US2371373A - Balanced frequency modulation system - Google Patents

Description

March i3, 1945. A,` BADMAIEFF 2,371,373

BALANCED FREQUENCY MODULATION SYSTEM Filed June 12, 1945- 4 sheets-sheet 1 1N V EN TOR. A2 vr/L5 5404/4/59? BY I A TTOR NE Y March 13, 194. A. BADMAIEFF 2,373,373

, BALANCED FREQUENCY MODULATION SYSTEM Filed June 12, 1943 4 Sheets-Sheet 3 IN VEN TOR. L E z/J 40AM/5F@ A TTOR NF V Max-ch 13, i945. A. BADMAIEFF 2,371,373

BALANCED FREQUENCY MODULATION SYSTEM Filed June 12, 194s 4 sheets-sheet 4 2 3 4567.891 2 3 4567894 Z 3 ooo wooo zooao Z 3 4 5 6 7 9 20 foo /ifauEA/cy /N cru es Peyssc ATTORNEY Patented Mar.' 13, 1945 UNITED STATES PATENT OFFICE BALAN CED FREQUENCY MODULATION SYSTEM Alexis Badmaie', Indianapolis, Ind., assignor to Radio Corporation of America, al corporation of Delaware Application June 12, 1943, Serial No. 490,614

14 Claims.

criminator are usually a short distance apart, or

even combined in one unit. Such systems have been applied to sound reproducing devices, and to measuring devices. In such prior systems an oscillator is provided with a frequency changing transducer, and the transducer is varied in response to a vibratory member thereby to frequency modulate the oscillator mean frequency.

The frequency modulated energy is then fed to a discriminator circuit, tuned to a slightly different frequency so that the oscillator mean frequency falls on one side of the discriminators resonance curve. The discriminator converts the frequencyvariable wave energy into lamplitude-variable wave energy. Subsequent rectification derives the original modulating frequencies from the amplitude-variable wave energy.

I have invented a method of greatly increasing the voltage output of such a system. The method is simple, and employs a minimum of elements. The method essentially comprises the concurrent variation, from one modulating source, of the oscillator frequency and the discriminator resonance peak in opposite directions.

From a generic viewpoint, it may be stated that it is one of the principal objects of my invention to provide a method wherein a properly constructed balanced transducer is employed to vary the oscillator and discrimlnator circuit tuning in opposite directions, or in opposite phase relation, simultaneously.

It is an important object of my present invention to provide a frequency modulated ("FM for brevity hereinafter) oscillator feeding into a discriminator whose resonance peak is subjected to concurrent FM action, the two frequency modulations being 180 degrees out' of phase whereby the discriminator voltage output will be twice as great as would be the case if only one of said FM actions had been employed.

The two forms of FM may be provided simply in an oscillator-discriminator network by employing a balanced transducer, more specifically a pushpull condenser, as the modulating device. One side of the condenser can then be connected across Vthe oscillator coil, while the other side is connected across the discriminator coil. Hence, if

the grounded center plate is moved either way, the direction of the frequency changes of the oscillator and discriminator circuits will be opposite.

Another important object of this invention is, therefore, to provide a novel FM circuit utilizing a symmetrical modulating device thereby providing cancellation of even harmonics, greatly reducing distortion, and doubling the output voltage from the discriminator.

The present invention has particular utility in an FM system for reproducing recorded sound waves. IOne such FM system has been disclosed, for example, by C. M. Sinnett in application Serial No. 459,375, led September 23, 1942. The particular Sinnett system is a single-ended frequency modulation network employing a capacity pickup device of the ribbon type for the reproduction of sound recordings. In the Sinnett system the frequency modulation network consists of an electron discharge tube which has one section thereof functioning as a high frequency oscillator of constant amplitude whose frequency is deviated by variations in the capacity of the pickup. In the plate circuit of the tube there are developed frequency modulated oscillations which are subsequently converted into audio modulation signals,

corresponding to the originally recorded audio signals, by a frequency discrimination process.

By applying the generic method of my invention to the particular ySinnett system, or in general to any FM system of record reproduction, various advantages are secured. The advantages in my system result in reduction of the mass of the vibratory pickup system; reduction of even harmonics; reduction of wear on the record surface; and an increase in the amplitude of the audio output voltage. I have found that these advantages may be realized by employing an additional capacitative variation across the discriminator circuit. 'I'he additional capacity variation is effected by the same capacitative vibratory system which is utilized across the oscillator tank circuit. Furthermore, the dual capacity-variations may be so correlated that the changes in the oscillatory frequency and the discrlminator circuit resonance occur degrees out of phase, whereby a push-pull frequency deviation eect is secured. Cancellation of even harmonics, reductioof inter-modulation and higher audio voltage ou ut are immediately realized by such a system. The higher audio voltageoutput can be employed to advantage, because the size of the pickup condenser plates can be reduced.

Accordingly, it may be stated that it is another object of my present invention to provide an FM "-network for with-asimple speciallyfdesigned f capacity pickup device,l which is'adapted to rev produce lateral or vertically cutrecords, and there being provided "a frequency modulated` signal whose frequency deviation is the cumulative. effect of oscillator and discriminator frequency variation in push-pull relation whereby the various aforementionedadvantages may be secured.

In a pickup it is most important to keep the mass of the vibratory system as low as possible. 'I'his raises the frequency of the peak and reduces the mechanical impedance, thereby permittinga. reduction of needle pressure and the accompanying reduction of surface noise.

Accordingly, it may be stated that it is another important object of my invention to provide al capacity pickup device wherein the stylus supporting wire acts as the center plate of a pushpull condenser working into a balanced, or pushpull, FM oscillator-discriminator network whereby the mass of the pickup is reduced to a. minimum, since nothing else` would be mechanically coupled to the stylus except its own support; and the supporting wire being small in diameter so that no extra mass would be added in transforming the mechanical vibrations to electrical frequencies. l Still otherobjects of my invention are to im prove generally the simplicity and emciency of record reproducing equipment of the FM type, and more especiallyv to provide a high delity, low cost and low distortion record reproducer system which is easily manufactured .and readily assembled. s f

The novel features which I believe to be characteristic of my invention are `set forth with particularity in the appended claims; the inven tion itself, however, as to both its'organization and method of operation will best be understood by reference to the following description, taken l a cathode tap on the grid coil may be-employed.

in connection with the drawing, in which I have indicated diagrammaticallv several circuit organizations whereby my invention may be carried into effect. Y

In the drawings:

Fig. 1 shows a generalized circuit embodying the invention,

Figs. la and lb schematically show modifications,

Fig. 2 graphically illustrates the FM action at the oscillator circuit, f

What is essential is that tube l be provided with v an oscillation-producing circuit to produce highA frequency oscillations of a certain mean frequency. g f

The oscillatory energy isv applied to the reso nant input circuit 5 of a second tube 4. vThe circuit 5 is slightly off-resonance relative tothe mean frequency of the oscillator. VThat is to say, the oscillator may be operated at a frequency located on somewhat either side of the resonance peak of circuit 5. This provides the Well known discriminator action at circuit 5. Tube 4 may be operated as a grid leak detector, or as a plate circuit-detector. For purely illustrative purposes, the leaky grid resistor 6 is shown in the grid circuit of tube 4. There is provided grid circuit rectification of any amplitude-variable energy developed across the discriminator circuit 5, The detector output may be utilized in any desired manner.

In the past it has been proposed to vary the frequency of the tuned circuit 2 by any form of periodically-variable y reactance device. For example, a variable condenser shunted across circuit 2 has been employed to provide a variation in frequencyi. e., a frequency modulation of the oscillatory energy. This may be accomplished, for instance, by having a vibratory member respond to a modulating source and vary the condenser capacity. The frequency-variable oscillations are applied to the off-tune circuit 5 l This aforedescribed action exists regardless of Whether the modulating condenser is part of a u microphone, `a stylus, a cutter, or a measuring Fig. 3 graphically illustrates the FM action at the discriminator, v

Fig. 4 illustrates the conjoint FM action at the oscillator and discriminator circuits,

Fig. 5 shows the circuit details of an FM reproducer of phonograph recordings 'embodying the invention,

Fig. 5a shows the circuit diagram of a preferred embodiment of the invention,

Fig. 6 shows an end view of the pickup;

Fig. 7 vshows a bottom view of the pickup;

Fig. 8 shows a detail of the stylus support wire;

Fig, 9 is a section along line 9--8 of Fig. 8;

Fig. 10 shows acoustic response curves of the present invention. i

Referring now to the accompanying drawings, wherein like reference characters in the diiferent iigures denote similar circuit elements, there is shown in Fig. 1 a generalized circuit arrangement embodying my present invention. The numeral l designates any type of electron discharge tube whose grid circuit 2 is tuned to a predetermined oscillatory frequency. The tube has a tickler coil 3 in the plate circuit to produce oscilinstrument. The general process of translating la. modulating signal into an FM oscillation, conversioninto an amplitude modulated (AM for brevity) oscillation, and re-translation into the original modulating signal is common to all the applications. Considering the operation more specifically, and referring to Fig. 2, there can be graphically depicted the relation between detector output voltage and FM oscillatory energy applied to the discriminator. In Fig. 2 the full line curve represents the discriminator resonance curve whose peak frequency is higher than the mean, or center, frequency Fc of the FM oscillations. By virtue of the slope of the side of the resonance curve, the frequency swings of the oscillations are converted into Variations in am plitude of the detector output voltage. put voltage amplitude will depend on the modulation of the FM energy.

Assume, now, that the variable condenser across the oscillator circuit 2 has been replaced by a fixed condenser of a value equal to the former condenser when not modulated, so as to produce a frequency equal to the former center frequency This discrimination action is very` The Out- `2,a71,s7s again graphically represented in Fig. 3, where the resonant peak of the discriminator will move back and forth along the horizontal frequency axis; andy the oscillator frequency remains xed. In the course of these illustrations, it is assumed that the L vto C ratios in both the oscillator circuit 2 and the discriminator circuit 5 are very nearly the same.

If we now combine both modulation actions, respectively depicted in Figs. 2 and 3, in one system, and provide modulation from a single modulating source there is secured a balanced, or pushpull, FM action. In Fig. 1 I have shown a simple method of securing such balanced FM action. Al double condenser is provided with its center plate 1 mobile and grounded. One xed plate 8 is conf nected to the high potential side ol oscillator circuit 2, while the opposite xed plate 9 is connected to the high potential side of discriminator circuit 5. Motion of plate 'I with respect to plates 8 and 9 will now result in a shifting of the oscillator frequency in the opposite direction from the discriminator resonance peak frequency.

Since the two frequency shifts, or modulations, are always 180 degrees out of phase, the voltage output from the detector will be twice as great as would be the case if only one of the frequency shifts had been provided. This is clearly depicted in Fig. 4, wherein there is shown the combination of the FM actions of Figs. 2 and 3. It will be noted that the detector output voltage in the case of Fig. 4 is double that of either Fig. 2 or Fig. 3. When the oscillator frequency is moved down along the frequency axis, and at the same time the discriminator resonance peak is moved up, the relative shift along the frequency axis is doubled thereby producing a double amplitude in the voltage output of the detector.

The frequency changing device', or balanced transducer, of Fig. 1 need not be of the capacitative type. In Fig. la I have shown how a variable inductance can be used for the Afrequency modulation process. More specifically, in Fig. 1a the modulating device is a shorted loop which is adapted to move between the oscillator and discriminator tuned circuits. The shorted loop acts as a modulator and inductive shield. In this way, the respective frequencies of the oscillator and discriminator are varied concurrently and in opposite senses. It will be understood that the shorted loop will be provided with a pivoted stylus, and the arrows adjacent the stylus and loop indicate the directions of motion of these elements.

In Fig. lb there is shown still another manner of concurrently Varying the oscillator and cliscriminator frequencies in opposite senses. In this case the iron cores of the oscillator and discriminator coils are actuated by a pivoted stylus to adjust the respective frequencies. Those skilled in the art are fully aware of the manner of constructing an electron discharge tube circuit so that it can function as an electronic reactance device. If an electronic transducer is required, a pair of electronic reactance tubes can be connected across the respective oscillator and discriminator tuned circuits for simulating the effects shown in Figs. 1, la or 1b.

It will, therefore, be'appreciated that my invention contemplates a generic method of varying the tuning of an oscillator and discriminator by a balanced transducer of appropriate construction, the tuning Variation being concurrent and in opposite phase. By the term balanced in this application'is meant any concurrent variation of two effects in opposite phase relation.

There. are man'y advantages which result from the utilization of this balanced FM action when `applied to a balanced capacity pickup device. In

the first Place, it is only necessary to employ a simple push-pull condenser. One side of the latter is connected across the oscillator coil, while the other `side of the condenser is connected across the-discriminator circuit coil. In that case if the center grounded plate is moved either way, the directid of the frequency changes'of the oscillator an discriminator circuits will be opposite. The advari age of such a system is primarily the cancellatio of even harmonics. When one condenser plate is moved away from the other, further movement'has less effect on its capacity because the capacity varies inversely as the distance of separation. This results in a non-linear relation between capacity and displacement with resulting distortion. By combining voltages from two similar non-linear elements in a balanced system there is secured a great reduction in distortion with practical elimination of the rectifi- .cation and even harmonic components of the distortion.

Another advantage of the balanced FM action is that the amplitude of the output voltage at the detector is doubled for a given amount of capacity change. In other Words, for a given amount of mechanical deflection of the grounded center condenser plate there is secured double the detector output voltage amplitude. 4If the output voltage amplitude is kept the same, the capacity increv ment can be half as much when using the FM balanced action. 'I'his has a practical result insofar as the mechanical construction of the modulating condenser is concerned. The condenser plates can be made smaller, and, in turn, it is not necessary to attach any plate to the wire which carries the stylus, assuming that the system is to be used in the reproduction of phonograph records. In the latter case there is not only secured considerable mechanical simplification, but the' mass of the vibratory system is reduced thereby lowering its mechanical impedance.

While in Fig. 1 I have shown a simple form of oscillator followed by ja discriminator-detector circuit, in Fig. 5 there is shown my generic method applied to the particular type of single-ended FM circuit referred to hereinbefore as the Sinnett system. The latter employs an'electron coupled v form of oscillator-discriminator circuit. It is again emphasized that my invention may be applied to any other type of FM record reproducer. Since the particular circuit of Fig. 5 in its singleended formhas been disclosed and claimed in the aforementioned Sinnett application, it is not believed necessary to describe the circuit details specifically, reference being made to Fig. 1 of the Sinnett application for a detailed disclosure of the circuit.

It is believed sufficient for the purposes of this application to explain generally the nature of the circuit, and the specific manner in which my present invention is advantageously applied thereto. The tube I0 in Fig. 5 is shown as of the pentode type. Its cathode is connected to a low potential tap on the oscillator coil II. The coil II is preferably of the variable inductance type, and hasan adjustableA core of comminuted iron. The control grid I2, which functions as the oscillator section grid, is connected through the direct current blocking condenser I3 to the high potential side of coil II and to one fixed plate I4 of ond fixed plate of the balanced condenser pickup is designated by numeral I3. Q

The mobile element IB of the pickup device is connected by lead I'I to the negative terminal, which may be ground, of the direct current energizing source. The positive terminal of the source may be at +300 volts. The positive terminal is connected through series resistors Il and I 3 to the electrode 20 of tube Ill. Electrode 20 functions as the oscillator anode of the tube. The output plate 2| has one connection through the direct current blocking condenser 22 to the fixed plate II of the balanced capacity pickup device. A second conductor connects plate 2| to the high potential end of the discriminator coil 23 in accordance with my invention.- The coil 23 is, also, a variable inductance having a core of comminuted iron, and the low potential end of coil 23 is connected to the junction of resistors I9 and I8. The third grid of tube lll may be connected back to lead II, and proper bypass asf/11,1178

discriminator circuit. Hence, the subsequent detector, or rectier, will have an input circuit whose operating point is along a linear portio "of the discriminator resonance curve. 1 Y Rectification of the resulting AM oscillatory energyis'effected bya simple diode circuit.' If

the 6R? type. 'The numeral 30 designates such-a tube. The latter comprises a cathode 3|,a plate 32, a control grid 33 and a diode auxiliary anode 3|. Thel anode 34 and cathode 3i function as the rectifier diode. The anode 34 isconnected to its `cathode 3| through a path comprising coil 35 and resistor 3B. Coil 35 is a filter choke to precondensers areemployed for the energizing leads feeding the anode and plate 2 I.

The cathode, grid I2 and anode electrode 2li of tube I0 provide the local voscillator section of the system. Coil II is resonated to the mean, or center, frequency Fc. This mean frequency is to be deviated in accordance with the amplitude of the modulation frequencies. The mean frequency of the oscillator may be chosen from a range of 30 to 60 megacycles (ma). The deviation range of the mean frequency is dependent on the capacity change and the mean frequency employed. A deviation up to about 1% of the mean frequency could be employed, but it should be noted that this total deviation is the sum of the oscillator deviation and the discriminator resonance peak deviation. The choice of the mean frequency of the oscillator is dependent on the sensitivity that is required from the pickup device. Since the capacity change, due to vibration, is of constant range as compared to the amount of deflection, then the higher the frequency the more this given capacity change will produce a frequency swing. Therefore, the mean frequency should be as high as possible, and yet not too high to be unstable, My invention is entirely independent of specific frequencies disclosed herein.

Of course, the balanced pickup device is schematically represented in Fig. 5. However, it will be understood that in accordance with my generic method the condenser I4-i5 is connected across the oscillator coil II, While the condenser IE-IG is shunted across the discriminator coil 23. The discriminator coil should` be slightly off-tune from the oscillator mean frequency, either above or below. For example, the oscillator mean frequency, as shown in Fig. 4, may be below the olis-v criminator peak resonance frequency. The amount of off-tune vdepends on the"Q of the discriminator resonance, which is the slope of the two sides of the peak. The oscillator mean frequency should be tuned to the middle portion of a linear part of the discriminator curve side. The maximum deviation could be of the order of 120 kilocycles (kc.) on either side of Fc. The generic expression angle modulation is used herein to designate that the modulation of the high frequency oscillations may either be frequency, phase or hybrids thereof. Preferably,it is desirable to adjust the iron core of coil 23 so that the peak frequency of the discriminator circuit is of a value such that the mean oscillator frequency Fc falls at approximately 70% of the response on one side of the resonance curve characteristic of the vent the radio frequency components from reaching the grid 33 of tube 30. The modulation voltage is developed across lresistor 36, and is ap-` plied to the control grid 33 through the audio coupling condenser 31. The control grid 33 is connected back to the lower end of the cathode bias resistor 40, properly bypassed for carrier currents, through the return resistor 4I.

The plate 32 of tube 3D is connected through the output resistor 50'to the positive terminal of the voltage supplyvsource. The condenser 5I couples the diode anode 34 to the high potential side of the discriminator coil 23. The audio modulation voltageI developed across resistor 36, is aml pliiied in the triode section of tube 30. The modulation voltagedeveloped across the plate load resistor 50 is transmitted by the audio coupling condenser 50 to any utilization circuit. For example, when reproducing a phonograph record an audio reproducer may be coupled to condenser 50', or one or more audio ampliers may be interposed between the condenser '50' and the audio reproducer.

The system shown in Fig. 5 embodying my generic method utilizes commercially available tubes, and possesses high fidelity. Further economy of tubes may be realized, and other advantages secured, by employing my preferred circuit of Fig. 5a. In this circuit the circuit components corresponding to similar components of Fig. 5 are distinguished by the prime designation. The cost of manufacture of a system as shown in Fig. 5a would be many times less than that of the conventional record reproducers` able to approach the high fidelity of reproduction secured by my present method. Considering the system of Fig. 5a, the tube I0' functions to provide the electron coupled oscillator, discriminator and detector. The tube may be one of the GSF? type, or any other type of tube which will provide a. cathode, oscillator grid I2', oscillator anode 20', suppressor 20", plate 2| and an Yauxiliary anode 34'. The auxiliary anode 34' cooperates with the cathode y of tube I0' to provide a diode rectification device similar to diode 34-3I of Fig. 5. The suppressor 20" acts as an electrostatic shield between the plate 2| and the other elements.

The cathode of tube Ill' in Fig. 5a is grounded, instead of floating as in Fig. 5. Furthermore, the screen grid 20 functions as an oscillator plate, and feeds into the lower part of the coil II'. By way of illustration only, the upper section of coil i I' may have 5 turns while the lower section has 2 turns. The discriminator coil 23' may have 5 turns. These are purely illustrative values, and do not affect the scope of my invention. Condenser C is the coupling condenser from the oscillator anode 20 to the oscillator tank coil II. The plate 2l' of tube I0' has the FM oscillations developed in its circuit, because of the electron coupling between the latter and the oscillator seccomponents from being presentrin the modulation signal output circuit. In the case of record reproduction the modulating signal is of audio frequency. Resistor 36' is the detector load resistor. The audio output energy may be amplied, and then reproduced.

The systems shown in Figs. and 5a owe their improvement to the generic balanced action which forms the basis of my presently-disclosed method, and which makes it possible to keep the mass of the vibratory system as low as possible. In a pickup it is most important to keep the vibratory system mass as low as possible. This raises the frequency of the peak, and reduces the mechanical impedance thereby permitting a reduction of needle pressure and theaccompanyingl reduction of surface noise. The balanced pickup employed in the presentl invention has the stylus supporting wire act as the center plate I5 of the push-pull condenser which actuates the balanced FM network. This results in a reduction of the pick-up mass to a minimum, since nothing else would be mechanically coupled to the stylus except its own support. Ihis supporting wire itself is very small in diameter, so that no extra mass would be added in transforming the mechanical vibrations into electrical frequencies.

Figs. 6, '1, 8 and 9 show the constructional details of the novel balanced pickup device. Considering Fig. 6, a sapphire stylus Bn is mounted at the forward end of a steel wire 6I. The wire 6 I, which is the supporting wire mentioned above, is bent into the shape shown in detail in Fig. 8. The upper end of the supporting wire 6I is mounted on a small brass block 62. This assembly of the stylus, supporting wire and brass block is shown in detail in Fig. 8. The stylus supporting wire is ground flat along its vertical plane on both sides and along its length to about 0.008 inch, and forms the center grounded plate electrode I5 of the balanced'condenser I4-I5-I6.

In Fig. 9 there is shown the cross-sectional appearance of the supporting wire 6I. Merely by way of illustration, it is pointed out that the vertical section of the supporting wire 6| in Fig. 8 may have a dimension ofv0.09 inch; the distance between the bend in the wire and the sapphire point may be equal to 0.13 inch, while the sapphire point may have a radius of 0.0028 inch. In Fig. 9 the long dimension may be 0.014 inch, while the short dimension may be 0.008 inch. A fixed condenser plate has been removed in Fig. 6 from in front of tbe' supporting wire 6I.

The assembly which has been described to this point isthen mounted on th front of a Bakelite block B3 by means of a screw member 64. Two xed condenser plates 65 and 66, both are shown in Fg. 7 while only plate 65 is shown in Fig. 6, are then mounted in the Bakelite block so as to be parallel to, and on each side of, the stylus support wire 6I. Each of the xed condenser plates is preferably spaced a distance of about 0.002 in'ch. or 0.003 inch, from the stylus support wire 6I. These two plates, 65 and 66, form the two outside plates of the balanced condenser,.

and correspond to plates I4 and I6 of Fig.' 5.

The addition of a small Viscoloid strip 61,

which is mounted between the stylus 60 and the mounting screw 64, completes the pickup assemtorsional resonance of the stylus support wire. 'I'he pickup assembly is secured to the tone arm 10 by means of the two spaced spring plates 80 and thatl permit it to be bent in a vertical.. t

direction. The purpose of this so-called compliance is to prevent damage to the pickup head when it is accidentally dropped on a record. 'I'he arm 10 consists of a conventional U-shaped channel, but has a supporting shield 9i running lengthwise and forming two separate channels.

The two leads from the pickup head, which may be regarded as radio frequency transmission lines,

are placed inside of the two corresponding chanconnected to the discriminator coil. The stylus support wire 6I is connected to the arm proper, and, in turn, to the chassis which is grounded.

In Fig. 10 there is shown a response curve secured with the present invention. The needle pressure required for proper trackingon the loudest passages was found to be only 0.2 ounce. In actual testing, however, and for a wider safety margin 0.33 ounce was used. The resonance frequency was found to be at 21,000 cycles, having a 10 decibel dip at 16,000 cycles due to the stylus support Wire developing a node at its middle. The rest of the response curve was fiat within one decibel to 65 cycles as shown by curve .A in Fig. 10. Curve B in Fig. 10 shows the response when the "Viscoloid damping member was used. The damping reduced the peak and dip, and also prevented the torsional resonance of the wire. 'I'he response in the useful frequency range, however, was unaltered, but otherwise stabilized the performance of the pickup. Curve C shows the response when a so-called "Orthacoustic network is inserted in the audio amplifier network of Fig. 5 or Fig. 5a. On actual hiss measurements it was found that hiss was very low, and about 7 decibels lower than with other types of commercial pickups which have been considered very satisfactory heretofore. p

The present balanced FM circuit has other anplications. For example, it may be used in connection with a record cutter.. Again, the present invention may be employed in connection with a push-pull form of condenser microphone. In that case the diaphragm would be the center plate of the push-pull condenser with the usual plate at the back of it. In front of the diaphragm would be placed another plate, but perforated with conically-shaped holes to allow the sound to pass through and deflect the diaphragm. The remainder of the system would be very similar to that shown in either of Figs. l, 5 or 5a. In general, the present invention is applicable to measurement devices where linearity and small pickup area are essential. Such uses would include pressure devices, liquid height measuring devices, elongation measurement, vibration measurement and the like. .When the pickup is a microphone, the system could be used in hearingaid apparatus, public address systems, 'binaural systems, and the like.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular circuits shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a tube provided with an oscillator section and an output electrode, means connected to said oscillator section to provide angle modulated oscillations which have a predetermined mean frequency, a resonant circuit coupled to said output electrode, means providing coupling between said oscillator section and said output electrode for developing said angle modulated oscillations in the circuit connected to said output electrode, said resonant circuit being tuned to a frequency sufficiently different from said mean frequency to permit the resonant circuit to function as a discriminator circuit, means mechanically responsive to operation of said firstnamed means for providing angle modulated oscillations at the discriminator circuit independently of said coupling, and means coupled to the discriminator resonant circuit for rectifying resultant amplitude modulated wave energy appearing across the discriminator circuit.

2. In combination with a capacity pickup device for phonograph records, an oscillator having a resonant circuit tuned to a predetermined mean frequency, a detector having a resonant input circuit coupled to said oscillator, said detector input circuit being tuned to a peak frequency which differs from the said mean frequency by a predetermined frequency value whereby the detector input circuit functions as a discriminator circuit, and said capacity pickup device consisting of a rst condenser connected across said oscillator resonant circuit and a second condenser connected across said discriminator circuit, said two condensers having a common mobile element for concurrently but oppositely varying the capacities of the two condensers. l

3. A method which includes producing oscillations of a predetermined frequency, transmitting said oscillations to a network whose frequency differs from said oscillation frequency by a predetermined frequency value, deviating said oscillations in frequency relative to said first-mentioned predetermined frequency in accordance with signals, concurrently deviating the frequency of said network with respect to its frequency, maintaining said frequency deviations in 180 degree phase relationship, and utilizing the resultant of the two frequency deviations,

. 4. In combination with an oscillator of a predetermined high frequency provided with a resonant circuit, a source of modulation signals, a discriminator circuit coupled to the oscillator, said discriminator circuit having a resonance curve Whose peak frequency is of a predetermined value different from the oscillator frequency, said oscillator frequency being located at a substantially linear portion of a ank of the resonance curve, a first capacity device provided across the oscillator resonant circuit, a second capacity device provided across said discriminator circuit, and said source of modulation signals being connected to said two capacity devices for varying the magnitudes thereof in opposite directions thereby to provide balanced frequency modulation.

5. In combination with a transducer device, an

-mean frequency by a predetermined frequency value whereby the detector input circuit functions as a discriminator circuit, and said transducer device consisting of a rst frequency changing component of said oscillator resonant circuit and a second frequency changing com- Ponent of said discriminator circuit, said two components having a common mobile element for concurrently but oppositely varying the values of said components.

6. In a frequency modulation system, a pair of tuned circuits normally tuned to different frequencies separated by a predetermined frequency value, and means for varying the frequencies of said circuits, respectively, simultaneously in opposite directions in accordance with a modulation signal.

7. In a frequency modulation system, a pair of tuned circuits, said tuned circuits being normally tuned to different frequencies spaced apart by a predetermined frequency value, and means for varying the frequencies of said circuits, respectively, simultaneously in opposite directions in accordance with a modulation signal, said varying means being a transducer.

8. In a frequency modulation system, an oscillator including a rst tuned circuit, a discriminator coupled to said oscillator andincluding a second tuned circuit, means for varying the frequencies of said circuits, respectively, in opposed phase relation in accordance with a signal.

9. In a frequency modulation system, an oscillator including a first tuned circuit, a discriminator coupled to said oscillator and including a second tuned circuit, means for varying the frequencies of said circuits, respectively, in opposed phase relation in accordance with a signal, said means being a transducer comprising a frequency changing component for each of said circuits, and a vibratile element common to said components.

10. In a frequency modulation system, an oscillator including a rst tuned circuit, a discriminator coupled to said oscillator and including a second tuned circuit, a transducer having a pair of frequency changing elements for said circuits, respectively, disposed in a plane, a vibratile element commento said pair of elements and supported for lateral movement relative to said pair of elements for varying the frequency of said circuits in opposed phase relation, and said vibratile element being constructed and arranged to that transverse movement of said element normal to said plane effectively cancels out the effect of said transverse movement.

11. In combination, a tube provided with an oscillator section and a positive output electrode, transducer means connected to said oscillator section to provide frequency modulated oscillations which have a predetermined mean frequency, a resonant circuit coupled to said output electrode, electron coupling between said oscillator section and said output electrode developing said frequency modulated oscillations in the circuit connected to said output electrode, said resonant circuit being tuned to a frequency sumciently different from said mean frequency to permit the resonant circuit to function as a discriminator circuit, a second transducer means responsive to operation of said first-named means for providing frequency modulated oscillations at the discriminator circuit independently of said electron coupling, and a diode, provided byl electrodes of said -tube, coupled to the discriminator resonant circuit for rectifying the wave energy appearing across the discriminator circuit whichis amplitude modulated.

12. In combination with a pickup device for phonograph records, an oscillator having a resonant circuit tuned to a predetermined mean frequency, a detector having a resonant input circuit coupled to said oscillator, said detect@ input circuit bein-g tuned to a peak frequency which differs from the said mean frequency by a predetermined frequency value whereby the detector input circuit functions as a discriminator circuit, and said pickup device consisting of a rst reactance connected across said oscillator resonant circuit and a second reactance connected across said discriminator circuit, said two reactances having a common mobile element for concurrently, but oppositely, varying the magnitudes of the two reactances.

13. A method which includes producing oscillations of a predetermined frequency. transmitting said oscillations to a network whose frequency differs from said oscillation frequency by a predetermined frequency value, deviating said oscillations in frequency relative to said predetermined frequency in accordance with modulating signals, concurrently deviating the frequency of said network with respect to its frequency, maintaining said frequency deviations in 180 degree phase relationship, subjecting the resultant of the two frequency deviations to frequency discrimination, and rectifying the resulting amplitude-variable energy.

14. In combination with an oscillator of a predetermined highlfrequency provided with a resonant circuit, a source of audio modulation signals, a discriminator circuit coupledt the oscillator,

Asaid discriminator circuit having a resonance m8 BADMAIEFF.

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