronizing signal separator 11, connected to first detector 14, is arranged for separating the horizontal and vertical synchronizing signals. Line scanning wave generator 88 and field scanning wave generator 8| are connected to synchronizing f Signal separator 11 for synchronizing the frequencies of the waves developed by generators 86 and 8| with the synchronizing signals developed by separator 11.

Picture signal reproducing tube 1i) includes electron gun 82 comprising cathode 83, control grid 84, ist anode 85 and second anode 86. The electron beam developed by electron gun 82 is focused upon luminescent screen 81 of reproducing tube 15]. The electron beam is deflected by defiecting plates 9d, 9o and 9|, 9i. Operating potentials are supplied for the various elements of signal reproducing tub-e 1e from a suitable source illustrated as battery S2 connected across potentiometer S3 and having its negative pole grounded. Control grid 8d is kept at a potential that is a few volts negative against that of cathode 83 by a suitable tap connected to potentiometer 93. First anode 85 and second anode 88 are maintained at increasingly positive potentials with respect to cathode 83 by suitable taps connected to potentiometer 93. Electrode 94 which may be arranged as a conductive coating on the inner surface of envelope 95 is connected to second anode 86.

Deilecting plates 9|, 9| are connected to line scanning wave generator 80 while defiectirig plates 90, 90 are connected to eld scanning wave generator 8| through pulse amplier 31. The leads of deflecting plates 90 and 9| are connected through resistors 96 and 91, respectively, each having a center tap connected to electrode 94. Thus, the average potential of deecting plates 9D and 9| equals that of second anode 86 and electrode 94.

The receiver system just described, with the exception of pulse amplifier 31, operates in a conventional manner. The video signal obtained from the output circuit of second detector 16 is impressed across resistor 98 arranged between control grid 84 and ground. Hence, the intensity of the electron beam developed by electron gun 82 is controlled by control grid 84 in accordance with the received picture signals. The electron beam is deflected across luminescent screen 81 8 in two directions normal to each other in accordance with the line scanning and eld scanning frequencies. The electron deflecting waves developed by generators and 8| correspond in frequency and phase to those developed by generators I5 and I6 of the transmitting system. The frequencies of the waves developed by generators 8@ and 8| are synchronized with those of the transmitting station by means of horizontal and vertical synchronizing signals developed by synchronizing signal separator 11, as is conventional.

In accordance with the present invention, there is provided electronic commutator I1 at the receiving station which may be identical to commutator I1 of Fig. 1. In the same manner as explained in connection with Fig. l, rectangular pulses are developed across resistors E38 which are amplified by pulse amplifier 31. Pulse amplifier 31 may be a direct current amplifier or an alternating current amplier Well compensated for low frequencies. In accordance with the invention, the connections between targets 24 and resistors .26 are the same as those of electronic commutator I1 of Fig. 1. Thus, target H5 has been shown connected between resistors IDI to IIIl and ground. Counters 44 and d2, which develop the electric fields for deflecting the electron beam of tube i1, are connected in. cascade to field scanning wave generator SI through which they are synchronized.

Hence, the electric pulses developed by pulse amplifier 31 are the same as pulses 53, illustrated in Fig. 3. Pulses 53 are superimposed upon the sawtooth voltage wave developed by field scanning wave generator 8| by connecting amplier 31 between one deecting plate Si] and generator 8|. Preferably, condensers lilo are arranged between amplifler 31 and the leads connecting it to one plate and to generator BI. Condensers |00 should have a capacitance large enough to pass pulses 53. They are provided for separating amplier 31 which is at a direct current ground potential from deflecting plates 9| which have a high positive average potential against ground.

The electron beam developed by electron gun 82 is deflected in accordance with the same interlaced scanning pattern as is the electron beam developed by electron gun I I of signal generating tube Ill. Thus, the twelve elds of which each frame consists are scanned, transmitted and received in predetermined irregular sequence.

It is to be understood that each frame may consist of more or less fields than twelve, the number twelve being chosen for purposes of illustration. It is also to be understood that targets 24 of electronic commutator I1 may be arranged in any other suitable manner. However, by arranging targets 24 in a plurality of rows such as 25, 26 and 21, it is particularly simple to derive the waves for deiiecting the electron beam of tube I1 from the field scanning frequency.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A scanning circuit for determining the order of interlace of successive scanning elds of a television tube comprising rst electron deiiectirig means for dening the horizontal component of a scanning trace, second electron deflecting means including a source of waves for dening the vertical component of said scanning trace, means for generating pulses equal in duration to that of one of said waves and of irregular amplitude to provide irregular order of interlace comprising a commutating device, a plurality of conductive targets in said device, and means connected to said source of waves for driving said device in synchronism with said waves, potential divider means including sections connected between diiierent ones of said targets for providing different potentials of irregular amplitude, and means for superimposing said potentials on said waves.

2. A scanning circuit for determining the order of interlace of successive scanning ilelds of a television tube comprising rst electron deflecting means for defining the horizontal component of a scanning trace, second electron deilecting means including a source of sawtooth waves for dening the vertical component of said scanning trace, means for generating pulses equal in duration to that of one of said sawtooth waves and of irregular amplitude to provide irregular ord-er of interlace comprising a commutating device, a plurality of conductive electrodes in said device, and means coupled to said source of sawtooth waves for driving said device in synchronism with said waves, potential divider means including sections coupled between different ones of said electrodes for providing diierent potentials of irregular amplitude, and means for superimposing said potentials on said sawtooth waves.

3. A scanning circuit for determining the order of interlace of successive scanning elds of a television tube comprising first electron deflecting means for dening the horizontal component of a scanning trace, second electron deflecting means including a source of waves for defining the vertical component of said scanning trace, means for generating pulses of irregular amplitude to provide irregular order of interlace comprising a cathode ray tube for generating a cathode beam, a plurality of electrodes in said tube arranged to be scanned successively by said cathode beam, and deflecting means coupled to said source of waves for deilecting said beam into impingement with said electrodes in predetermined sequence, potential divider means including impedances coupled between different ones of said electrodes for providing different potentials of irregular amplitude incident to impact of electrons on said electrodes, and means for superimposing said potentials on said waves.

4. A scanning circuit for determining the order of interlace of successive scanning elds of a television tube comprising rst electron deflecting means for defining the horizontal component of as canning trace, second electron deflecting means including a source of waves for defining the vertical component of said scanning trace, means for generating pulses of irregular amplitude to provide irregular order to interlace comprising a cathode ray tube for generating a cathode beam, a plurality of electrodes in said tube arranged to be scanned successively by said cathode beam, and deecting means coupled to said source of waves for deilecting said beam into impingement with one of said electrodes after another at the same periodicity as said waves, po-

tential divider means including sections coupled between diiierent ones of said electrodes for providing diierent potentials of irregular amplitude incident to impact of electrons on said electrodes, and means for superimposing said potentials on said waves.

5. A scanning circuit for determining the order of interlace of successive scanning fields of a television tube comprising first electron deflecting means for defining the horizontal component of a scanning trace, second electron deflecting means including a source of waves for defining the vertical component of said scanning trace, pulse generating means for generating pulses equal in duration to that of one of said waves and of irregular amplitude to provide irregular order of interlace comprising a cathode ray tube for generating a cathode beam, a plurality of electrodes in said tube arranged to be scanned successively by said cathode beam, and deflecting means connected to said source of waves for deecting said beam into impingement with one of said electrodes after another at the same periodicity as said waves, potential divider means including sections connected between diierent ones of said electrodes for providing diierent potentials of irregular amplitude incident to impact of electrons on said electrodes, and means for superimposing said potentials on said Waves.

6. A scanning circuit for determining the order of interlace of successive scanning fields of a television tube comprising first electron deecting means for defining the horizontal component of a scanning trace, second electron deflecting means including a source of sawtooth waves for defining the vertical component of said scanning trace, pulse generating means for generating pulses equal in duration to that of one of said sawtooth waves and of irregular amplitude to provide irregular order of interlace comprising a cathode ray tube for generating a cathode beam, a plurality of electrodes in said tube arranged to be scanned successively by said cathode beam, and deilecting means connected to said source of sawtooth waves for deecting said beam into impingement with one of said electrodes after another at the same periodicity as said waves, potential divider means including sections connected between diierent ones of said electrodes for providing different potentials of irregular amplitude incident to impact of electrons on said electrodes, and means for superimposing said potentials on said sawtooth waves.

LOUIS F. MAYLE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,203,520 Cawein June 4, 1940 2,224,677 Hanscom Dec. 10, 1940 2,250,479 Goldmark July 29, 1941 2,251,525 Rosenthal Aug. 5, 1941 2,265,216 Wolf Dec. 9, 1941 2,277,192 Wilson Mar. 24, 1942 2,280,572 Farnsworth Apr. 21, 1942 FOREIGN PATENTS Number Country Date v 819,883 France Oct. 28, 1937

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