US2438948A - Modulating system - Google Patents

Description

April 6, 1948.

R'. R. RIESZ MODULATING SYSTEM BV Mgw A TTORNEY R. -R. RIESZ MODULATING SYSTEM April 6, 1948.

3 Sheets-Sheet 2 Filed Jan. 2l, 1944 (aaoow) INCREASING B/J' CURRENT oc /A/Pur .s/c/uL- db ww) #5 4/ l VARY/N6 MPLTUDE SIGNAL COMPLEX CARR/ER WA VE (aaqaoow) ro o THE/i Mom/1.4 roes Jb 36 INPUT CARR/ER LEVEL OF /250'v COMPONENT=ldbm w Z N R5 R 0E 0 T/ T MR N 3 WR. N a /R l F V B muzmwmk n n. w u u m a 2 o m -m m- .t-u

April 6, 1948. R. R. RlEsz n MODULATING SYSTEM Filed Jan. 21, l1944 s sheets-sheet 's F/a la D C BIAS 65 INPUT CRR/ER LEVEL 0F [350m COMPONENTE-J5 d'bm /Nl/ENTOR By R. R. R/ESZ 35 c/bm L ATTORNEY I '50 45 /NPUT CARR/ER LEVEL 0F /350'V COMPONENT Patented Apr. 6, 1948 2,4sa94s MODULATING SYSTEM Robert R. Riesz, Chatham, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application January 21, 1944, Serial No. 519,109

3 Claims.

The invention relates to a Wave modulating system, and particularly to a wave modulating system employing a modulator comprising a plurality of physically inert elements having non-linear voltage-current characteristics, connected in a bridge arrangement.

When the usual modulator of that type is employed for combining direct current signals of varying amplitude or alternating signals of a band of frequencies and of varying amplitude with a complex carrier wave, such as one comprising a continuous Iband of frequencies or a series of harmonies of a given base frequency, it has been found that for a given input signal level the relative amplitudes of the wave components in the modulator output are not the same as they are in the carrier supply; and that the amount of l this non-linear distortion changes as the relative amplitudes of the components in the carrier supply Wave change.

An object of the invention is to reduce the distortion introduced by the modulating devices in such a modulating system.

A more specific object is to combine varying amplitude direct current or rectified alternating signals with a complex (multi-frequency) carrier wave in such manner as to provide substantially perfect product modulation, that is, so

that the combination products are directly .pro-

portional to the product of the input signal and carrier voltages over a wide range of signal input voltages. y

Other objects are to increase the linear operating range and to provide a high degree of carrier balance in a, modulator of the above-described type.

These objects are attained in accordance with the invention mainly by reducing the carrier input level to the modulator bridge to a value such that the variation of impedance of its non-linear elements is small throughout the carrier cycle; and by applying a small direct current bias to the modulator bridge in the conducting direction such as to reduce the resistance of its non-linear elements to a sufficiently low value thatsatisfactory carrier balance can lbe attained by relative small adjustments of the impedances of adjacent arms of the bridge, and to provide the optimum value of |bias current to give the maximum modulator operating range.

The various objects and features of the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

Fig. l shows a single line, functional schematic of a. multi-channel voice frequency carrier wave signaling system in which the use of channel modulators in accordance with the invention is desirable;

Fig. 2 shows an experimental test circuit whichA was employed for determining the signal distortion which wouldbe introduced in a channel of the system of Fig. 1 by a known type of modulator; l

Fig. 3 shows graphically test results obtained with the test circuit of Fig. 2;

Fig. 4 shows schematically the circuit of a modulator embodying the invention;

Figs. 5 to 8 show diagrams and curves used in connection with an explanation of the invention; and

Fig. 9 shows curves illustrating test results obtained with a test circuit similar to that shown in Fig. 2 except for the substitution of a modulator in accordance with the invention as shown in Fig. 4 for the known modulator illustrated in Fig. 2. l

Fig. 1 shows in single line, functional schematic lform the essential features of a system ofthe general type disclosed in my Patent 2,183,248, issued December 12, 1939, or in Dudley Patent 2,151,091, issued March 21, 1939, utilized for analysis and synthesis of speech or other vocal sounds, frequency range reduction and restoration or for other purposes as set forth in the patent specifications, in which the use of modulators in accordance with the invention is particularly desirable. The system as shown includes at its transmitting or analyzing end, a, telephone transmitter T for transforming applied speech sounds into an electrical wave comprising a continuous band of speech frequencies, say, of the frequency range 0-3000 cycles. This frequency band is amplified in the amplifier Al and the amplified wave impressed in common on the inputs of a plurality of wire transmission circuits or channels 1 10 extending to a receiving point. The band-pass lters F1 F10 inthe inputs of the respective channels 1 10, respectively select from the impressed waves a. different one of a plurality of contiguous frequency subbands, which may, for example, comprise the frequency ranges, 0-250, 250- 550, 550-850, S50-1150, 1150-1450, 1450-1750, 1750-2050, 2050-2350, 2350-2650, 2650-2950 cycles per second, respectively. The selected frequency suibbands are rectied by the rectiers Rl4 R10 in the respective channels 1 10, and the resultingv direct current components in these channels are extracted by the respective lowpass (0-25 cycles) filters `F11 F20 and are transmitted to the receiving or synthesizing end of the system where they are respectively applied to the signal inputs of identical modulators M1 M10 in the individual channels, corresponding to the shaping networks SN1, SNz, in the individual channels of the system shown in Fig. 1 of my aforementioned patent. j

The carriers supplied to the carrier inputs of the modulators Ml MIU, conjugately connected with respect to the signal inputs, are the 3 appropriate frequency subbands ltered by the band-pass iilters Fl Fill', respectively having the same pass frequency rangee attire-transmittingichannel band lters FI E10., fromthe output of the electrical energy sources ES supplying a iiat spectrum of either continuous .o r. discrete frequency components which arederiyed by the pitch control channel PC from the original speech Wave as indicated-. The myeclrianisms of the electrical energy sources E;.and thepitchz 10;

need not be discussed here. The receiving"band` 4 the pass frequency band of the energy source band-pass filter F5 for the modulator M5 of channelrNo. 5- in the system of 1. The outputsiof': the:l two single frequency carrier sources Cl and C2 are coupled by a hybrid circuit H,

whioh may comprise the usual hybrid coil and .associated balancing network, in conjugate relalters Fl .FI B, having frequency pass ranges respectively corresponding t those of the transyputting.hand.iilters Fl.. P110, .and to -the.energ source'. blanc cuers al.' Fini, select. incappropriate .frequencyeuhiiands from .the .combination products appearing in the outputs of the -modulatorsMl Mill in.cl11annels..l. lli, v,respectively. The selected-frequency subbandsare .superposed in the input of .the amplifier AZ.- to form a wave simulating the4 originalspeech wave applied to the input. off the system, which. wave. after-amplification inthe amplifier A24 to thede.- sired level, is transformed into speech sounds vby the .telephone receiver R.

A. series. of experiments.. weremade by appli.- .cant tcueterminetne signal .distortion .resulting inthe channelsxof asysteinsuch as .shownin Eig.. 1, by the. failure to. employ. periect product channel modulators, using a test circuit such, as `shown in Figv2f Tire particular. .eetfu'p illustrated inr'ig 2 is similar. totnat used to test channel No, A5 of the system QfFig. 1 inlwhicl; the channel .modulator M5. was of. a Wellfknown type. employing. a. lplurality. of. metal. oxide rectiiiers connected in, a. balanced. .bridge circuit. which modulator .is adapted .for producingan output` practically independent .of Variations. .in

carrier4 supply .leveiwnen used for. combining adireotcurrentinput .Signale/.ave oi a given v.amulitude level anda .single ,frequency carrier. wai/.e of. auamplitude level; Sufiiciently high. to vary, at. thecarrier. .frequent/n the resistance of its component. .rectier .elementsirom .a very. .higlito a. very, .lowA value compared with. the. generator andload resistance conneotedto the modulator. They particular. modulator. illustrated is., of .the general^ type 'disclosed in. Cowan. `Patent No. 1,959,459. iesuedliday. 2.2, 19.3.4., de.. shown. the ,modiuator comprises' a Wheatstope bridge. networuof copperfoxide. rectier elements- .l tod, conneetedin shunt. relationto a` pair. af iilterelisindicated Y by, the arrows. the, rectiers I and. .4 inonepair. of adjacent .arms gareconductiye in a direction away. from. their. common bridge. terminal, and the .rectiers 2.and,3 in the other two adjacent arms. are conductive. inthe .direCLOn towards their. common .bridge terminal. resistance potent.iometerA 5., provided for. adlustingthe balen-ceci the bridge to compensate.. for .small differences in the 'rectiner elements. hae...its..2ll.0.. clam resistance element connected series loe.-l tween. the. adjacent. arms of the modulator bridge,` including. the rectiers Land. ,3, and` its Sliding arm .connected to one terminal of the carrier supply.

To .simulate immuni-.frequency carrier supply. for the modulator employed inthe .system ofiliig, l. the carrier supply to thecopper-oride rectifier modulator bridgeinthe testfcircuitof Fig. 2 comprisesvthe two sources Cl andCZjgeneratingl constant frequency waves of 125.0 cycles and 1350 cycles, respectiyely, which are Within tion with each other and in yenergy transmission relationfwitli tile-carrier supply leads CS includ- -whichiimpress both carrier components across the terminating, resistor R1 on the horizontal diagonal of the modulator bridge. By suitable selectionv of circuit constants, the power level of each carrier component from C'I and C2 was set at..-..1,1 dbnl. (decibels referred to I milliwatt) so that. with both components applied, theinput carrier .power level` was-about f8 dem. .After balancing. the-modulator .bridge with tliepotentiorneter l5, a direct .current .signal .input of about 0.4 milliampere from a direct .currentsourca rep,- resentedby the battery BI` and series lL200-'ohm resistor. B2, .wasintroduqed throughthe 25,-cycle ylow-pass .filter F15, corresponding to the similarly` designated lterof channeLNo. 5.0i Fig. l, and the resistance network RN1,.connected across the Vertical diagonal Yof the modulator bridge,` this signall input being of a value .wellup on the .op- .eratingL range of.modulator` M5. The productsci ,modulation pass from. the same. -bridge diagonal .through the resistancenetwork. RNz .to the input of the Yreceiving .band filter, F5 .corresponding to the Similarly designated .filter .in channel No- .5 .ef 121g. l, the 1250..-cycleand lfcyclecomponents falling Within the pass-bandof that filter. The waye .components in 4.the output ofdlter- F5" are jappliedthrough the resistance network RNs to a measuring circuit including inorder, thel current analyzerQA, the. amplier et. andthe. volume in- .dicatorV VI. which .may be ci any. of theWellknowntypes. The output. amplitudes ofthe 1.35.0 and lzk-.cycle components. were measuredv in this .measuring circuit. as the 1259.-.cyc1e com,- nonentwasheld constant at-1i db m. andthe carrier input .level of. .the 13.59.,-cycle component is reduced..

The results of these measurements are shown graphically in.Fig-. 3- .It will be `noted from-the curyes of. .that .ligure that .as the carrier ampli.- tude of. the. ldilfcyclecomponent is reduced, the output amplitude for that component decreases loyA aboutthe. same number. of decibels- The output. amplitude of the 1250.-.cyc1e ,component .does net stay .constant put ieeeento increase.. When the amplitudeo thel lkfcyclecomponent has been. reduced 18. decibels, the output amplitude of .the 125,0-cyc1e component has been increased 4.8 decibels. A curve Kof the decibel difference inoutputllevel. between the 12,50 and 1350-cycle components is also shown in Fig. 3. For the modulator. to produce no amplitude distortion, the. latter. curi/e .Should be aetraiglit. line with a slope. Qi. -i degrees. putA actually it varies from the .Straight l-degreeline aSehov/n- To visualize the type of distortion introduced by the Vmodu ulatgr, assume thatwat some time the harmonics fall inthe pass-.band ojone of the channels of Fig. l, Then, .due to a rising inflection of the voice, let the fundamental frequency begin .to rise. The lower harmonic (1250 cycles) will remain inthepaee-band of the receiving citer, but the upper Vharmonic (1350` cycles.) will begin to moyeinto4 the suppressed band of the channel and will be attenuated by the energy source .bande page .illico Due to distortion .in the modulator,

the difference in level between the two harmonics at the modulator output will be greater than that produced .by the attenuation of the energy source band-pass filter. The over-all effect is to decrease the relative amplitude of a component falling in the cross-over frequency region between channels compared with components falling in the pass-band. This will tend to increase the attenuation peaks which may be introduced in the transmission characteristic by the failure of the filters to satisfy the relation (a1')2+(2)3=1 (l) where el and a2 are the respective ratios of the amplitude of components measured at the output of a lter to the amplitude measured at the input of that filter.

One method of eliminating this non-linear distortion is to employ in each carrier channel a substantially perfect product modulator,` that is, a modulator providing output products which are directly proportional to the product of the applied signal and carrier voltages over a wide range of signal inputs. The circuit of one such modulator in accordance with the invention is shown in Fig. 4.

The modulator of Fig. 4 differs from the modulator shown in Fig. 3 essentially only in the following particulars. The complex carrier wave is applied to the horizontal diagonal of the balanced copper-oxide rectifier bridge through a circuit including the repeating coil T2 and the condenser CI (4 microfarads) and resistor R2 V(600 ohms) ln series with the secondary winding of that coil, of such circuit constants as to reduce the carrier input level to the bridge to a suiiciently low value as to make the variation in impedance of copper-oxide varistors I to `4 in the modulator bridge small throughout the carrier cycle. When the carrier input level was reduced sufficiently to satisfy this condition, it -was difficult to secure a high degree of balance in the bridge modulator without the use of an external balancing condenser, particularly in the higher frequency channels. This is due to the fact that at low voltages the phase angle of the 1%" copper-oxide rectifier discs used in each varistor was found to be about -45 degrees at 3,000 cycles. The use of an external balancing condenser with a bridge modulator is troublesome, and experience has shown that the stability of balance is not high enough to warrant its use in a commercial system.

'I'he effect of capacity on the balance of the modulator in Fig. 4 is minimized by applying a suitable small biasing voltage to the copper-oxide rectifier bridge in the conductingr direction, such as to reduce the resistance of the copperoxide rectifier elements l to 4 to a suliiciently low value so that satisfactory carrier balances can be obtained by adjustment of the 20G-ohm resistance potentiometer 5 and without the use of an external balancing condenser, since the capacity of each rectifier is effectively in shunt with its resistance. For the particular bridge used, this was accomplished by connecting the 45- volt battery B2 through the large (200,000 ohm) series resistors R3 and R4 and the series resistor (600 ohm) R2 across the horizontal bridge diagonal with the poling shown. The direct current input signal is applied to the other diagonal of the bridge, as shown; then in such a direction that adjacent arms of the bridge are changed oppositely to higher and lower resistance values than the normal balance values, as

the amplitude of the'applied signal varies, and so unbalance the bridge by an amount proportional to the amplitude of the applied signal current.

For a balanced` copper-oxide bridge modulator of the type shown in Fig. 4, there is an optimum value of bias current that will yield the greatest modulator operating range. A typical family of modulator input-output curves for various values of bias current applied to such a modulator bridge is shown in Fig. 5. For all of these curves, the carrier input level is assumed held constant. Power output levels from the modulator in the pass-band are plotted in decibels as ordinates and the direct current signal inputs to the modulator in decibels as abscissae. show that there is a definite carrier leak level for low signal inputs and a common maximum saturate value of output signal power. Between these two limits lies the useful operating range of the modulator. A short portion of the upper and lower. end oi the characteristic, as shown in each curve, is not usable because of the departure from a straight line fof a 45-degree slope.

The carrier leak level is due to capacity unbalance in the modulator rectifier discs and decreases as the bias current increases; for then the resistance of the discs idecreases and the. capacity `unbalance has a smaller eiect, To visualize Why the curves all showa common saturate output level, consider the simplified balanced bridge modulator A in Fig. 6. At a sufficiently high value of signal current Ine, rectier discs 2 and 4 in opposite bridge arms will have a negligibly small resistance, and ldiscs I and 3 in the other two opposite bridge arms will have the high non-conducting value of disc resistance, so that the bridge degenerates to that shown at B in Fig. '7. When this situation prevails, there will be a fixed loss between the carrier input and modulator output terminals that is independent of the initial modulator bias current. Plotting this ratio, in decibels, between the saturate output power and the output carrier leak power level as a function of modulator bias current, the curve A of Fig, 8 is obtained. However, as the system of Fig. l is used, there is a maximum direct current signal, Io, that can be furnished by each channel rectifier and low-pass filter, Plotting theratio of the maximum output power obtained from the modulator when Io is the input signal, to the output carrier leak level, as a function of bias current, the curve B of Fig, 8 is obtained. The operating range is seen to have a maximum value at some bias current i. This optimum bias current will be that bias current producing the input-output modulator curve that just gives the saturate output power for the signal direct current In. For the particular system of Fig. 1 used, Io=0.67 milliampere. The table below shows values of modulator operating range as a function of modulator bias current for 10:0.67 milliampere into the modulator circuit of Fig. 4,

Ratio of Output Power for Io= 0.67milliamperc Bias Current f to Carrier Leak Abias current of 0.11 milliampere was taken as the optimum value and the constants of the These curves circuit'elements' in the biasing circuit were se'- lected to produce that value of bias current at the bridge. The receiving band-pass iiters, to which the modulator outputs are connected in a 'system in accordance with Fig'. 1, which was built., are unbalanced with one side grounded, so that the low side of each modulator is grounded. In order to leave the balance point of each modulator conveniently near the center of the 20G-ohm resistance potentiometer in the modulator of Fig. 4, the pairof series 200,000-ohrn resistances in the bias supply circuit for each channel modulator was selected to be Within i5 per cent of those values.

In the system of Fig, 1, the signal applied to thesignal input of the modulator in each chan nel is not truly a direct current one, inasmuch as each of the low-pass filters FII F20 used in the channels passes all frequency components from zero to 25 cycles per second. If a modulator in accordance with the invention, as sho-Wn in Fig, 4, is used in each channel of the system of Fig. 1, and is operated as described above, it will operate as a perfect product modulator for a Wide range of alternating signal voltages. Raising the cut-01T frequency of the low-pass lter will enable the modulator to work as a perfect product modulator -for higher alternating signal voltages. The Vinput signal voltage must always be on the linear portion of the curves of Fig. 5 for the output to be proportional to the product of the input and carrier voltages. For the case of a single frequency carrier and a complex input signal," this is illustrated by the equation:

= (aero COS mAH-aal GOS out COS wet-iag cos mi cos wt+.

Carrier Lower sideband A test Vcircuit similar to that of Fig. 2 was set up, except that the biased perfect product modulator of the invention, as shown in Fig, 4, Was used. The carrier input levels into that modulator of the 1250 and 1350-cycle components were each set to -36 dbm. so that the total carrier power input level to the modulator was -33 dbm, Then, the output amplitudes of the 1250 and 1350-cyc1e components were measured as the carrier input amplitude of the 1250-cycle component was held constant and the amplitude of the 1350-cycle component reduced. The results are shown by the curves of Fig. 9. As shown, the 1250-cycle component in the output wave remains constant in amplitude and the amplitude of the 1350-cycle component is proportional to a synthesized speechtoecarrier leak ratio of about 54 decibels was measured, which was about a 12-decibel improvement over the original'highcarrier-level modulators used. The reduction -of vcarrier leak background produced a' noticeable improvement in the naturalness ofthe synthesized speech, and a perceptible reduction of the nasal quality of the synthesized speech as well.

Because the perfect product modulator of the invention described above provides y-alarge linear operating range and an unusual degree of carrier balance, its use is desirable in all carrier systems, of which the system of Fig. 1 is one example, where the carrier cannot be suppressed by filtering without suppressing wanted modulation products. Also in systems Where the carrier is suppressed by filtering, the amount of ltering required can be reduced by the use of 'the described type of modulator.

Although the improved modulator circuit of the invention as described above employs copper-v oXide varistors in the modulator bridge, any other type of physically inert non-linear impedance element may be used provided the direct current bias `applied can bias the elements in the bridge to a resistance value low compared to the reactance of the capacity of the element used, and it is not necessary that these elements be rectifiers; for example, each non-linear element may be one whichconducts equally Well in both directions, such vas the composition `known as Thyrite prepared as described in the United States patent to K. B. li/iclilachron,` 1,822,742, issued September 8, 1931. Other modifications of the circuits illustrated fandV described above which are within the Spirit and scope ofthe invention will occur to persons skilled in theart.

What is claimed is: Y

1. The method of operation of a modulator of the type consisting oiv a plurality of non-linear impedance elements connected in a balanced bridge arrangement across the two diagonals of which sources of signal and carrier waves to be combined are respectively connected, which'consists in reducing the input level of Vthe carrier wave applied to th'e bridge arrangement to a sufciently low value so that the impedance variation of said non-linear impedance elementsis 10W throughout the carrier cycle, and applying aY di rect current bias to the bridge arrangement in the conducting direction such as to minimize the effect of capacity on carrier balance and to provide the optimum amount of bias current to give the' maximum modulator operating range.

2. A system for combining a carrier wave of given frequency with a multi-frequency alternating signal wave of Varying amplitude so as to produce combination Waves of an amplitude level proportional to the product of the signal and carrier voltages over a wide range of signal input levels, comprising a plurality of non-linear impedance elements connected in a Wheatstone bridge arrangement across the two conjugate diagonals of which the sources of sign-al Waves and carrier waves to be combined are respectively connected, means to reduce the carrier input level to the bridge arrangement to a sufficiently `low value so that the impedance variation of said non-linear impedance elements is low throughout the carrier cycle, and means to apply a direct current bias to said bridge in the conducting direction, such as to reduce 4the effective resistance of said non-linear impedance elements to a -suiciently low value as to'enable satisfactory carrier balance to be attained by slight adjustment of the resistance values of adjacent bridge arms, said direct current bias being of the optimum value which will provide maximum modulator operating range.

3. A system for modulating a complex carrier wave with a direct current signal wave of varying amplitude, so as to produce combination products of amplitude level proportional to the product of the signal and carrier voltages over a wide range of signal input levels, comprising a Wheatstone bridge having anon-linear impedance element in each of its four arms, a resistance potentiometer in series with' two adjacent bridge arms for making small adjustments in bridge balance to reduce carrier leak, means for impressing said signal wave on one diagonal of said bridge, and said carrier wave on the other diagonal of said bridge at such a .level as to make the impedance variation of the non-linear impedance elements in said bridge low throughout the carrier cycle, means to apply to said bridge in its conducting direction a biasing direct current voltage such as to reduce the effective resistance of said non-linear impedance elements to such a low value as to enable satisfactory carrier balance to be attained by adjustment of said resistance potentiometer, and to provide the optimum amount of bridge biasing current for maxi- .mum modulator operating range, and an output circuit for taking oi the combination Waves.

ROBERT R. RIESZ.

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

UNITED STATES PATENTS 1,959,459 Cowan May 22, 1934

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