US2334510A - Record reproducing control circuit - Google Patents

Description

' Nov. 16, 1943. w. VAN B. ROBERTS RECORD REPRODUCING CONTROL CIRCUIT 2 Sheets-Sheet 1 Filed Feb. 18, 1942 lw A|| lNvENToR W ra/Z.0Ze/Z T'TORNEY 2 Sheets-Shawl 2 Nov. 16, 1943. w. VAN B. ROBERTS RECORD REPRODUCING CONTROL CIRCUIT Filed Feb. 18, 1942 INVENTOR ATTORNEY d K awww@ Patented Nov. 16, 1943 RECORD REPBODUCING CONTROL CIRCUIT Walter van B. Roberts, Princeton, N. J., asslgnor to Radio Corporation of America, a corporation of Delaware Application February 1s, 1942, sei-iai No. 431,320

1o claims. (ci. 17o-100.4)

My present invention relates to sound record reproducing systems, and more particularly to methods of, and means for. controlling record reproduction so as to prevent pitch changes, or wow' In the conventional sound record reproducing system it has always been dilcult to eliminate "wow, or changes o! pitch in reproduced musical tones, as it has been necessary, in order to obtain perfect reproduction, to maintain absolutely constant speed both during recording and reproduction. In the case of disc systems, it has usually been the angular speed of the disc that had to be kept constant. In the case of sound on lm, or magnetic records, it is, of course, the linear speed of the film, or magnetized wire, that must be kept constant. It will be obvious that if the record is not made at a truly constant speed it will be impossible to compensate by correspondingly varying the speed during reproduction, for the reason that there is no criterion available in the ordinary record for the speed required for perfect reproduction.

Therefore, the best that can be done in ordinary reproduction is to hold the speed as constant as possible. Thus. the music reproduced can be no better than permitted by the excellence of the recording speed regulation device, and, in particular, if the hole 'in the disc record is not accurately centered, as is often the case, there is no way to compensate for the resulting Wow. Especially is this true as the orientation of the eccentricity with respect to the turntable is usually a matter of chance, since the record is put on the turntable with random orientation.

Hence, it may be stated to be one of the main objects of my present invention to provide a method of reproducing sound records, whether recorded at constant speed or otherwise wherein a control means is provided to wipe out variations of record speed from its correct value in response to such variations arising during reproduction.

Another important object of my invention is to provide in a record reproducing system a sound record having superposed on the sound track during recording a super-audible, or sub-audible, constant amplitude-constant frequence wave; the voltage produced by the latter during record reproduction being converted into a control voltage for automatically regulating the record speed in such a manner as to prevent wow effects.

In my copending application, Serial No. 369,829, filed December 12; 1946, I have disclosed and claimed methods of recording, and reproducing, sounds by frequency modulation of a constant amplitude,superaudible carrier. The mean frequency, or carrier, of such recorded waves constitutes a definite criterion for the speed of the reproducing equipment. Hence, another object of my invention is to provide means for controlling the speed of the reproducer so that the mean frequency of the frequency modulated carrier waves picked up by the electrical pick-up device is maintained constant whereby the audio pitch of the detected output will remain constant regardless of undesired speed variations which may have occurred during recording, and even regardless of eccentricity of the record. In fact, the recording may be deliberately carried out at an angular speed which increases as the center of the record is approached, so as to maintain a more nearly constant linear speed of the cutting head. In this case the speed control at the reproducer will cause the speed to vary in a similar fashion. Furthermore, with a suiilciently close speed control the same reproducer can play records made at constant angular speed, or constant linear speed-or any other speed variation, without change in the reproducing mechanism.

In the case of phase modulated carrier recordings, as disclosed in my copending application, Serial No. 428,810, filed January 30, 1942, the sound track is a composite of the phase modulated carrier wave and an unmodulated wave of constant amplitude employed for phase comparison at the phase detector. It is another object of my present invention to utilize such recordings in the present control methods; the unmodulated wave being employed in such case to provide the reproducer speed control voltage.

Still other objects of my invention are to improve generally the efiiciency of reproduction of sound records, and more especially to provide economical and simple means for insuring correct reproduction of pitch of musical output despite variation in recording speed, eccentricity of the record, or the like.

The novel features which I believe to be characteristic of my invention are set forth in partlcularity 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 schematically one system embodying the invention,

Fig. 2 illustrates a Inodication,

Fig. 3 shows another modiiication based on features common to Figs. 1 and 2',

Fig. 4 shows still another embodiment applicable to frequency modulated carrier wave recordings,

Fig. 5 illustrates a further embodiment applicable to phase modulated carrier recordings.

Referring now to the accompanying drawings, wherein like reference characters in the diiferent gures denote similar circuit elements, in all the embodiments shown hereinafter certain common features are present. In the following figures there is illustrated a relatively high-speed motor having a small amount of inertia, and driving a turntable through a rubber roller pressing against the rim. This motor works against a brake, whose braking force is controlled by an electro-magnet. In the absence of current through the electro-magnet, the motor speed will increase to a value considerably above any speed ordinarily desired. In the presence of maximum available current in the magnet, the motor speed will be reduced to less than any value normally desired. The moment of inertia of the motor system and the normal braking force are so proportioned that the motor speed may be changed by an appreciable amount in a small fraction of a second.

The brake control current for the magnet is supplied from a vacuum tube energized from a well regulated source of voltage, and this current is controlled by the grid potential. The latter is obtained through a low pass filter from a discriminator used for converting the speed control wave picked up from the record into a direct current voltage of variable amplitude. It is well known in discriminator circuits that the average potential of the discriminator output is zero so long as the mean frequency of the applied Waves does not vary from a predetermined frequency.

It is known, too, to filter out audio frequency fluctuation from a discriminator output, and to employ the filtered output to control the frequency of the local oscillator in a superheterodyne receiver. That is known as automatic frequency control. In this present case, the same filtered control voltage is used, but it is applied t the control of motor speed rather than oscillator frequency. The filter used is preferably so constructed that variations of average potential will be transmitted through the filter, even though they take place rather rapidly-although not so rapidly as the fluctuations due to the lowest frequency audio modulations. Thus, in case there is eccentricity in the record, it would produce a frequency fluctuation having a period of a little less than one second. The speed control action would take place sufficiently rapidly to impose just the right amount of motor speed variation to offset the effect of the eccentricity.

In the foregoing it has been assumed that the speed control action would result in producing a truly constant mean frequency of the picked up waves, but it will be understood that this condition cannot be attained in theory; although it may be approachedv sufliciently closely for all practical purposes by making the control action sufficiently stiff. To this end, for example, a stage of direct current voltage amplification may be inserted between the output of the filter and the input of the tube which supplies current to the braking magnet.

In the case of frequency modulated carrier recordings the mean frequency of the waves generated by the transmitter employed in making the record must be held sufiiciently constant. This, however, is well within the possibilities of the art as carrier frequencies can easily be maintained to a much closer percentage of an assigned value than the percentage variation of pitch which the human ear can detect. The ear, to be specific. does not notice a change of pitch less than a few tenths of 1%, so that the governing mechanism of the present invention needs only to keep the speed within a similar percentage of the value which would produce a truly constant average frequency at the input to the discriminator.

Considering, now, the various embodiments shown in the figures, the numeral I denotes the usual amplitude-variable record. This record has the sound tracks, or grooves, engraved or embossed in the usual and well-known manner. In the usual record there is nothing in the recorded passages that can be used as a criterion for proper reproducing speed. Hence, it will be understood that during the recording process there has been added to the music, or speech, channel to the cutting head a tone of constant frequency and constant amplitude. The tone is preferably super-audible. The manner of injecting the super-audible pilot tone may be according to any known in the art. What is important is that there be recorded a composite audible wave and super-audible tone.

The record is placed on the usual turntable 2 for reproduction; the table is driven by a motor 3. The latter is schematically represented, since any conventional motor device may be used. Preferably the motor device 3 is of the travelling magnetic wave type which acts to drag the conductive disc 4, made of copper, by eddy current action. If desired, any type of induction motor may be used to drive disc 4, the synchronous speed being above any required speed of the disc. However, the moment of inertia of the disc assembly should be as small as possible so that the speed can follow changes in brake drag rapidly. The table 2 has a depending rim 2' which is in peripheral contact with the rubber drive roller 5 pressing against the rim. The roller is axially and rigidly secured to vertical drive shaft 6 whose lower end is rigidly secured to the center of the disc 4.

The numeral 'I designates in a purely schematic manner an electrical pick-up device of the magnetic type. Of course, a pick-up of the piezoelectric type may be utilized instead. In well known manner, rotation of the record I, centered on pin I', causes currents to flow through the coil of the pick-up device. These currents, which include the audio Waves to be reproduced and the super-audible monitor wave, are transmitted through an amplier 8 if desired. The amplified currents may then be passed through an audio lter 9. The latter permits only the waves of audio frequency to pass, and prevents the tone wave from passing to the following reproducer. Further amplification, as at I0, may follow the audio filter 9, the latter may be a low pass filter passing audio frequencies up to about 10,000 cycles.

The speed control tone is derived from the amplified output of network 8 by a tone filter I I. The latter is the initial network of the speed con- -trol channel. The filter II may be a high pass filter network rejecting all audible frequencies. After optional amplification at I2` the tone current is passed through a limiter I3. The latter eliminates any amplitude variations in the tone current so that the discriminator output will be solely a function of variation of the frequency of the tone current. The limiter is well known in the art; for example, there may be used a readilysaturable amplifier whose output rises proportionately with input up to a predetermined amplitude and thereafter remains constant.

The limited tone current is applied to discrimcontrolled by an electromagnet.

aseguro inator Il. The latter is constructed' and arranged to convert a frequency change of tone current into a direct current voltage whose magnitude and polarity are respectively a function of the extent and direction of the tone current frequency departure from its correct value. For this purpose there may vbeemployed any well known ,discriminator o1' the type commonly used in automatic frequency control (AFC) of superheterodyne receivers. For example, there may be employed the type of discriminator shown by S. W. Seeleyln U. S. Patent No. 2,121,103, granted June 21, i938.

The low pass filter I5 filters out audio frequency fluctuation; the filter is preferably so constructed that variations of average potential will be transmitted through the filter even though they take place rather rapidhr--although not so rapidly as the fluctuations due to the lowest audio frequency modulations. Thus, where there exists record eccentricity, which would produce a frequency fluctuation having a period of a little less than one second, the speed control action takes place sulciently rapidly to impose a motor speed variation to offset the effect of the eccentricity.

The output of the low pass filter I5, which is direct current voltage, is applied to the control y grid of tube I6. A negative biasing source Il is in circuit with the grid, and source I'I may be adjustable in magnitude so as to provide any desired plate current flow in the absence of applied voltage from filter I5. This bias shall be so adjusted that the motor will run at approximately the correct speed in the absence of control voltage from low pass filter I5. The plate of tube I8 is connected to the positive terminal of a direct current source, not shown, through coil I8. The latter is wound around the magnet core I9 so as effectively to energize the electromagnet. The -disc 4 has its periphery arranged to rotate between the opposed poles I9' of the core I9.

It will now be seen that the electromagnet I8-I9 acts as an eddy current brake for the motor driven disc 4. In other words, the motor 3 works against a brake, whose braking force is In the absence of current flow through winding I8 the motor speed will increase to a value considerably above any speed ordinarily desired. In the presence of maximum current flow through winding I8 the motor Speed will be reduced to less than any value normally desired. A slight overspeed of the turntable tends to increase the pitch of the control tone, and, hence, the discriminator output voltage causes an increase in current flowing through the winding of the eddy current brake. This is more than enough to prevent the assumed overspeed.

Similarly a speed decrease of the turntable reduces the braking action, and permits the motor 3 to speed up. It will, therefore, be understood that the bias of source I1 is adjusted so that in the absence of discriminator output voltage there will be enough current flow through winding I8 to make the speed approximately correct. The effective moment of inertia of the rotating system I, 8, 5, 2', 2, I, I', 3 and the normal braking force are so relatively proportioned that the motor speed may be changed by an appreciable amount within a small fraction of a second. Should it be desired to make the control action of increased stiffness, a stage of direct current voltage amplification may be inserted between the output of low pass lter I3 and the control grid of tube i6.

It will now be seen that no matter what speed variations may have occurred during recording, the reproducer speed control holds the superaudible control tone substantially constant and thus prevents wows or pitch varia-tions in the reproduced music. It is, also, emphasized that the record could have been made at approximately constant linear speed, yet the reproducer will follow it correctly. The present speed ecntrol action will also wipe out variations of linear speed due to eccentriclty of the hole in the record.

It will be appreciated that the control tone frequency may be stepped down to a lower frequency value by utilizing a conventional oscillator-mixer network. Such a frequency reduction network may be inserted, for example, subsequent to the tone filter II. This results in a larger percentage change in frequency for a given percentage change in record speed, and thus increases the stiffness of the governing action. It is, also, to be understood that there are many other ways to apply a governor brake in accordance with the frequency of the generated control tone.

In Fig. 2 there is shown a somewhat different speed control arrangement embodying the use of a frequency reducing network subsequent to the tone lter II. In Fig. 2, it will be seen that a local oscillator 20 generates local oscillations of frequency fo and feeds the oscillations to a mixer 2 I. The latter has applied to it the superaudible control tone whose frequency is f. Hence, a beat frequency output of frequency fo--f will be produced in the output circuit of mixer 2i. It will be understood that well known conventional networks may be used for networks 20 and 2I. The beat frequency energy is applied to the control grid of tube I6. A negative biasing source I1 is inserted in the grid circuit of tube I6. In this form of the invention the plate of tube I6 is connected to the primary winding of transformer 22. The secondary winding is connected to one pair of contacts 23 of a double-pole, double-throw switch S. A second pair of contacts 24 are connected to the usual alternating current source which may, for example, be of volts at 60 cycles. The contacts 25 will be understood as being connected to the adjustable arms of the switch which are to be thrown into connection with either of the pair of contacts 23 or 24. The contacts 25 are connected directly to a driving motor. Hence, the motor may either be connected to contacts 23 or to contacts 24.

In explaining the operation shown in the arrangement of Fig. 2, let it be supposed that at the correct record speed f equals 15,000 cycles and that this speed is obtained when the motor i is fed with current of a frequency of 50 cycles. Assume that fn is 15,050 cycles. Assuming, also, that the motor 3 is a self-starting synchronous motor capable of holding synchronism throughout a range of frequencies including 50 cycles, then throwing S to the right-hand contacts 24 will start up the motor which will then run too fast. Throwing switch S quickly to the lefthand contacts 23 will then allow the motor to slow down until f equals 15,000 cycles. The current supply to the motor will, then, become synchronous with the motor speed at that instant. Hence, the motor can hold synchronism. Thereafter, a slight increase in the frequency f will reduce fo--f and thus slow down the motor. and

vice versa. Only when the motor runs at such speed as to make f equal 15,000 will the current supplied to the motor be synchronous with the motor. The speed to which the turntable is governed can be varied by a fine adjustment of the frequency of the local oscillator 20. Tube I8 is indicated as a simple triode. Preferably, this tube may be replaced in practice by one or more devices of the thyratron type arranged in known fashion to give relatively large power output suitable for driving the motor at a frequency controlled by the mixer output.

In Fig. 3,1 have shown another modification using the magnetic brake device of Fig. 1 together with the frequency reducing network of Fig. 2. It will be understood that the amplifier 30 is connected to the output terminals of the mixer 2| in Fig. 2. A limiter 3I follows the amplifier 30, and the output of the limiter is applied to a low pass filter network 32. A diode 33 has its cathode coupled through a condenser 34 to the high potential output terminal of filter 32, while the anode of the diode is connected to the low potential output terminal. The resistor 35 is connected in shunt with the diode, while the control grid of the energizing tube IB is connected through resistor 36 to the cathode end of resistor 35, the by-pass condenser 31 being connected in shunt with resistor 35. The grounded end of condenser 31 is connected to one end of the brake winding I8 through a potentiometer resistor 38. The brake core is designated by the numeral I9. The cathode of tube I6 may be connected by an adjustable contact to any desired point along the potentiometer 38.

In the modification of Fig. 3, as the motor for the turntable starts up from rest, the value of j increases. The beat frequency fc-f thereby decreases to a value that will pass the low pass filter 32, and is rectified at 33. The rectified output voltage developed across resistor 35 causes current to flow through winding I8. A steep cut-off in the filter 32 assures a steep governing action. By making the cut-off frequency low compared to fn, the cut-olf can be made to occur within a frequency interval that is very small compared to the normal value of f. Hence, variations of f are held to a Very small percentage. Of course, amplification may be added to, or can be made a part of, the low pass filter. The speed to which the record is governed may be adjusted by slightly altering the value of fo.

In the foregoing arrangements it was assumed that a special super-audible pilot wave was recorded along with music, or speech, on an ordinary amplitude record. The methods of control hereinbefore disclosed are not limited to such records, however. In fact these methods may even more readily be applied to records upon which are recorded, not audio frequency waves, but super-audible waves modulated either in phase or frequency in accordance with the desired audio signals. For in either of these cases the super-audible wave may be utilized, quite independently of its demodulation to form audio currents, in exactly the same way as the pilot wave heretofore assumed as especially included on an ordinary record.

Fig. 4 shows a circuit arrangement which will reproduce, and govern the speed of, a record bearing frequency modulated (FM) waves wherein the same circuits are used both for demodulation and generating the governing voltage. The electrical pick-up device 1 in this case feeds the FM carrier waves of super-audible frequency to the amplifier 40. In order to eliminate amplitude variations, the limiter 4I is utilized. If desired, a frequency converter 42 and/or a frequency divider 43 may be inserted between the amplifier 40 and the limiter 4I. By the use of conversion and/or.divlsion, the frequency swing and the mean frequency can be independently adjusted. The discriminator 44 is then used to convert the limited FM waves into two components. One of these components is the audio modulation originally applied .to the superaudible carrier wave, while the other component is the direct current voltage which corresponds to the mean carrier frequency of the FM waves. The audio modulation voltage is utilized in any following audio frequency amplifier, and is then reproduced. The direct current voltage component developed across the output resistor 45 is utilized to govern the speed of the turntable as shown in Fig. 1. From this point on the method may be exactly as described in the case of the speed control circuit of Fig. 1, because the direct current voltage is a function of the frequency deviation of the super-audible carrier from the predetermined mean frequency. In this case the super-audible carrier frequency is analogous to the super-audible tone frequency in Fig. 1. The low pass filter 46 is designed to pass only such low frequency variations of potential as result from variations of record speed from its proper value. The direct current amplifier 41 may be inserted prior to the filter 46, in order to increaseA the stiffness of governing action. Following the low pass filter 48 the elements shown in Fig. 1 may be utilized in exactly the same way.

It is to be understood that the reproducing for the FM recorded Waves may follow the teachings of my aforesaid application Serial No. 369,829. The governing mechanism in the case of Fig. 4 need only keep the speed within a small percentage of the value which produces a constant average, or mean, frequency at the input to the discriminator 44. It is emphasized that the ear does not notice a change of pitch less than a few tenths of one percent. In the case of any record having a wave of constant mean frequency, the voltage of this wave may be selected out, amplied, and impressed on the discriminator shown in Fig. 1 for the speed governing function. This is true because the phase, or frequency, variations of the wave representing audio signals occur at a higher frequency than the frequency of record speed variations so that such audio frequency variations are readily filtered out of the voltage supply to the brake control tube I6.

In Fig. 5 there is shown still another modification wherein the record I is of the type wherein the record waves are a composite of a phase modulated carrier wave and an unmodulated reference Wave. This type of recording and record reproducing has been referred to as disclosed in my aforesaid application Serial No. 428,810. The electrical pickup 1 in this case feeds the composite phase modulated carrier wave (PM) and unmodulated reference wave to the amplifier 50. The amplified waves are fed to a filter on the one hand, which is designated by numeral 5I. This filter is constructed and arranged to pass only the unmodulated reference wave. Thereafter, the reference wave is limited and then fed to a discriminator as shown in Fig. 1 for example.

The remainder of the system following the discriminator I4 in Fig. 1 may be used in this case; the reference wave in this case would be analogous to the control tone embodied in the recording of Fig. 1. The output of amplifier SII is also fed through the main reproduction channel as disclosed in my last mentioned co-pending application. Since the construction and operation of the main reproducing channel is disclosed and claimed in the latter application, itis only briefly described herein. In general, separate selectors separate the reference wave from the modulated carrier wave. The modulated carrier wave is re the frequency multiplier, and the PM and reference waves are applied to the PM detector, which may be a pair of opposed rectifiers, in normal quadrature phase relation. This results in conversion of the PM wave into an AM carrier wave, which is rectified by the opposed rectiflers to produce the original audio modulation.

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 withoutdeparting from the scope of my invention, as set forth in the appended claims;

What I claim is:

1. A method of reproducing records including waves of constant mean frequency which includes deriving from the reproduced waves a direct voltage which is a function of the mean frequency of the reproduced waves, and utilizing said voltage to control the record speed to oppose any deviation of said mean frequency from said constant,

frequency. l

2. A record reproducing system, means for rotating a record, means for converting the record impressions into currents, means for reproducing the currents,l means for deriving from the currents a control voltage whose magnitude is a function of departure of record rotation speed from a` predetermined constant speed value, and means responsive to said voltage for automatically regulating said rotation speed in a sense to maintain said constant speed.

3. In a record reproducing system for sound records having superposed on the soundtracks thereof through recording a wave ofJ constant frequency and constant amplitude; ,the method which includes subjecting the record to rotation relative to an electrical pick-up thereby to reproduce the sound tracks, deriving a control voltage from said constant frequency wave, and automatically regulating the record speed in response to said control voltage in a sense such as to prevent variations in pitch of reproduced sounds due to variations in record rotation speed.

4. In a reproducing system for sound records of the type having recorded thereon a pilot wave of constant frequency, means for producing motion of said sound record relative to an electrical pick-up at a predetermined speed thereby to reproduce the recorded sounds, means for deriving from the reproduced currents a control voltage whose magnitude is a fimction of deviations of said pilot wave frequency, and means responsive to said control voltage for controlling the motion ofsaidsoundrecordsoastomaintainsaidprcdetermined lpecd.

5. In a reproducing system for sound records of the type having recorded thereon a pilot wave of constant frequency, means for producing motion of said sound record relative to an electrical pick-up at a predetermined speed thereby to reproduce the recorded sounds, means for deriving from the reproduced currents a control voltage whose magnitude is a function of deviations of said pilot wave frequency, means responsive to said control voltage for controlling the motion of said sound record so as to maintain said predetermined speed, and means reducing the frequency of said pilot wave priorto control of said predetermined speed.

6. The method of reproducing records wherein audio signals are recorded as frequency modulated carrier waves of super-audible frequency, means for producing rotation of the record with respect to an electrical pick-up thereby to provide frequency modulated carrier wave currents corresponding to the sound tracks of the record, means for detecting said modulated waves to provide a sound voltage component and an independent unidirectional voltage component whose magnitude is a function of the mean frequency of said modulated waves, and means for controlling the rotational speed of said record in response to variations in magnitude of said uni-directional voltage.

7. In a system for reproducing recorded signals from a record upon which waves of constant mean frequency are recorded, driving means for said record, and means for controlling the speed of said driving means, said last named means being actuated by the mean frequency of currents picked up from said recorded waves in such sense as to oppose departure of the mean frequency of said picked up waves from a predetermined constant value.

8. In a system for reproducing recorded signals from a record upon which waves of constant mean frequency are recorded, driving means for said record adapted to drive the record too fast, braking means for reducing the speed of said driving means, translating and limiting means for said waves, selector means for converting variations of frequency of said translated waves into amplitude variations, rectifying means for producing variable direct current from said variable amplitude waves, and means controlled by said variable direct current for varying said braking means in such sense as to oppose changes of the frequency of said translated waves from a predetermined frequency.

9. In combination with a disc record having recorded thereon sound and a superaudible pilot tone of constant superaudible frequency, means vent undesirable frequency shifts of the currents.v

WALTER VAN B. ROBERTS.

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