US2349886A - Phase modulation recording and reproducing system - Google Patents

Description

y 30, 1944- w. VAN B. ROBERTS 6 PHASE MODULATION RECORDING AND REPRODUCING SYSTEM Filed Jan. 30, 1942 2 Sheets-Sheet 1 7 6 Phase, 6" Mnrogkone No a 5 Er I I QIKc.

. Man 70' g -ay r INVENTOR Wag aroBJ2oZerb ASI'TORNEY ay 1944- w. VAN B. ROBERTS PHASE MODULATION RECORDING AND REPRODUCING SYSTEM Filed Jan. 50, 1942 2 Sheets-Sheet 2 IA'ILI'ORNEY Patented May 30, 1944 UNITED sTATEs PATENT OFFICE PHASE MODULATION RECORDING AND EEPRODUCING SYSTEM Walter van B. Roberts, Princeton, N. 1., assignor to Radio Corporation of Delaware of America, a corporation Application January to. 1942, Serial No. 428,810

15 Claims.

My present invention relates to recording and reproducing systems wherein the modulation exists on the record as a timing modulation of a carrier wave, and more particularly to systems of the aforesaid type wherein the audible signals are caused to phase modulate the carrier wave.

In my application, Serial No. 369,829, flied December 12, 1940,"I have disclosed and claimed re cording and record-reproducing systems wherein timing modulated carrier waves are provided on the record. Specifically, the audible signals frequency modulate a super-audible carrier wave, and the modulated waves are recorded on the record. Various advantages are outlined in the aforesaid application for such systems. entirely independently of the general advantages of timingm'cdulation in recording systems there are certain special advantages to be secured in. phase modulation (PM) of a recorded carrier wave over phase modulation of a radio wave. To

appreciate these special advantages. consider first v an ordinary phase modulation radio system. In

such a system a single carrier is phase modulated and radiated.

The generation and transmission of such, a

wave involves no ditllculty, but the demodulation of the wave is by no means easy. This demodulation is accomplished ,by' deriving, from the phase modulated carrier, an unmodulated carrier and then combining this unmodulated carrier with the phase modulated wave in suitable phase relation so that the vector sum of the two varies in amplitude as the modulated component varies in phase. The variable amplitude vector sum difficult part of the above process is obtaining the unmodulated carrier which acts as a voltage of reference phase. This step is made relatively easy when a phase, modulation recording system is involved by recording, in accordance with the 40 present invention, both a phase modulated wave and also an unmodulated wave of different but integrally relatedfrequency. In demodulating, these two waves are separated from each other by selective, circuits and then by the process 01-"45 frequencymultiplication or division or both, are brought to a-common frequency. The relative phase is then properly adjusted an the/two waves, now at the same frequency'andof suitable phase relation, are combined to form a single 5 variable amplitude vector sum which is rectified as previously described in connection with radio systems. It will be seen that in the recording system the required unmodulated wave having a frequency always absolutely identical with the 5 Now, 15

mean frequency of the modulated wave and having an unvarying phase relation thereto, is much more easily obtained.

Accordingly, it may be stated to be one of the main objects of my present invention to provide,

"in a system of recorded signals, a method wherein a pair of waves of integrally related frequencies are applied to a record-producing member, one of the waves being super-audible and being subjected to phase modulation by signals to be recorded.

Another important, object of my invention is to combine a phase modulated carrier wave and a reference wave in harmonic relation as a physical record; and reproducing the record by separating the reference wave from the modulated wave, and phase comparing the separated waves after frequency multiplication of the reference wave to and there being utilized phase comparing means for the aforesaid output waves thereby to provide for the demodulation of the phase modulated waves. voltage is then rectified to produce signals. The

Still other objects of my invention are generally to improve the efficiency and reliability of recording and record-reproducing systems, and more especially to provide systems utilizing phase modulated super-audible carrier waves for recording purposes in an economically-manufacturable manner.

The novel features which I believe to be characteristic of my invention are set forth in 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:

Figure 1 shows schematically a recording sys-- tem employing the invention;

Figure 2 shows a modification network of th system of Figure 1,

Figure 3 shows schematically a record-reproducing system employing the invention;

Figure 4 shows a modified system that can be used in place of the reproducing system of Figure 3.

Referring now to th accompanying drawings, wherein like reference characters in the different figures designate similar circuitielements, it is to be understood that numeral I designates the source of audio signals to be recorded. It will, of course; be understood that while the audio source is shown as a microphone, it may well be any other desired audio signal source. For example, it may be the audio output of the detector of a radio broadcast receiver, or it may be an audio frequency distribution line, or it may even be the reproducer of another phonograph. The numeral 2 designates the turntable upon which is supported the record3 on whose upper face the recording is to be out. A motor 4, of any desired and well known type, rotates the turntable 2; the spindle I passing through an opening in the record 3 so as to prevent slippage.

These are devices well known to those skilled in theart of recording. It is to be understood, of course, that the record i'mayuse a base either of metal, fibre composition or glass. The cutting-of the record can be of any desired type commonly employed in the art of recording. It will, also, be understood that either embossing or engraving may be employed for the production of the lateral recording. Specifically, there is disclosed an engraving cutter. While the cutter has been schematically represented as one of the electromagnetic type, it will be understood, of course, that a crystal cutting head may be utilized, as disclosed in my aforesaid co-pending application.

Of. course, if a crystal cutting head is employed, then the windings 1 and 8 might be replaced by a pair of crystal actuating electrodes, and a common grounded electrode, one actuating electrode being connected to 6' while the other actuating electrode is connected to lead It). It will, therefore, be appreciated that the specific manner of cutting the record, and the cutting devices utilized; may be of any well known type.- The cutting stylus 9 may be of any desirable and well known type. Reference is, also, made to my aforesaid application wherein there are described various methods of providing the recording proper on the record.

Referring, now, to the method of translating the audio signals from source I into physical displacements of the cutting device, there is shown in Figure l a master oscillator III which is operated at a predetermined frequency, as, for example, 'l kilocycles (kc.). The master oscillator frequency has a predetermined relation to the super-audible carrier wave frequency which is to be directly modulated by the audio signals. The oscillations from oscillator M are transmitted over conductor l directly to the winding 8 whose lower end is grounded. These oscillations, which are unmodulated and of constant amplitude, provide the reference wave.

Oscillations from master oscillator I0 are concurrently applied to a frequency multiplier net- The phase modulator 6 has applied to it audio signals from source I, and the third harmonic oscillations from the master oscillator. The phase modulator produces, at its output, phase modulated carrier waves whose mean frequency is 21 kc. The PM waves are transmitted over a conductor 6' to winding l of the cutting device, the lower end of winding 1 being grounded. It will, therefore, be seen that the stylus 9 is actuated by virtue of the energization of windings I and 8, and, therefore, there will be provided a recording which is a composite of the reference waves fed to winding 8 and the PM waves fed to winding I.

The master oscillator ill, the frequency multiplier ll an dphase modulator network 6 have all been schematically represented since they are networks which are well known to those skilled in the art. The phase modulator network may be of any well known type, and reference is made to Communication By Phase Modulation" by M. G. Crosby, proceedings of the I. R. E.; February 1939; pages 126 to 136, for

various types of phase modulators that may be employed. It will now be seen that two waves have-been superposed on the record 3; one of these waves is unmodulated and of constant am; plitude, while the other wave has a mean frequency, in harmonic relation to the reference wave frequency, and is phase modulated in accordance with the audio signals which may be voice or music.

Of course, the unmodulated reference wave and the superaudible carrier wave may both be derived by frequency multiplication from a single low frequency wave. As shown in Figure 2, the master oscillator, by way of illustration, is operated at a low frequency of 2 kc. The low frequency oscillations are fed to a frequency multiplier network l2. For example, a frequency multiplication of four times may be utilized thereby providing an 8 kc. reference wave to the lead Ill. The output of the frequency multiplier [2 may be supplied to an additional frequency multiplier analogous to multiplier H of Figure 1 so that the super-audible carrier wave for modulation is provided.

The record now being made, the recording is readily reproduced by the system shown in Figure 3. In this figure the record 3, or a duplicate thereof, is placed upon a turntable 2' cperated by motor 4'. The pick-up device is schematically represented by numeral l3. Here, again, while the pick-up is shown as an electromagnetic pick-up, it is to be clearly understood that a crystal pick-up device may be used to transform the mechanical vibrations into electrical energy of a phase modulated super-audible frequency voltage whose amplitude is more or less constant.

However, as shown in Figure 3, after suflicient amplification it is preferred that the modulated waves be passed through a limiter stage for removing substantially all amplitude modulation. While a constant amplitude has been referred to heretofore, what is mean is that no attempt is made to vary the amplitude. As a matter of fact, even if the cutting stylus or reproducing network has an inherent tendency to operate more efilciently at one frequency than at another, it will make little or no difference in the final result because the limiter networks used in Figure 3 nullify the effect of amplitude variations.

Considering the circuit of Figure 3 more spe- 'quency-is 21 kc. The pass band of the selector network l should be wide enough to pass with high efilciency the entire spectrum of the super audible carrier wave.

The selector it, since it is transmitting the unmodulated reference wave, may be sharply tuned to 7 kc., which is the frequency of the reference wave. For example, the selector it may be a piezo-electric crystal tuned to 7 kc. This is of especial value, as shown in Fig. 5 of my copending application serial No. 431,320 filed Feb.

18, 1942, where automatic means are provided for keeping the turntable speed sufficiently constant. The outputs of selector networks I! and i6 may then be passed through the limiter stages i5 and i6 respectively. The function of the limiter stages is to suppress all amplitude variations, and thus prevent any amplitude variations in the respective channels from affecting the PM detector tubes. Those skilled in the art are fully aware of the construction of the limiter stages. Above the limiter I! there is shown the characteristic of a limiter stage. It will be seen that'the output rises proportionally with input up to a predetermined amplitude, and beyond that the curve flattens off., Since limiter devices are very well known to those skilled in the art, it is not believed necessary to include any further description thereof.

1 The output of limiter i5 is applied to the primary winding of transformer 11 which feeds the PM carrier wave to the detector. As shown in Fig. 3, the secondary winding l'l' has its opposite ends connected by direct current blocking condensers i8 and I8 to the anodes 0f the oppositely. connected diodes II and II. The cathodes of the two diodes are connected in common, and the common cathode connection to the midpoint of input coil il' includes the secondary winding 20 of transformer II which feeds the nowsynchronous reference wave into the detector circult. The primary winding of transformer ii is connected in the output circuit of a phase adjuster device 22, the input terminals of the phase adjuster being fed with the frequency multiplied output of limiter It.

The frequency multiplier 23 multiplies the 7 kc. reference wave to 21 kc., since the reference wave must be injected into the PM detector circuit at the same frequency as the mean carrier frequency of the PM carrier wave. The phase adjuster 22, which may be of any type well known to those skilled in the art, is adjusted so that in the absence of any modulation on the superaudible carrier wave the voltage components on each of diodes I! and I! are in quadrature. In other words, the phase adjuster 22 phases the reference wave voltage fed into winding 20 so that the reference wave voltage is in quadrature with the PM wave voltage applied to the anode of each of the diodes when there is'no modulation.

Another way of considering the phase relations between the PM voltage and the reference wave voltage is to consider that the PM wave voltage is in one quadrature phase with the reference wave voltage on one of the diodes, while it is in the opposite quadrature phase relation on the other diode. Of course, these phase relations are considered for the super-audible carrier wave in unmodulated condition compared to the unmodulated reference wave. Accordingly, when modulation exists on the super-audible carrier. wave, the detector outputs yield a differential voltage which corresponds to the original audio modulation signals. To secure this differential modulation voltage, a resistor 24 is conected between the anodes of diodes i9 and IS. The midpoint of the resistor is connected to the common cathode connection of the two diodes. The differential voltage developed across resistor 24 is applied through the audio coupling condensers to one or i more stages of audio frequency amplification.

The phase modulation detector circuit is purely illustrative, as there are other types of phase modulation detector circuits that can be utilized. Reference is made to the aforesaid Crosby publication to show such other types of phase modulation detector circuits. The function of the injection of the frequency multiplied reference wave into the input circuit of the detector circuit is to convert the applied PM carrier wave into an amplitude modulated wave. As explained previously, this translation of the PM carrier wave into an AM carrier wave is accomplished by rotating the carrier vector degrees with respect to the modulation side bands. Injecting the multiplied reference wave as shown provides a standard of phase against which to compare the phase of the PM carrier wave. The rectifiers i9 and IQ, of course, function to provide differential rectification of the AM wave, which results from combining the unmodulated reference wave with the PM wave. It is not believed necessary to describe the functioning of the phase modulation detector circuit in any further detail, as those skilled in the art are well acquainted with the manner of demodulating a PM carrier wave.

While I have described a particular embodiment of the invention in Fig. 3, it will be realized that considerable variation is possible within the scope of the invention. For example, the unmodulated reference wave voltage may be applied differentially to the diodes, and the phase modu lation carrier voltage may be applied in like phase relation. Again, if the amount of phase swing recorded is too little, or too great, for the voltage comparing and detecting network, then the phase swing may be increased by frequency multiplication, or decreased by frequency division in the channel which feeds the phase modulated carrier wave to the detector. In any case there must always, of course, be such frequency multiplication or frequency division of the unmodulated carrier wave or reference wave, or both, as to provide components at the detector of identical frequency.

Thus, in Fig. 4 there is shown a modified arrangement which embodies the aforesaid modifications. It will be seen that the selector i5 feeds frequency divider 30 which divides the mean frequency of the PM carrier waves by three (3),

and thereby also proportionally decreases the phase deviation. The 7 kc. waves are fed through the limiter-l5 and a frequency doubler ll to the phase adjuster 22, and the adjusted PM waves are applied to the winding .20 whereby the PM carrier waves are now applied in like phase to the differentially connected diodes. The selector II separates out the reference waves of 7 kc., and

after limiting at I6, these waves are frequency multiplied at 23 to provide a reference wave frequency of 14 kc. The reference waves are applied to the diodes of the detector in differential manner.

Of course, in Fig. 4 there is illustrated the situation where the phase swing recorded is too great for the voltage comparing and detection network, and, hence, frequency division is employed as at 30. Where the phase swing is too small, frequency multiplication will be utilized at 30 instead of frequency division.

While I have indicated and described several systems for carrying my invention into eil'ect, 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:

1. In a system for recording signals, means for generating a pair of waves of integrally related frequencies, means to phase modulate one of said waves, means to combine the remaining unmodulated wave and the phase modulated wave, and means to translate the combined waves into a physical record.

2. In a system for recording signals, means for generating a pair of waves of integrally related frequencies, means to phase modulate one of said waves, means to combine the remaining unmodulated wave and the phase modulated wave, means to translate the combined waves into a physical record, and said phase modulated wave being of super-audible frequency and bearing an harmonic reiationto the unmodulated wave.

3. In a reproducing system for a record upon which is recorded an unmodulated reference wave and a phase modulated wave of super-audible frequency which is integrally related to the frequency of the unmodulated wave, means to derive said waves from the record, selective means to separate the waves into separate channels, frequency changing means in at least one of said channels adapted to bring the output waves of said channels to a common frequency, and phase comparing means for said waves including means to derive the modulation on the wave of super-audible frequency.

4. A method of recording audible signals which includes generating a pair of waves of integrally related frequencies, one of said waves being of super-audible frequency, phase modulating said super-audible wave, and recording the combined modulated and unmodulated waves.

5. In a method of reproducing a record upon which has been recorded a phase modulated carrier wave and an unmodulated reference wave in harmonic relation, separating the reference wave from the modulated wave, frequency multiplying the reference wave to the modulated wave frequency, and phase detecting the combined modulated and multiplie waves.

6. The method of reproducing signals which comprises generating a pair of alternating currents of integrally related frequencies, phase modulating one of said pair in accordance with said signals, recording said modulated current and the other current of said pair, reproducing the recorded currents, separating the reproduced currents into separate channels, bringing the separated currents to a common frequency, combining the common frequency currents in nor- 16 mally quadrature phase relation, and rectifying the combined currents to reproduce said signals.

7. In a system for recording signals and reproducing the recorded signals, means for generating a pair of waves of integrally related frequencies, means to phase modulate one of said Waves with the signals, means to combine the remaining unmodulated wave and the phase modulated wave, means to translate the combined waves into a physical record, means to convert the record to said waves, means for separating the waves, means to bring both wave to a common frequency, and phase detecting the latter waves.

8. In a reproducing system for a record upon which is recorded an unmodulated reference wave and a phase modulated wave of super-audible frequency which is integrally related to the frequency of the unmodulated wave, means to pick up said waves, selective means to separate the wave into separate channels, frequency multiplier means in at least one of said channels adapted to bring the output waves of said channels to a common frequency, phase comparing means for said waves, and means coupled to the comparing means to derive the modulation on the wave of super-audible frequency.

9. In a method of reproducing a record upon which has been recorded a phase modulated carrier wave and an unmodulated reference wave in harmonic relation, separating the reference wave from the modulated wave, frequency multiplying the reference wave to the modulated wave frequency, combining the modulated wave and multiplied frequency wave in normally quadrature phase relation, and rectifying the combined waves.

10. In a system for recording signals, means for generating a wave of a desired frequency, deriving therefrom a pair of waves of integrally related frequencies, means to phase modulate the higher frequency one of said waves, means to combine the remaining unmodulated wave and the phase modulated wave, and means to translate the combined waves into a physical record.

11. In a system for recording signals, means for generating a wave of low frequency, frequency multiplying the low frequency wave to a higher frequency, deriving from the multiplied wave a pair of waves of integrally related frequencies, means to phase modulate one of said pair of waves, means to combine the remaining unmodulated wave and the phase modulated wave, means to translate the'combined waves into a physical record, and said phase modulated wave being of super-audible frequency and bearing an harmonicrelation to the unmodulated wave.

12. In a reproducing system for a record upon which is recorded an unmodulated reference wave and a phase modulated wave of super-audible frequency which is integrally related to the fre quency of the unmodulated wave, electrical pickup means to pick up said waves, selective means to separate the waves into separate channels, frequency changing means in at least one of said channels adapted to bring the output waves of said channels to a common frequency, means to combine the latter output waves in normally quadrature phase relation, and opposed rectifiers coupled to the combining means for deriving the modulation signals.

13. A method of recording audible signals which includes generating a super-audible frequency wave of constantamplitude, phase modulating said super-audible wave, recording the modulated Wave, reproducing said record upon which has been recorded said phase modulated carrier wave, deriving from the reproduced wave a reference wave, combining the reference and modulated wave in normally phase quadrature relation, and detecting the combined waves.

14. A method which includes generating a super-audible frequency wave of constant amplitude, phase modulating said super-audible wave with audible signals, recording the phase modulated wave, converting said recorded phase modulated wave, converting said recorded phase modulated carrier wave into corresponding modulated wave energy, deriving from the modulated wave energy a reference wave, and combining the reference and modulated waves for phase detection.

15. A method which includes phase modulating super-audible frequency oscillations with audio signals, translating the phase modulated oscillations into a physical record, converting the record into phase modulated oscillations corresponding to said original phase modulated oscillations, producing unmodulated oscillations having a frequency equal to the mean frequency of said converted oscillations, and phase detecting the combination of the converted phase modulated oscillations and saidlunmodulated oscillations in predetermined phase relation to produce said audio signals.

WALTER wm B. ROBERTS.-

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