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Dec. 3, 1946.' c. M. sl‘NNET-r ` 2,412,015 _ . ANGLE MODULATED WAVE REPRODUCING SYSTEM @Lw LIM lNvENToR ¿Zw-fn? M JPM/V577: BY ATTORNEY Dec. 3, 1946.v .2,412,015 c. M. slNNETT ANGLE MODULATED WAVE REPRODUCING SYSTEM Filed March 20. 1944 3 Sheets-Sheet 2 T1 :l .4.. aa/a Mme'. n 4mM l ns .m C 0 , M/ ß/ CD f f N 0 .F M H 6 f. ß/. H M M œ ßn M . ko iN4 F c..a m ‘ INVÉNTOR (yf-fra?? M „ÜN/V577.' BY ATTORNEY Dec. 3, 1946. l v ' l c. M. slNNl-:TT 2,412,015 ANGLE MODULATED WAVE vREPRODUCING SYSTEM Filed March 20, 1944 l I 3 Sheets-Sheet 5 45 N ¿a 5/7 '_ ' 62;* T1515. V BY ATTORNEY 2,412,015 UNITED STATES PATENT oFFlcE Patented Dec. 3, 1946 "J v 2,412,015 ANGLE MODULATED WAVE REPRODUCING SYSTEM _ . Chester M. Sinnett, Westmont, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 20, 1944, SerialNo.. 527,264 19 Claims. (Cl. 179-1004) 1 _ My present invention relates generally to a whose condenser element acts as an electrostatic novel method of, and systems for, reproduction reproducer. of audible frequencies existing in the form of stated to reside in the provision of a frequency A further object of the invention may be ` angle modulation on a superaudible wave. responsive network, adapted to have angle modu In the past it has been proposed to reproduce sound records by translating the recorded varia tions into reactance variations, angle modulating a superaudible oscillator in accordance with the reactance variations, converting the angle modu lated oscillations into corresponding amplitude modulated oscillations, and ñnally rectifying the lated waves applied thereto, which consists of a coil and a condenser providing together a resonant circuit tuned off-resonance relative to the mean frequency of the applied waves, and either the coil or condenser acting as a repro ducer element whereby the angle modulation on the applied waves is directly converted into sound. latter and reproducing the audio frequency cur rents resulting from rectification. The generic term “angle modulated” is intended to include A still further specific object of this invention is to provide a phonograph, or other sound re frequency modulation (FM) and phase modula 15 producer device, operating onl the frequency modulation principle and employing a super tion. The aforesaid converting and rectifying audible frequency modulated carrier, wherein the steps have been common to other sound repro modulated carrier is amplified and fed into a ducing methods and systems, whether the source tuned discriminator circuit, one reactive element4 of input energy was a record bearing impressions representative of audio signals or angle modu 20 of which functions as an electrostatic loud speaker. lated superaudible oscillations. In the case of A further object of my present invention is to` frequency modulation radio reception, as well, provide a. novel method of operating a condenser discrimination and rectifying steps are employed type of sound reproducer, wherein the polariz to provide modulation signals for ñnal reproduc ing voltage of the reproducer is provided by tion. A general object of my present invention is to provide a system for reproducing frequency modulated superaudible waves, wherein the. dis criminator of the system has a reactive element thereof which functions as a modulation repro superaudible wave energy applied to the repro ducer. 30 whether condensive or magnetic, wherein angle ducer thereby eliminating the need for the con ventional form of discriminator-rectifier network commonly employed in frequency modulation systems. . Still other objects of my invention are to pro. vide novel sound reproducer . arrangements, v According to another important object of my present invention, I utilize an electrostatic type of reproducer as an essential reactive element of a simple tuned circuit acting to convert angle modulated superaudible waves into correspond ing amplitude modulated waves with concurrent acoustic reproduction of the modulation on the amplitude modulated waves. Still another important object of my present invention is to provide a novel method 4of re producing sound, wherein angle modulation of a 45 superaudible wave is employed to represent the currents corresponding to the sounds to be re produced by a transducer, and the transducer modulated superaudible carrier waves are directly applied to the reproducer for acoustic reproduc tion of the modulation. Still other features of my invention will best be understood by reference to the following de scription, taken in connection with the drawings, in which 1 `have indicated diagrammatically several circuit organizations whereby my inven tion may be carried into effect. f Fig. l schematically shows one embodiment of the invention, Fig. 2 is a graphic representation of an ideal resonance curve of the discriminator network of Fig. 1, . Fig. 3 is a modification of the system of Fig. 1 wherein further circuit details are shown, Fig. 4 presents an idealized graphical analysis of the functioning of the present> system, Fig. 5 shows a further embodiment of the in itself acts as an essential reactive component of a discriminator network for the >angle modulated 50 vention applied to reproduction of frequency waves. ‘y A more specific object of my invention is vto provide a system of reproducing sound records, modulation-recorded impressions, Fig. 6 schematically shows the invention ap plied to a modification of the system of Fig. 1, wherein a pickup is employed to convert the Fig. 7 'illustrates an embodiment of the in record impressions into capacity variations of a 55 vention showing both condenser and magnetic superaudible oscillator circuit thereby to provide frequency modulated superaudible oscillations; the essential improvement in the system includ ing the replacement cf the usual discriminator rectiñer network by a discriminator network 60 reproducer elements, ‘ ' Fig. 8 shows a modification ofv the circuit of Fig. '7, ' Fig. 9 illustrates schematically the manner of 2,412,015 embodying the invention in a frequency modu lated carrier wave receiving system. While the present invention is of wide utility, as will be clear after a reading of lthe followingdescription of .the various aspects and features of the invention, the embodiment of .the invention shown in Fig. 1 will be presented with a View to explaining the functioning of my novel discrimi 4 lated wave energy. The term “discriminator” used in the description and claims denotes a cir cuit which functions to produce an amplitude variation in the frequency modulated wave energy, which amplitude variation is representa tive of the frequency deviations of the wave energy. The latter wave energy is then capable of being rectified in order to provide the modula nator-reproducer network. f AAs stated above., it tion signals for reproduction. This is true has been proposed in the past -to reproduce sound 10 whether the sourceof the frequency modulated records by employing a superaudible carrier energy is directly responsive to a record, or which is frequency modulated by reactance' vari whether a frequency modulation radio receiver is ations derived from the record impressions.' Ac being employed. cordingly, in’Fig. 1 I have schematically repre According to my present invention, acoustic sented by the numeral I a superaudible oscillator 15 reproduction of the modulation signals occurs si network which is provided with the usual resonant multaneously with discrimination of the angle tank circuit 2. The latter consists of a coil 3 and modulated wave energy. Stated differently, I shunt condenser 4, and is tuned .to a predeter have provided a discrimin-ator circuit which mined normal oscillation frequency Fc. The pre . automatically and concurrently acoustically re determined frequency of the superaudible oscil 20 produces without the usual rectification .the am lator may be chosen from a wide range of fre plitude modulated wave energy produced by dis quency values depending upon the design of the crimination. Reference to Fig. 1 will show that other networks of the system. The oscillation fre the FM wave energy, subsequent -to amplification quency may be in the megacycle (mc.) range, or - v- at 9, is applied to a condenser C and coil L pro it may be in the lower kilocycle (kc.) values. By 25 viding a parallel resonant circuit. The condenser way of specific example, let fthe-assumed that C is actually an electrostatic .type of acoustic re the oscillator I has its tank circuit 2 tuned to a producer. Those skilled _in the lart of sound frequency of 20 kc. The frequency of ythe tank reproduction are acquainted with the manner of circuit 2 is modulated or deviated by virtue of Y designing an electrostatic type of reproducer, such reactance variations caused by the tracings of 30 as a condenser loud-speaker. the needle or stylus through the record grooves. A .condenser loud-speaker is .a loud-speaker in By way of specific illustration, the numeral 5 which the mechanical forces result from electro denotes a record on a turntable 6. An electro~ static reactions. The -plates of a condenser loud static pickup device is schematically shown, and speaker are relatively charged .due to polarizing comprises a small condenser one of whose plates 35 voltage. The polarizing voltage may be as high ‘I is relatively fixed. The plate 'I is connected to as +200 volts or more. The mobile electrode of the high alternating potential side oi’ tank cirn a condenser loud-speaker radiates the sound di cuit 2. The mobile electrode 8 is indicated as rectly into the air. Provision must be made to grounded, and, therefore, is effectively connected keep the electrodes of the .condenser separated to the grounded side of lthe oscillator tank circuit. without at the same time incurring much me The mobile electrode has operatively associated chanical stiffness. 'I‘o obtain an efficient system therewith a stylus which runs through .the record the mass of the mobile electrode is best kept low. grooves. Hence, the capacitance of condenser The impedance of the amplifier .tube driving the 'I-8 will vary in accordance with the physical condenser speaker is preferably made approxi displacements of the stylus. These capacitance mately equal to the capacitative impedance at the variations are translated into' frequency devia» highest audio frequency to be reproduced so .that tions of tank circuit 2. The electrostatic pickup the capacity current drawn by the speaker will device may be constructed in accordance with the - not cause excessive falling-off of voltage at the teachings of my application Serial No. 414,305, higher audio frequencies. It is an impor-tant and ñled October 9, 1941, now Patent No. 2,376,456 50 novel feature of my invention .that the modulated issued May 22, 1945. However, the invention is superaudible voltage applied to the condenser not limited to this type of Idevice .as any other form of pickup maybe used, as long as it'l is capable of transforming the physical displace ments of the stylus into reactance variations of the oscillator tank circuit. The extent or range of frequency deviation with respect to Fc, the predetermined normal oscillator frequency, will be representative of the speaker C serves to provide polarizing voltage as required by the electrostatic speaker. A special or auxiliary polarizing voltage source is thereby eliminated. The showing of the reproducer .construction in Fig. 1 is highly illustrative and schematic, since the particular type of reproducer is well known, and my invention is not dependent upon any par- . amplitude of the sounds which are recorded on 60 ticular reproducer construction. In general, it record 5. while the rate of frequency deviation will be representative of the audio frequencies per se. ’I'he maximum frequency swing on either side of the mean frequency will depend also upon the design of the oscillator circuit. For example, the overall frequency swing or range may be some 10 kilocycles (kc). The frequency modulated oscil lations may then be amplified as at 9. Accord ing. to my invention, Ithe amplified frequency comprises a metal backing plate I0 of wavy con figuration so as to provide spaced peaks. The lead II connects the plate I0 to the lower end of .coil L. The condenser I2 establishes both the lower end of coil L and the plate'I 0 at ground potential for alternating current. The upper end of coil L is connected to the output electrode (not shown) ofthe amplifier 9. The +B terminal of the direct current supply source (not shown) is connected modulated wave energy is subsequently treated in 70 to the lower end of coil L. The second electrode an entirely unconventional and novel manner. of condenser C is provided by a thin metal sheet In the prior art, it is usual to subject frequency I3, as for example, tin or aluminum foil. It is modulated carrier wave energy to discrimination obvious Ithat this metallic surface could be com in order to translate the frequency modulated posed of sputtered or deposited gold or silver lof wave energy into corresponding amplitude modu 75 essentially molecular thickness as long as .con 6 tinuous contact were maintained. This sheet I3 is. stretched across an insulation sheet, such as a sheet of rubber I4., The latter -functions as an elastic dielectric for the condenser C. I . . ter voltage is applied, tothe input grid of the am Dliñer2l. ~ ’ ' » The input grid of ampliflertube 2| is connected by the direct current blocking condenser 20 to the ‘ I8., The remainder'of the , plate of oscillator tube; The vlead >I5 connects the electrode I3, the mo circuit elements electrically connected to tube 2I bile or diaphragm electrode, to the high potential are conventional in nature, and need not. be de end of coil L. Hence, the spaced metal electrodes scribed in detail. The plate circuit of tube 2| in I0 and I3 are effectively connected in shunt with cludes a tuned circuit 22- which is resonated to. ' coil L. Since the elastic dielectricy sheet I4 is 10 the mean frequency Fc. Hence, across tuned cir stretched over the outer face of the ñxed elec cuit 22 there will be developed amplified fre trode I0, there is mechanical contact'between the. sheet vHI and ,electrode Iû only atgthe -peaks of the _ latter. The metallic or metallized sheet I3~is in contact With the dielectric sheet I4 at all points. ' Hence, the metallic surface is rendered effectively elastic clue to the subjacent dielectric. As stated previously; the specific structure of the condenser speaker C is not part of my present invention; See, for example, U. S. Patent No. 1,644,387 to Kyle for a reproducer construction which is sim ilar to that shown herein. ` quency modulated superaudible oscillation volt age, >This voltage may be further amplified in the output tube 2'5. The input grid of the latter is coupled by direct current blocking condenser 23 to the plate side of the resonant circuit 22. A suitable choke coil 24 connects the input grid of 'tube 25 to ground. The plate of tube I1 is con nected to the +B terminal of the direct current 20T supply source through the ¿coil L which was de scribed in connection with Fig. i. The condenser - speaker C, schematically represented, is shown The' ideal discriminator characteristic of the connected in parallel with coil L. As stated previously, it is desirable to have the output impedance of tube 25 match the capacita tive impedance of the condenser speaker C at the highest audio frequency to be reproduced. The circuit L~-C acts as a high impedance in the plate circuit offtube 25. The tube 25 may be operated circuit L--C is shown in Fig. 2. It is to be under- ‘ stood that the graphicrepresentation of Fig. 2 is purely illustrative. . The curve in Fig. 2 is a single peak resonance curve Whose peak fre quency is the resonant frequency of the circuitl which includes the coil L in shunt with the con denser C. The predetermined mean or center as a class A, class B or class C ampliiier. There frequency Fe of the tank rcircuit 2 of the super 30 are advantages in class C operation; for example, audible oscillator is off-tune with respect to the peak frequency of circuit L-C. The extent of off-tuning will be determined by the shape of the resonance curve. ' u considerable power output could be secured even though tubes of relatively reduced dimensions were used. If desired, the power output stage may use a pair of-,tubes in push-pull relation. The bypass condenser I2 is connected from the lower end of coil L to ground, as in Fig. 1. Since the particular construction of the condenser re producer C is no part of my invention, the repro - The peak frequency of the electrostatic repro ducer circuit L--C is preferably so chosen that the mean frequency of the modulated Waves ap plied to the reproducer circuit will fall on a point on either flank of the resonance curve which has 40 ducer is schematically represented in Fig. 3. 'I'he a substantial slope. Hence. it will be seen that mobile electrode, or diaphragm, is denoted by the applied frequency modulated superaudible oscillations will be translated into correspondingv amplitude modulated oscillations. Such ampli-l tude modulated wave energy will be reproduced ` by the electrostatic reproducer C. Accordingly; my method generically contemplates the trans lation of angle modulated superaudible oscilla tions into corresponding amplitude modulated oscillations and concurrent reproduction of the audio modulation which exists on the translated wave energy, In Fig. 3 I have shown ’specific circuit elements which may be employed in the schematic net works of Fig. 1. A morev detailed explanation of the operation of the condenser reproducer will be given in connection with Fig. 3. The electro static pickup device 8-,-1, which constitutes a condenser, is shown. as having its fixed electrode l connected to the junction of the coil I6 and blocking condenser I1. The coil I6 and condenser 8--1' provide a resonant grid circuit for the oscil lator'tube I3. The resonant grid circuit will nor- l mally be tuned to the predetermined mean fre quency Fc. The plate circuit of tube I8 includes a resonant circuit I9 which is also tuned to the mean frequency Fc. It will, therefore, be recog nized that Athe superaudible oscillator shown in Fig. 3, is of the well known tuned grid-tuned plate type. The variations in capacitance of the pick up device 1_8 in response to stylus motion cause _ frequency modulation of the oscillations produced in the oscillator circuit. Hence, there will be de veloped across the plate circuit I9 frequency mod--y ulated super'audible oscillation voltage. -Thelat numeral I3', While the numeral I0’ designates the fixed electrode corresponding to electrode III of Fig. 1. Circuit L--C is tuned to the peak fre quency of the curve shown inFig. 2. The fre quency Fc Will fall on either flank of the curve; at about 70% of resonance. ' ' - Before explaining the functioning of circuit L-C, let it be understood that the following analysis is equallyapplicable to an‘ electromag netically-operated device of the diaphragm type. The only requirement is that the original dis placement of the diaphragm be secured in re sponse to carrier Voltage. In any event the car the system should be of superaudible frequency, and is required'to be'of an intensity capable of providing initial displace ment of the diaphragm. Under these conditions, assumethe sound reproducing element is either - riez` wave employed in the coil or condenser of the resonant discrimina tor circuit, and that Fe of the applied frequency variable waves falls on either flank of the dis criminator resonance curve, The variations in frequency of the modulated Wave energy Will cause a change in polarizing voltage, and conse quently the diaphragm displacement will change. If the variations in frequency of the FM energy applied to circuit I_r-C occur at an audio fre quency rate, sound will be emitted by the re producer reactance element. . Another Way of looking at the action of vthe discriminator circuit L-C is as follows: Regard less of the polarity of the super-audible fre quency current, the diaphragm elect-rode will be displaced in the same direction thereby-produc-4 2,412,015 7 8 ing second'harmonic output of the applied super audible carrier frequency. This can bethought (3+ and -~)_` thereto. 'In circuit L-C there will appear across coil L, and'consequently across C, an alternating voltage having the frequencyFc. Since the opposite ends of coil L must 'havefop of 4as a form of rectification. Once the diaphragm electrode has been :displaced by the carrier cui' ren‘t„ variations in carrier amplitude produced posite‘polarit’y potentials, the opposite electrodes of C must be of opposite'polarityl thereby caust-> by, thedetunin’g-'of?the L-C circuit (which is madeup impart by the reproducer element per ing attraction between them. se) v_willV result in _acoustic output from the re- ` producer element in step with carrier frequenc changes. _, ’ f ~- `In order more clearly to depict the functioning ' ‘ This gives therequired initial displacement of the speaker` diaphragm electrode I3. The amount' 1,0 ofdisplacement increases as the voltage across-` coil L increases. 1‘The carrier voltage of _mean frequency Fc will produce a displacement of-.the the-curves A, B, C and D of Fig. 4. These curves diaphragm for each half cycle, since the only. re are ideal representations, and are purely illus quirement for the attractionv between the »di trative.- K'Cu'rve A shows a modulating audio Wave 15 aphragm I3 and fixed electrode I0 is that oppo assumed »to represent the recorded sound wave. site polarity potentials be applied. Thus, as far. This curve can represent the change of capacity as the diaphragm is concerned the pull on it will of-»the -FM pickup 1-8 which produces changes' occur at double the carrier frequency Fc. Since in oscillator frequency as I*shown by curve B. The Fc is fundamentally above audibility the> vari curve A' could also 'represent the voltage output 20 ation in pull on thediaphragm at 2Fc cannot be of piezo-electric `crystal 4I) of Fig.'6. The curve heard. Therefore, the same effect is produced, pictorially shows the relative values of the mod asfar asv the electrostatic speaker is concerned, ulating and modulated frequencies. Curve B a's though direct current voltage were applied to» represents the frequency modulated wave re the electrodes in the well known manner. of the present system, attention is directed to» sulting fromY variation of the pickup device 1_8 Having initially displaced the diaphragm I3 in response to the variations of curve A. vThe against the resilient -sheet or darn I 4, there is created a restoring-force due to the stressed dam. This force tends to return -the diaphragm I3 to its original. normal position'. If we now change oscillator voltageduring modulation would gen erally follov’i7 the wave form of curve B. The am plitude is constant, while the frequency is vari able. vThe effect 'of' circuit L-C on the 'wave the magnitude of the polarizing voltage, the sound output must result. >This acoustic output form of curve Bl is depicted'in curve C. It will be> observed that the frequency-variable wave now has variable amplitude as well. It will fur ther be seen that the variable amplitude of curve occurs as the attraction between the electrodes I0 and I3 varies with the applied voltage, and the variation Will occur'at an audio frequency rate C follows the variation of the modulating audio correspondingv to the modulating frequency yap wave of curve A. Finally, curve D illustrates the plied to the carrier wave. There may be some improvement in acoustic output by operating at point Fc on the curve of Fig. 2 rrather than at point Fc’ on the high fre quency ilank ofthe curve. It has previouslybeen pullïonthediaphragm electrode (I3 in Fig. 1 or I3' in Fig;v 3) of' condenserreproducer C. Note that the pull is always in one direction, at double the carrierffr'equency and of a variable ampli tude `determined by the voltage across L-C as shown byl curve C. Due to the audible variations `explained .that as the voltage applied to speaker electrodes increases, the pull on the diaphragm increases. As the diaphragm is displaced in this fashion by a change in the instantaneous value of the applied modulated energy, the capacity of condenser-C .increases thereby to lower the fre. quency ofL-C.. y.This causesa.further'increase in pullon the diaphragm> the sound emitted by the'diaphragm 'will be a' reproduction of the re corded sound.v Curve D is more closely spaced than the prior curve C, because it represents the pull on the diaphragm, and this occurs at double theA carrier frequency shown in curves B and C. in voltage and further displacement of electrode ' The frequency'range of the superaudible oscil lator is preferably Vbetween 15 and 30 kc., al though notnecessarily limited thereto. Gener I3. Obviously this cannot'go on very long, since 50 there is not necessarily a linear relation between capacity change, displacement of the diaphragm ally, the’ oscillator frequency should be several times 'higher than the highest audio frequency and the polarizing voltage. `Some of the eiîect is present, however. Similarly as the instan which it v'is desired to reproduce. 'For an oscil lator having a frequency of 15 kc., it is most likely that -frequency deviations up to a maximum of plus orminusA 2 'to 3 kc. should be suilicient. At an oscillator frequency of 30 kc. it might be de sirable'to employ frequency deviations up to al maximum of» plus or minus 10 kc. These -fre 60 quency values are not intended to be limiting in nature, but are merely illustrative. ' YIt has been previously stated that one of the main features of my invention is to utilize the appliedicarrier‘voltage as the polarizing voltage taneous frequency is Lshifted downward by modu lation, the Voltagenacross condenser C decreases. This causes decrease in displacement of >electrode I3, with concurrent upward increase of> the resonant frequency of L-C thereby causing a further decrease in voltage across condenser C. Thus, tuning the circuit L-C sc_.that Fc falls on the low frequency flank of the resonance curve of circuit L--C, should result in some increase in output. . ' ' Assume, now, that the operating point is at 65 Fa'.y This means that Vthe normal Vfrequency of oscillator' is 'Fc'. If the instantaneous frequency of the FM voltage applied' to 'L-C is above ‘Fe’ then a'decrease ofv outputvolta’ge occurs. The' iliary'direc't current `polarizing voltage is need displacement of lthe `diaphragm will decrease ed in my condenser reproducer. If like voltages 70 thereby causinga- decrease in effective capacity for the condenser. The following explanation is not necessarily exclusive of other theoretical ex planations of the demonstrated fact that no aux (+ and -l-‘) are applied to electrodes I0 and I3, there will, >of course, be set up a mechanical force tending to push or repel the electrodes from each of condenser C.: This will, in turn, cause circuit L--C- to tune to a higher frequency resulting in' other. ' 4The electrodes »are attracted to each condition occurs when. the modulated Lenergy lhas stillîless voltage and displacement. A similar other uponk the application- of unlike 'voltages 75 an -instantaneous:frequency less than Fe'.; There 21,412,015 10 The resulting FM currents may be amplified. as atamplifier 42 tuned to the mean frequency Fc, and then applied to the output amplifier £3. Fc on the resonance curve of circuit L-fC». The latter has the circuit L-C in its plate circuit My invention has many possibilities. The pick exactly as described in Fig. 3. The action of cir up device need not be electrostatic, but may be cuit L-C is the same as previously explained, of the variable inductance type. The records used and the audio frequency current repro-duced by may be of the conventional laterally cut type; the condenser reproducer C-is emitted as sound. “hill and dale” type; or nlm recordings of either As stated previcusiy,. the present invention is amplitude-variable or frequency-variable types. applicable to a resilient diaphragm type of 10 Moreover, the records may be of the frequency is thus obtained less acoustic output'from-an overall viewpoint when operating at Fc’ than at modulation type, wherein frequency modulation electromagneticaliy-operated device. The dia signals are directly recorded. In general, any transducer device can be employed in conjunction phragm may have sufficient inherent resiliency to spring back, or restore, into normalA horizontal with a superaudible oscillator to vary the fre quency of oscillations thereof, and the resulting chanical restoring device may be used. The frequency modulated oscillations after amplifica. position, orA any well known and suitable me only requirement is that the original displace ment of the diaphragm from normal position be obtained by means of carrier-derived voltage. The ampliiied voltage at carrier frequency Fc (applied to the driving coil) should be sufiiciently pickup device whether of the electrostatic, mag 20 high to provide the necessary polarization and netic or piezo-electric crystal types or any other initial', minimum or threshold displacement of mechanism capable of translating acoustic or re the diaphragm. In Fig. 7 I have shown the corded waves into electric currents of audio fre output ampliñer 5o of a system fed with FM tion can be reproduced in a discriminator-repro ducer circuit L-C as described herein. By the term “transducer” I include a microphone, a quency. In Fig. 5 I have shown a modincation of the system or” 3 to the extent that the amplifier 25 is supplied with ampliñed frequency modulated signals from a piezo-electric crystal pickup de vice. The record in this form of the invention is generally of the type disclosed by W. van B. Roberts in his application Serial No. 369,829, ñled December 12, 1940. The record 5 has cut or en graved in its face impressions corresponding _to FM signals whose mean carrier frequency is superaudible. Here, again, the recorded FM sig- ‘ nals may have a mean frequency chosen from a waves produced in any of the ways shown in Figs. 1, 3, 5 or 6. The amplifier tube 50 has the circuit L--C arranged in its plate circuit asv previously explained. The condenser C is a re producer which has a high audio frequency re spons@ characteristic. In other words it is of the so-called “tweeter” type. The coil L is the primary winding of a transformer 5l Whose sec ondary winding 52 is arranged in a closed circuit with a low impedance coil 53. The coil 53 isA provided with a diaphragm 5s which is initially displaced or polarized by virtue of the carrier derived voltage developed across coil 53. Coil range of 15 to 30 kc., with a maximum deviation range from 2 to l0 kc. The recorded FM signals are converted into FM currents by the crystal material readily adapted to providea magnetic standard amplitude vrecord of the laterally-cut ‘ ‘ winding 53 may be provided with a core 53’ of magnetic pickup 3B. The latter is schematically repre 40 pull on diaphragm 5s against the illustrative leaf spring restoring device 55. The magnetic repro sented, since it is very Well known to those skilled ` ducer 53-54 is of the low audio frequency re in the art of reproducing recordedrsound.. The sponse, or so-called “woofen” type. » FM currents produced at the output terminals of This form of dual reproducer system operates pickup 3l) are amplified in one or more stages of in a manner similar to the prior circuits L--C. amplifica-tion 3l and 3i'. The ampliners 3| and f4.5 In the present case the primary circuit of trans 3l’v are each tuned to a predetermined normal former 5l is tuned t0 the peak frequency of the frequency Fc. 'I‘he aforesaid mean carrier fre curve shown in 2. The mean -frequency of quency is Fc. The ampliñed FM signals applied the applied FM signals falls on either flank of to amplifier 25 are reproduced by the conjoint the curve as previously explained. The-repro discrimination and reproduction action of L--C ducer C functions in response to the envelo-pe of as previously described. l the rectified amplitude-variable wave, as depicted In Fig. 6` I have shown a dirferent method of in Fig. 4 by curve D. The variable amplitude producing the FM signal energy for application to carrier currents flowing through primary wind the circuit L-C. In this system the record 5 is a ing L will induce similar currents in secondary type or of the “hill and dale” type. The pickup de vice 43 is a suitable form of piezo-electric crystal pickup, as in the case of Fig. 5, and feeds its audio frequency output current to the input terminals of a superaudible frequency oscillator ci. ri‘he latter is normally tuned to produce oscillations ci a frequency Fc. The enîect of the audio input current is to vary the frequency of oscillation of the oscillator di. Any formof relaxation oscilla tor. such as the well known resistance-capacity -' oscillator or thel multi-vibrator oscillator, may be These variable amplitude currents will vary the position of the initially-displaced diaphragm Dotted line A' showsl the dis placed positionl of the diaphragm at the peaks of the curve D in Fig. e, while dotted line B’ shows the minimum signal, or threshold, displaced posi tion at the valleys of curve D of Fig. 4. It will be seen that the diaphragm is pulled towards the-„magnetic core in response to the minimum signals flowing-through coil 53. As the signal current modulatesv therestoring element will be caused to vary-the diaphragm position. » i . used at I prefer tc employ the R'-C type of In Fig. 8 there is shown a modiñcation of the oscillator disclosed and claimed for the same dual reproducer system of Fig. '7. Here the FM function in my application Serial No. 463,349, signal current output of tube Bil-flows through 70 nled Cctcber 26, 1942. The last-named applica the-»discriminator circuit C'---,-L’. Condenser- Cf tion explains the manner in which the audio fre isl a reproducerA whose mobile or diaphragm quency- currents,_ derived from a sound record, electrode-is designated by numeral I3', and the may be used to frequency modulate the oscillatory static electrode is l0’. Coil L’ is a high imped output current of-an R-C oscillator of the-¿form ance coil _and- maybevprovided with core¿i3?4 ,_ disclosed-therein.; . .f ' ' ' - ' 2,412,015 l1 - and initially displaces diaphrag-m 54’ due to the carrier-derived polarizing voltage. Circuit C’--- 12 for circuit; L--C to handle. Hence, I utilize cas caded frequency dividers 6| and 62 to reduce L’ is tuned to the peak frequency of a resonance concurrently the mean frequency and frequency curve (Fig. 2) upon either of whose flanks may swing to more practicable values of 172 kc. and fall Fc, the mean frequency of FM signals ap 5 i3 kc. respectively. This method is generally plied to amplifier 50. Of course, the reproducers may have different, such as opposite or com in accordance with the teachings of M. G. Crosby in his U. S. Patents 2,230,231 and 2,230,232 plementary, frequency response characteristics. granted February 4, 1931. The frequency di The plate of tube 50 is connected to the +B vider 6I reduces the I. F. energy by a factor of terminal through isolation coil 6I, and direct cur 10 5, and divider 62 reduces the FM energy output rent blocking condenser 60 connects the plate of divider 6I by a factor of 5. to electrode I3'. Additionally, the condenser C' Each of dividers 6l and 62 may, if desired, be may be a normal condenser and non-reproducing, constructed in accordance with the teachings of the coil L’ and its diaphragm acting then as the G. L. Beers in application Serial No. 430,588, reproducer. 15 filed February 12, 1942, patented Aug. 22, 1944, To recapitulate the operation of the systems as U. S. Patent No. 2,356,201. Beers shows a of Figs. 7 and 8, the circuit L-C performs the locked-in oscillator whose input terminals has same as previously described. In Fig. 7 the coil FM signal energy applied thereto (I. F. stage), L forms the primary of an output transformer and in the output circuit there is produced FM 5| which matches the speaker coil 53 to the 20 signal energy Whose mean frequency and fre output of tube 50. Thus, as the voltage across quency swing have been reduced by a like factor. coil L varies with modulation there will be a Of course, any other form of frequency divider variation in voltage across coil 53. Consequently may be used at 6l and 62. It is to be clearly there will occur a change in displacement of dia understood, however, that additional frequency phragm 54. The particular advantage of the 25 division may be employed to reduce the 172 kc. system of Fig. 7 resides in the fact that dia mean frequency, or heterodyning action may be phragm 54 can reproduce low frequencies, where used for the same purpose if the deviation is not as it is diiiicult to obtain satisfactory reproduc to be affected. Further, the receiving system tion of low audio frequencies from an electro may operate in any FM transmission band. static speaker. Coil 53 could be the voice coil 30 Again, either of the reproducer systems of Figs. of a loudspeaker, and serves to displace the dia 7 and 8 may be employed in place of that shown phragm 54 initially under influence of the car in Fig. 9. The functioning of the circuit L-C rier voltage. ' in Fig. 9 will be similar to that described in In Fig. 8 the set-up is somewhat different. In 1 and 3. Circuit L-C is tuned to a fre this instance L’-C' performs normally. Coil 35 Figs. quency above or below 172 kc., and as depicted Lf is, however, magnetically coupled to dia in Fig. 2. The carrier-derived voltage functions phragm 54' so that displacement of the latter as polarizing voltage in this case as well. varies with changes in voltage across coil L’. While I have indicated and described several v'I'his has been explained in connection with Fig. systems for carrying my invention into effect, 7. Again, the purpose of diaphragm 54’ is to 40 it will be apparent to one skilled in the art that reproduce the lower audio frequencies while con my invention is by no means limited to the par denser C’ reproduces the higher audio frequen ticular organizations shown and described, but cies. It is to be clearly understood that the in that many modifications may be made without herent restoring characteristic of a springy dia departing from the scopel of my invention. phragm may be used in either Figs. 7 and 8. 45 WhatIclaimis: Hence, in Fig. 8 I omit the leaf spring restoring 1. In a system for reproducing frequency device of Fig. 7. modulated carrier waves, a resonant descrimi My invention is applicable to the reception of nator circuit including as the capacitative ele- , FM broadcast waves in the presently-assigned ment thereof an electrostatic reproducer, and 42-50 megacycle (mc.) band. Such waves have 50said discriminator circuit being tuned to a fre a considerable deviation range. For example. at quency which is substantially off-resonance with present an FM transmitter in the 42`-50 mc. band respect to the mean frequency of frequency is permitted a maximum frequency deviation up modulated waves applied to the discriminator to '.75 kilocycles (kc.) on either side of the trans circuit. mitter normal or carrier frequency. Assuming 55 2. In a method of reproducing sound records superbeterodyne reception of such FM waves at wherein angle modulation of a superaudible the receiver system shown in Fig. 9. those skilled wave is employed to represent currents corre in the art are fully aware of the fact that the sponding to sounds to be reproduced by a loud mean frequency of the received signals is re speaker; the improved step comprising the simul duced to a lower frequency value, say of the 60 taneous translation of angle modulated super order of 4.3 me., without changing the range in audible Wave energy into corresponding ampli absolute figures of the frequency swing of the tude modulated wave energy and reproduction intermediate frequency (I. F.) energy. Hence, of the modulation existing on the latter Wave I. F. amplifier El) is to b-e understood as having energy in a single discriminator circuit. its inout terminals coupled to the I. F. output 05 3. A frequency responsive network, adapted circuit ci’ a prior converter. The I. F. amplifier to have angle modulated superaudible waves ap passes FM energy whose mean frequency is 4.3 plied thereto, comprising a coil and a condenser mc.. and whose frequency swing may cover a loudspeaker, said condenser loudspeaker being rance of plus or minus 75 kc. adapted directly to convert the modulation on The discriminator-reproducer circuit L-C is 70 the aplied waves into sound, and said coil and arranged in the output circuit of the last tube condenser being tuned substantially olf-reso of the system. It is not practicable to apply nance relative to the mean frequency of applied the FM current output of I. F. amplifier 60 to modulated Waves. circuit L-C. The mean frequency of 4.3 mc. 4. In combination with a source of frequency is too high, and the i-75 kc. swing is too great 75 modulated oscillations, a. condenser reproducer, 2,412,015 - and a circuit connecting the reproducer to said source for reproduction of the modulated oscil lations; the improvement comprising said cir cuit including a coil which 4forms a tuned fre quency discriminatcr circuit with the condenser reproducer, said coil and condenser being tuned to a frequency sufficiently different .from the mean frequency of the modulated oscillations to secure frequency discrimination. the angle modulated wave energy to a resonant circuit tuned substantially off-resonance relative to said Wave frequency, and providing in said reso nant circuit the simultaneous translation of the angle modulated superaudible wave energy into corresponding amplitude modulated Wave energy and reproduction of the modulation existing on ` 5. In a system for receiving frequency modu lated carrier waves, a resonant discriminatcr circuit including as the active capacitative re actance element thereof an electrostatic repro ducer, and said discriminator circuit being tuned to a frequency which is substantially off-reso nance with respect to the mean frequency of frequency modulated carrier voltages applied to the discriminatcr circuit. 6. In a method of reproducing sound wherein angle modulation of a superaudible Wave is em 14 to the mean frequency of frequency modulated carrier waves applied thereto. 12. A method of reproducing sound records whose impressions are provided by angle modula tion o-f a superaudible wave, comprising applying the latter wave energy. 13. A frequency responsive network, adapted to 15 have angle modulated superaudible waves applied ployed to represent modulation voltages corre sponding to sounds to be reproduced; the irn provement consisting of the concurrent transla thereto, comprising a coil and a condenser, one of said latter reactive elements being Ia transducer, and said coil and condenser being tuned substan tially off-resonance relative to the mean fre quency of applied waves. ì 14. In combination with a source of angle mod ulated oscillations, a condenser reproducer, an amplifier Ycircuit connecting the reproducer to said source for reproduction of the modulated os tion of angle modulated superaudible wave en cillations, a tuned discriminator circuit consisting ergy into corresponding amplitude modulated 25 of a coil in shunt with the condenser reproducer, Wave energy and reproduction of the modula said coil and condenser being tuned to a fre quency sufficiently different from the mean fre quency of the modulated oscillations to secure dis tion existing on the latter wave energy in a simple tuned circuit off-resonance with frequency. said wave crimination. 7. A frequency responsive network comprising 15. In combination with a reactive transducer, a. superaudible oscillator, means for varying the frequency of the oscillator in accordance with the transducer output energy, a discriminatcr circuit substantially off-tune with the mean frequency of a coil and a condenser, at least one of the two reactances being adapted to convert modulation voltages into sound, and said coil and condenser being substantially off-resonance relative to a desired mean frequency of frequency modulated the oscillations, and the capacitative component carrier waves adapted to be applied to said network. 8. In combination with a source of angle modu lated oscillations, a condenser reproducer, a circuit coupled to said source and including a coil which forms a tuned discriminatcr circuit with the condenser reproducer, said coil and con denser being tuned to a frequency substantially diñ'erent from the mean frequency of the modu of the discríminator circuit being an electrostatic reproducer, 16. A frequency discriminatcr circuit consisting of two reactive components of opposite reactive sign related in magnitude to tune to a predeter mined peak frequency, and at least one of the reactive components being a transducer. 17. A discriminatcr circuit including as an ele ' ment thereof an electrostatic reproducer, and lated oscillations to secure discrimination. 9. In combination with a reactive transducer, a superaudible oscillator, means for varying the frequency of the oscillator in accordance with the transducer output energy, means for repro ducing the frequency modulated oscillations without rectification, said means comprising a said discriminatcr circuit being tuned to a fre quency which is adapted to be substantially cif resonance with respect to the mean frequency of modulated carrier waves applied to the circuit. 18. In combination with a source of angle mod ulated oscillations, a condenser reproducer, a cir cuit connecting the reproducer to said source for discriminatcr circuit substantially off-tune with reproduction of the modulated oscillations, said the mean frequency of the oscillations, and the circuit including a coilV which forms a tuned dis capacitative component of the discriminatcr cir criminator circuit with the condenser reproducer, means for polarizing said condenser with voltage derived from the oscillations, said coil and con denser being tuned to a frequency substantially cuit being an electrostatic reproducer, 10. In combination in a receiver of frequency modulated carrier waves having a. high mean fre quency and a relatively high frequency deviation different from the mean frequency of the modu ratio, and at least one frequency divider for re ducing the said mean frequency and deviation ratio by a like factor; the improvement which in cludes means for reproducing the divided Wave energy without rectification, said last means com prising a tuned discriminatcr circuit composed of at least two reactive elements of opposite sign, at least one of said reactive elements being a modu lation reproducer. 11. In a system for reproducing frequency mod ulated carrier Waves derived from sound records, (il) lated oscillations to secure discrimination. 19. In combination with a reactive transducer, a superaudible oscillatc-r, means for varying the frequency of the oscillator` in accordance with the transducer output energy, means for repro ducing the frequency modulated oscillations with out rectification, said means comprising a dis criminatcr circuit substantially off -tune with the mean frequency of the oscillations, ther capacita tive component of the discriminatcr circuit being an electrostatic reproducer, and means ïresponsive a discriminatcr circuit including as a resonating 70 to voltage derived from said frequency modulated reactive element thereof a transducer, and said discriminatcr circuit being tuned to a. frequency which is substantially off-resonance with respect oscillations for polarizing said electrostatic re producer. CHESTER M. SINNETT.