US2361634A - Record reproduction circuit - Google Patents

Description

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RECORD REPRODUCT ION C IRCUI T Filed Oct. 6, 1942 2 Sl'leeLs-Sl'xeerI l l 1 //9 T Hl ./0 .1

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RECORD REPRODUCTI ON CIRCUIT Filed Oct. 6, 1942 2 Sheets-Sheet 2 1 ZL HM @5c/LLAMA 704mm -frfc/a/F C/Aca/T 1 f/EL. CQA/MQL 20V ATTORNEY Patented Oct. 31, 1944 RECORD REPRODUCTION CIRCUIT Winiield R. Koch, Haddoniield, N. J., assigner to vRadio Corporation of America, a corporation o! Delaware Application october s, 1942, serial no. 460,940

9 claims. (ol. 11s-100.4)

My present invention relates generally to phonograph record reproduction circuits, and more especially to circuits of the general type utilizing a frequency modulation oscillator having a ca pacity pickup.

C. M. Sinnett has disclosed in his application Serial No. 459,375, filed Sept. 23, 1942, a system I is not shown, and the latter is schematically represented as consisting of `a pair of spaced metal plates. However, it should be clearly understood that the pickup device I may be replaced by any variable reactance device. such as a microphone or pressure-responsive element. The stylus plate (usually a ribbon) is connected to the grounded for reproducing phonograph records with high delity and high gain, but with minimum production cost. In general, the system comprises a capacity pickup device capable of converting recorded sound waves into frequency modulated high frequency waves, a discriminator-rectifier network being used to derive from the modulated waves the original audio signals which had been recorded. The system has other uses. Microphones and pressure responsive pickups can be employed as the input devices.

One of the main objects of my invention is to provide various improved circuits of the same general type, but wherein sensitivity, fidelity and economy are extended to a further degree.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 shows an embodiment of the invention, wherein the modulated wave energy across the discriminator is greatly increased,

Fig. 2 shows a modification wherein an amplifier of the modulated waves is concurrently used as a detector, I

Fig. 3 is a modification of the circuit of Fig. 2, the oscillator functioning simultaneously as a rectifier,

Fig. 4 is an embodiment wherein the detector is of the multi-grid type,

Fig. 5 is a variation of the circuit of Fig. 4, the oscillator acting as a detector.

Fig. 6 is an embodiment of the system, wherein dynamic range control is employed.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, it will be understood that the systems disclosed herein are generally of the type described and claimed in the aforesaid Sinnett application. The record to be associated with the stylus of, capacity pickup sheath' of the high impedance coaxial cable 2. The immobile plate is connected to the inner conductor; the latter is connected to the high alternating potential side of the oscillator tank circuit.

The oscillator tube 3 may be a 6SM type tube whose cathode is returned to an intermediate point on the adjustable inductance coil 4. The latter is schematically represented as being of the powdered-ironcore type; condenser 5 shunts the coil to resonate the same to the mean, or center, frequency of the oscillator. In general, the pickup I and capacity of .cable 2 and condenser 5 provide the normal resonating capacity for the coil l. The first grid 6 is connected back to the high alternating potential side of coil 4 by the usual blocking condenser 1, and grid return resistor l connects the grid 6 to ground. The second, third and fourth grids are tied together to function as an effective single electrode, and are supplied with approximately volts thereby providing a positive oscillator anode which is designated by numeral 9.

Circuit 5 4 has a normal or mean frequency which is varied by changes in capacitance of the pickup. The capacitance changes occur by virtue of motion of the stylus produced by the riding of the latter through the record grooves. As a result of the capacitance variations of pickup I, the frequency of the oscillator tank circuit will be varied. The frequency deviation` relative to the mean frequency may extend to any desired frequency value. For example, an overall frequency deviation of 30 kilocycles (kc.) may be employed. The mean frequency can be of the order of 30 megacycles (mc.). These are purely illustrative Values.

The frequency modulated (FM hereinafter for brevity) high frequency oscillations appear in the circuit connected to plate l0 by virtue of electron coupling between the oscillator section of tube 3 and plate I0. The electron coupling phenomenon is well known, and it is suillcient for the purposes of this application to point out` that the FM signal energy appearing in the plate circuit of tube 3 is amplified. A second tube is provided, and this tube is designated by numeral Il. The tube may be one of the pentode-diode through leakage.

type wherein there is an electrode I2 functioning as a diode anode. The anode I2 is adjacent cathode I3 which ls grounded. The pentode section of the tube comprises the cathode |3, plate I4 and three intermediate grids designated I5, I6 and I1.

The intermediate, or screen, grid I6 is connected to the source of potential of oscillator anode 9, the connection between these electrodes being bypassed to ground by condenser I8. for high frequency currents. The control grid I is coupled to the high potential side of the oscillator tank circuit by coupling condenser I9, resistor 20 returning grid I5 to ground. Plates I0 and I4 are connected in common to a point of approximately +200 volts through the coil 2| of the discriminator circuit. This coil 2| is schematically represented as having a powdered-iron core. The condenser 22 shunts coil 2|, and provides a resonant circuit.

The core of coil 2| is adjusted so that the discriminator circuit 2I-22 has a single peak resonance curve. The peak frequency is either above or below the mean, or center, frequency of the FM signals. By way of illustration, the center frequency may be located at an intermediate point of either slope of the resonance curve of circuit 2 |-22. It is desirable to have these slopes as linear as possible. Hence, it is seen that discrimination occurs, and that the FM signal energy developed across the plate circuit of the oscillator is transformed at circuit 2|-22 into corresponding amplitude modulated wave energy. The frequency deviations of the FM signals correspond to the sound waves of the phonograph record, and the corresponding amplitude modulated wave energy similarly corresponds tothe recorded sound waves. 1

The amplitude modulated wave energy is rectified by the diode |2--I3. Anode I2 is coupled by condenser 23 to the high potential side of coil 2|. The modulation voltage is developed across a load resistor 24 arranged in series with the radio frequency choke coil 25 between anode I2 and ground. The high frequency bypass condenser 26 shunts resistor 24. The modulation signals derived from resistor 24 may be transmitted to any form of audio utilizing circuit.

By using the pentode-diode tube II it is possible to have the pentode section in parallel with the amplifier part of the oscillator tube 3. radio frequency voltage is thereby increased across discriminator circuit 2|-22 to a value of over double the magnitude that would be secured by feeding the diode directly from the plate of the oscillator. In this way sufficient audio voltage is derived from a simple diode rectifier to operate the audio amplifier. The circuit is free from possible noises which would ordinarily be introduced in a voltage-doubler diode circuit Because of the high frequencies employed, short leads are desirable. By giving a grid resistor 20 for a value of about 40,000 ohms the grid is driven further positive, and con'- siderably more radio frequency output voltage is obtained across the discriminator circuit. The choke coil 25 affects the Q of the discriminator very little. In general, then, it will be seen thaty in the circuit of Fig. 1 the FM signal energy is fed into the discriminator circuit by electron coupling intube 3, and is concurrently transmitted from tube I| by the amplifier action of the pentode section of tube In the arrangement of Fig. 2, the plate I0 of oscillator tube 3 is connected to the positive volt- The'y age source which feeds the oscillator anode 9 and the screen grids of the following amplifier tube. Otherwise, the oscillator circuit of Fig. 2 is similar to that shown in Fig. 1. The amplifier tube is a pentagrid tube, and may be of the 6SA7 type. The tube is designated by numeral 30, and its cathode 3| is connected to an intermediate point on the coil 2| of discriminator circuit 2I-22. In this modification the high potential side of the discriminator circuit is connected to the plate 32 of tube 30 by the coupling condenser 33, while the low potential side of the discriminator circuit is grounded. The choke coil 25 and series load resistor are arranged between plate 32 and ground.

The control grid 34 of tube 30 is coupled, as in Fig. 1, to the high potential side of oscillator circuit 4 5. It will be noted that the second, third and fourth grids of tube 30 are tied together, and effectively function as a common screen electrode. The cathode 3| and plate 32 of tube 30 cooperate to provide diode rectification. It will be noted that the discriminator circuit 2|-22 is arranged in the cathode circuit of tube 30. This is very similar to the oscillator circuit used, except that in the oscillator the control grid is affected from its own cathode circuit instead of from an external source of voltage as in the case of amplifier operation. The voltage gain is all supplied by the step-up secured from the tuned circuit 2|-22. The discriminator circuit is, of course, slightly mistuned as in the case of Fig. l. so as to secure an amplitude variation corresponding to the frequency deviation introduced by the capacity change in the pickup device.

In Fig. 2, the plate current of tube 3 does not vfeed directly into the discriminator circuit 2 l-22.

The oscillator voltage is applied to the control grid 34 of tube 30 from the high potential side of circuit 4-5. The tube 30 acts as a cathode follower amplifier. Theoutput voltage of tube 30 is stepped up and shifted in phase because of the tapping point of cathode 3| on coil 2|. 'i'he said output voltage .is applied to the plate 32 of tube 30. The plate 32 cooperates with cathode 3| to provide a rectifier, and in this respect is similar to diode I3-I2 of Fig. l. However, the rectifier of Fig. 2 is grid-controlled. The rectifier path between cathode 3| and plate 32 is controlled in phase with the voltage of grid 34. The detector of Fig. 2, therefore, is a grid-controlled rectifier. When grid 34 is positive relative to cathode 3| space current flows through the tube, and diode rectification can take place. When grid 34 is negativer no space current flows. Rectification by 3|-32 cannot take place. Because of the phase shift between the voltage applied to grid 34 and that applied to plate 32 (which changes in accordance with the change in oscillator frequency) the output voltage across resistor 24 contains audio frequency currents.

Each of Figs. 2, 3, 4 and 5 employs the passive discriminator circuit 2|-22 in the cathode lead of the amplifier tube. The difference in these four circuits is in the manner of securing detection of the resulting, amplitude-modulated wave energy. Because of the slight mistuning there is also present a difference in phase of the oscillator and discriminator circuit voltages, which varies when the frequency is varied: This is utilized in the detector circuits to give a greater output than would be secured by depending on amplitude variations alone. This action is explained as follows: When the grid of either the oscillator or amplifier tubes is driven extremely far. the plate current will iiow rent to vary between these extremes.

in the tube only for short intervals during the extreme positive operation of the radio frequency voltage. When there exists a phase difference between these impulses of current arriving at the anode and the voltage applied to the anode by the discriminator circuit, only during a short period can this plate current flow. If the phase of the voltage differs sufficiently then no plate current can flow. If it is in phase with the current, u

the maxirnumplate current can flow. Variation of the oscillator frequency will cause plate cur- By using the voltage drop across the series resistor the audio output voltage can be secured.

Referring then to Fig. 3, it is pointed out that in this modification the plate l acts as the anode of a diode rectifier consisting of anode Ill and cathode 3| of tube 30. The action is similar to that described in connection with Fig. 2. In this arrangement the plate 32 is tied to the positive screen assembly of tube 30. The discriminator circuit 2|-22 is in the cathode circuit of tube 30. In other words, in Fig. 3 besides functioning as the output electrode of the amplifier section of oscillator tube 3, the plate I9 additionally has the function of acting as the anode of a grid-controlled rectifier of the amplitude modulated wave energy developed by the discriminator circuit 2|-22. It is, again, emphasized that by virtue of the phase shift noted above, sharper discrimination is secured. The variation in voltage applied to plate l0, because of the selectivity of the discriminator circuit, isgaccompanied by a change in phase thereby enhancing the rectifier output variation responsive to frequency change.

In Figs. 4 and 5, rectification is secured by a multi-grid detector. In the modification of Fig. 4, the plate I0 of tied tube 3 is connected to the oscillator grids 9. The detector tube is designated by numeral 49, and may be a pentagrid tube of the 6SA7 type. The control grid 34 is connected to the oscillator grid of tube 3. The second and fourth gridsV of tube 40, designated by numerals 4| and 42 respectively, are connected to the plate Ill and grids 9. The control grid 43, located between grids 4| and 42, is connected to ground by a resistor 44, and condenser 45 couples the grid to the high potential side of the discriminator circuit. The plate 32 of tube 40 provides the audio voltage across the output load resistor 50, and the lower end of resistor 50 is connected to a source of positive potential. 'Ihe load resistor 59 is shunted to ground by the high frequency bypass condenser In Fig. 4, as'the frequency of the oscillations changes, the phase of the voltage applied to lgrid 43 will vary from that applied to grid 34. If the oscillator frequency changes enough to produce resonance of the discriminator circuit, the two grid voltages will be in phase and maximum plate current will flow. By limiting the swing to one side of the discriminator, the plate current will vary from this maximum to a low value. Both grids are, of course, supplied with sufficient voltage so that the tube is operated on a non-linear portion of its characteristic. Tube 40, in this modification, acts as a cathode follower amplifier. A voltage from the oscillator tank circuit 5--4 is applied to grid 34. The stepped-up and phase-shifted voltage is applied to grid 43. The variable phase between the alternating voltages impressed on grids 34 and 43 yields a plate current containing the audio frequency components.

The arrangement of Fig. 5 differs from that shown in Fig. 4 in that the oscillator tube 3 is given the additional function of acting as the multi-grid detector which was described in connection with Fig. 4. This is accomplished by having the second and fourth grids of tube 3 acting as the oscillator anode 9, while the intermediate grid 9 is connected by lead 60 to the junction of condenser 45 and resistor 44. The plate of tube 40 is tied to the second, third and fourth grids of the tube, and have in common a positive potential applied thereto. The plate I0 of the oscillator tube 3 is connected to the load resistor 50, across which latter resistor is developed the audio frequency voltage. In Fig. 5, the amplifier tube 40 acts as a cathode follower amplii'ler. The stepped-up and phaseshifted voltage developed across 2|-22 is applied to grid 9 of the tube 3. The variable phase between the voltages applied to the first and third grids of tube 3 causes the plate current of tube 3 to vary in accordance. In this way there is secured across the load resistor 5|) plate current having audio frequency components.

The circuits shown in Figs. 2, 3, 4 and 5 have the advantage of employing standard commercial types of tubes, and only 100 volts of B supply is required. It should, also, be noted that the pair of tubes required in these circuits can be of the same type,`t hereby simplifying manufacture and servicing. The tubes used (6SA'7) have helical heaters thereby reducing any hurn modulation. If desired, in Figs. 4 and 5 the plate resistor 59 may be returned to ground, instead of to the positive potential point.

In Fig. 6, I have shown the reproducing system of the aforementioned Sinnett application. Briefly, the discriminator ,circuit 2|-22 is arranged in circuit with the plate I0 of thev oscillator vtube 3. The diode rectifier 10 is connected across the series arranged choke coil 1| and load resistor 12. The audio voltage developed across resistor 12 is amplified by the audio frequency amplifier 13. The remainder of the circuit elements, between the pickup I and the output circuit of the audio amplifier 73, is believed to be obvious in function. The intermediate grid 80, in this modification, is given a gain'control function. It may act to expand, or compress, the dynamic range. The dynamic range control is effected by means of a control circuit now to be described.

A portion of the audio voltage output of amplifier 13 is transmitted by the audio coupling condenser to the diodes 9| and 92. These diodes are connected in opposition. In this way a transformer is avoided. The input has one side grounded, yet the output affords both and voltage relative to ground. The anode of diode 9| is connected to the cathode of diode 92, and condenser 99 is connected to the junction of these two electrodes. The anode of diode 92 is connected to the cathode of diode 9| through a resistor 93. The latter resistor is shunted by a resistor 94. The midpoint of resistor 93 is connected to ground by condenser 95. Each half of resistor 93 is, also, shunted by a respective bypass condenser. 'I'he midpoint of resistor 93 is connected to the junction of the anode of diode 9| and the cathode of diode 92 by a resistor |00. The midpoint of resistor 93 is connected by an adjustable tap Illl to any desired point of a potentiometer |02. This potentiometer is connected between spaced negative potential points of -3 volts and -20 volts. The

potentiometer |||02 functions as a general acoustic level control.

'I'he gain control electrode 80 is connected by lead |03 and resistor |04 to the adjustable tap |05. The grid end of resistor |04 is shunted to ground by condenser |00. The tap |05 is slidable along resistor 94. -When the tap is moved toward the upper end of resistor 94 there is secured an expansion of the dynamic range by virtue of positive voltage being supplied to gain control 80. On the other hand as the tap |05 is slid downwards compression of the dynamic range is secured, since negative voltage is supplied to grid 811. Hence, the tap |05 functions as a dynamic range control device. It w-ill be appreciated that the diodes 9| and 92 function as a voltage-doubler rectification means, and that the voltage applied to the grid 80 is rectified audio voltage.

Hence, the control of the oscillator gain is in accordance with the audio envelope of the amplified audio signals. Variation of tap |0I will vary the general acoustic level, because as the tap |0| is moved toward the least negative portion of resistor r|02 the gain of oscillator 3 will increase, whereas it will decrease as the tap |0| is moved toward the more negative end of resistor |02. Condenser |06and resistor |04 serve to filter olf any audio frequency voltage from getting back to grid 80, and to prevent too rapid changes in volume. Resistor |00 is a direct current path, and could be replaced by an audio choke. Condenser 95 completes the audio path from the amplifier through condenser 90, through the rectiiiers, through the capacitors across resistor 93, to ground.

The circuit shown in Fig. 6 permits volume expansion or compression with low distortion. It is sometimes desirable to have a different dynamic range o n the acoustic reproduction than that recorded on the phonograph record, for exam-` 'up as shown in Fig. 6, such changes in the dynamic range can be easily secured with less distortion than occurs where the amplification of an audio amplifier tube is controlled. should be noted that because there is no variation in the envelope of the carrier, since it is FM, no distortion of the audio wave shape can occur due to the characteristic curvature of the controlled amplifier tube.

Various modifications are, of course, desirable. For example, instead of the voltage doubling rectier network shown in Fig. 6, a single diode circuit may be employed. Also, time delay may be introduced so that compression will occur only on diode voltages above the threshold of delay. Separate audio amplifiers may be used to feed 'the audio rectifier circuit thereby to avoid any possibility of the diode current introducing distortion in the audio channel. It

'should be noted that the general acoustic level control is secured by an ordinary potentiometer. The low per cent amplitude modulation of the signal resulting from the discriminator permits the diode rectifier to work at a relatively high level and under favorable circuit constants, so that little distortion will result from the detection process. n

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 organizations 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:

4l. In combination with a capacity pickup device for phonograph records, an oscillator tube provided with electrodes forming an oscillator section, a resonant circuit tuned to a predetermined operating high frequency coupled to said oscillator electrodes, said pickup device being connected to said oscillator resonant circuit whereby variations in capacitance result in frequency modulation of the high frequency oscillations, said tube including a positive output electrode which has electron coupling to said oscillator section, a discriminator circuit connected to said output electrode, said discriminator circuit being mistuned with respect to said operating frequency, rectifier means coupled to said discriminator circuit for producing audio voltage, and means responsive to unidirectional voltage derived from said audio voltage for varying the gain of said oscillator tube.

2. In combination with a capacity pickup device for phonograph records, an oscillator tube provided with electrodes forming an yoscillator section, a resonant circuit tuned to a predetermined operating high frequency coupled to said oscillator electrodes, said pickup device being connected to said oscillator resonant circuit whereby variations in capacitance result in frequency modulation of the high frequency oscillations, said tube including a positive output electrode which has electron coupling to said oscillator section, a discriminator circuit connected to said output electrode, saidr discriminator circuit being mistuned with respect to said operating frequency, rectifier means-coupled to said discriminator circuit for producing audio voltage, a second rectifier arranged to produce a unidirectional voltage from said audio voltage, and means for varying the flow of electrons to said oscillator output electrode in response to the magnitude of said uni-directional voltage.

3. In a phonograph including a capacity variation pickup producing frequency modulation of a carrier wave, demodulating means, and audio frequency amplifying and reproducing means; a method of control of the dynamic range of the output Which includes rectifying a portion of the audio frequency current to produce a positive voltage, rectifying yanother portion of the audio frequency current to produce a negative voltage, and regulating the gain of an electron tube amplifying said frequency-modulated carrier wave by applying a manually adjustable portion of either of said voltages to a grid of said electron tube.

4. In a frequency modulated carrier wave utilization system, the method of control of the volume of the modulation frequency output which includes demodulating the frequency modulated carrier wave to provide modulation frequency output energy, and controlling the frequency modulated carrier wave amplitude applied to the demodulation portion of the `system in response to the envelope of said modulation output.

5. In a phonograph, including a capacity-variation pickup ,producing frequency modulation of a carrier wave, demodulating means, and an electro-acoustic transducer; the method of control of the volume of the acoustic output which consists of demodulating the frequencyb modulated carrfier energy, rectifying the modulation output energy, and varying with the rectified energy the transconductance of an electron tube amplier of said frequency modulated carrier wave by change in bias voltage of a grid in said electron tube.

6. In a system comprising a source of high frequency oscillations of a predetermined frequency, means for modulating the oscillations by deviating its frequency in accordance with modulation signals, and means for deriving from the modulated oscillations modulation signalrepresentative energy; the improvement comprising means for deriving from the signal-representative energy a control voltage proportional to the :intensity of the latter, and means responsive to the control voltage for regulating the amplitude of said modulated oscillations.

7. In a system as defined in claim 5, means for manually adjusting the amount and sense of said regulation.

8. In combination with a. variable capacity device, an oscillator tube provided with electrodes forming an oscillator section, a resonant circuit tuned to a predetermined operating frequency coupled to said oscillator electrodes, said device being connected to said oscillator resonant circuit, said tube including a positive output electrode which has electronic coupling to said oscillator section, a discriminator circuit connected to said output electrode, rectifier means coupled to said discriminator circuit for producingV audio voltage, and means responsive to' the envelope of saxid audio voltage for varying the gain of said oscillator tube.

9. In combination with a capacity pickup device for phonograph records, an oscillator tube provided with electrodes forming an oscillator section, said pickup device being connected to said oscillator section whereby variations in capacitance result in frequency modulation of high frequency oscillations, said tube including a positive output electrode which has electron coupling to said oscillator section, a discriminator circuit connected to said output electrode, rectifier means coupled to said discriminator circuit for producing audio voltage, means arranged to produce a control voltage from said audio voltage,

and means for varying the flow of electrons to said oscillator output electrode in response to the magnitude of said control voltage.

WINFIEID R. KOCH.

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