US2361658A - Sound recording and reproducing system - Google Patents

Description

Oct. 31, 1944. Q s TT 2,361,658

SOUND RECORDING AND REPRODUCING SYSTEM I Filed Oct. 26, 1942 T'li il.

OSCILLATOR T0 RECORDER AMPLIFIER TO SOURCE of AUDIO WAVES AMPL.

INVENTOR CHESTER M.

DISCRIMINATOR RECTIFIER R E W L P M A AMPLIFIER T0 .70 RECORD PICKUP .SINNETT vww A'TroRNEY Patented Oct. 31, 1944 SOUND RECDRDING AND SYSTEM nnrnonucmc .Chester M. Sinnett, west'montn. 1., asslgnor to Radio Corporation of America, a corporation of Delaware Application October 26, 1942, Serial No. 463,349

6 Claims.

My present invention relates to the recording of frequency modulated signals,- and more particularly to a novel and improved type of system for recording and/or reproducing frequency modulated oscillations.

It has been proposed in the past by W. V. B. Roberts, in application Serial Number369,829, filed December 12, 1940, to record sound waves by frequency modulating carrier energy with such sound waves, and the resulting frequency modulation signal energy then being recorded. Such records are reproduced in a system employing a discriminator-rectifier network commonly employed in the art of receiving radiated frequency modulated carrier energy. There are various practical disadvantages in the system disclosed in the aforesaid Roberts application. Ideally, the recording of frequency modulated signals should not require record cutters of special design, nor should special types of record surfaces be utilized. Further, it is desirable to have a system, such as the present, applicable to film, magnetic tape or any other recording system capable of 20 kilocycle (kc.) reproduction.

It can be stated, therefore, that it is one of the main objects of my present invention to provide a'system of recording sound waves by means of frequency modulated signals which are produced by modulating local oscillations with the aforesaid sound waves. One of the outstanding advantages of th present invention resides in the fact that the record cutter may be of any well known type adapted for recording frequencies from 10,000 to 20,000 cycles.

Another important object of this invention is to provide a method of recording sound waves wherein the latter are employed to frequency modulate a carrier, at 15 kilocycles (kc.), up to a maximum deviation of 2 to 3 kc. on either side of the mean carrier frequency,

Yet another important object of this invention is to provide a system for recording sound waves by virtue of a frequency modulation method, wherein the oscillator 'to be modulated infrequency is of the resistance-capacity type.

acteristic of my invention are set forth with particularity in the appended claims; the invention to the input grid 24, the cathode 25 being con-' (Cl. 179L00.1)

itself, however, as to both its organization an method of operation will best be understoodby reference .to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit diagram of a recording system embodying the invention,

Fig. 2 shows a reproducing system that may be used for'records produced by the system of Fig. 1.

Referring to Fig. 1, there will first be explained the oscillator circuit which is to be modulated in frequency by the sound waves which are to be recorded. The oscillator is of the resistancecap'acity (R-C) type. It comprises a pentode I, which may be of the 1852 type (or 6A0? type) and a triode 2 which may be of the 6J5 type. The +B terminal of the direct current supply source is connected through voltage reducing resistor 3 to the plate 4 of tube I, and to the control grid 5 of tube 2. The plate 6 of tube 2 is connected to the lower end of resistor 3, and the screen grid of tube i is also connected to this lower end. The cathode l of tube i is connected to ground through a biasing resistor 8, and-thelatter is shunted by a condenser 9 of low impedance to the oscillator frequency currents. The cathode ID of tube 2 is connected to ground through a resistor II which is an element of the resistancecapacity network of the oscillator circuit.

The resistance-capacity (R-C) network of the oscillator circuit comprises the shunt resistors ll, l2, l3, l4 and IS. The condensers l6, II, It .and i9 arearranged in series relation between the cathode end of the resistor H and the lead 20. The lead 20 is'connected between the upper end of resistor l5 and the input, or control, grid 2| of tube l. The resistor I2 is connected between the junction of condensers l6 and I1 and ground, The resistor i3 is connected from the junction of condensers l1 and I8 to a lead 22. Resistor I4 is connected from the junction of condensers i8 and ill to ground.

A twin triode tube 23, which may be of the 608G type, has one triode section thereof arranged to act as the audio wave input section. Hence, the audio 'waves to be recorded are applied nected to ground through the unbypassed resistor 26. The plate 21 of triode section 25-24-21 is connected in common to the cathode 28 of the other triode section 23. The lead 22 is connected quency of oscillations.

to the common connection of 21-28. Cathode 28 supplies electrons "to the plate 30, while control grid 3| is connected to an intermediate point. on the resistor 32 connected to ground from the plate 30. Positive voltage can be applied to grid 3|, because the cathode of this section is also at positive potential with respect to ground by an amount determined by the bias existing on grid 24 and the position of the tap on resistor 32. The plate 30 may be connected to any desired source of direct current. By using the two triode sections in series it is possible to provide in effect a tapped resistor, the bottom effective resistance of which depends upon the bias existing on grid 24 either as fixed bias or due to the incoming audio signal. Since the lower triode section is in series with R13, whose normal value is about one half that of the other resistors, we are able to effectively change Rn from a little more than one half value to full value of the other resistors in the network, depending upon the magnitude of the audio voltage applied to grid 24.

The frequency modulated oscillations are derived from a path comprising condenser 40 and resistor 4| connected in series between the upper end of resistor II and ground. The adjustable contact 42 connects to the power .amplifier. Contact 42 may be adjusted to any point along resistor 4I thereby to regulate the magnitude of the frequency modulated oscillation energy applied to the power amplifier which precedes the record cutter. The resistor-capacity network connected between the cathode circuit of oscillator triode 2 and the input grid 2| of tube I has a natural, or mean, frequency Fc of approximately 15 kc. The resistor I5 is made adjustable so as to set the initial, or mean, frequency of the oscillator;

The oscillator circuit itself provides sustained oscillations of substantially constant amplitude 40 at its chosen frequency by virtue of the regenerative feedback from the cathode circuit of tube 2 to its grid circuit through the resistor-capacity network and tube I. The oscillator as such consists of both the tubes I and 2. The R-C delay network comprising condensers I6, I'I,' I8, I9 andresistors II, l2, I3, I4 and I5 is used as the cathode load circuit of the tube 2. 'I'he propagation constant of the R-C network is such that for a given required frequency the grid of tube I will be positive, when the cathode of 2 is negative. In other words a half cycle delay occurs between the cathode of 2 and the grid of I. Since this is the case, and due to the amplifying properties of tube I, sustained oscillations can be produced the frequency of which depends upon 5 the time delay characteristics of the R-C network. For a frequency value of about 15 kc. the resistor branches are about 20,000 ohms, and the capacity values are of the order of 300 micromicrofarads. It can thus be seen that by changing the effective values of R13 and R15 it is possible to change the propagation constant of the R-C network, and consequently the fre- Tube I is used as an am- 5 plifier, and tube 2 as a directly-coupled cathode follower amplifier, the combination producing oscillations by virtue of thetime delay R-C network and regenerative feedback to tube I.

Application of the audio waves to grid 24 acts 7 to vary the magnitude of the frequency Fc. This variation is in accordance with the amplitude of the sound waves. The source of audio Waves connected to grid 24 may be a microphone, the detector output terminals of a radio receiver, a 75 telephone line and the like. The mean frequency of the oscillator will be varied, or deviated, in direct proportion to the amplitude of the sound waves, while the rate of deviation will be dependent upon the audio frequencies per se. It is desirable to have a maximum deviation of 2-3 kc. to either side of the value Fe. In this way the output of the power amplifier may be fed to a record cutter of the type capable of recording frequencies from 10 to 20 kc. Such cutters are well known in the art of recording sound waves.

One of the main advantages in recording the sound waves as frequency modulated signals resides in the fact that background noise and scratch noise is effectively eliminated during reproduction. The manner of reproducing the record will be clear to those skilled in the art. Any of the reproducing systems shown in the aforesaid Roberts application may be employed. In Fig. 2 there is schematically shown a sys tem which can be employed. In this case the record is placed on the usual turntable, and there is employed a record pickup device which feeds the output thereof to a potentiometer 50. The amplifier 5I amplifies the frequency modulation signal energy, and the amplified signal energy is fed to a tube 52 which has its cathode 53 connected to ground through the resistor 54.

The output circuit of amplifier 52 is a resonant circuit 55 tuned to the mean frequency of 15 kc.', and it is shunted across the cathode resistor 54. The main advantage of using tube 52 as a cathode follower lies in the low resistance of resistor 54, which means that the primary circuit 55 may be of low impedance thus requiring smaller components and resulting in 'a higher "62 circuit. The numerals 56 and 51 designate a pair of resonant circuits which are respectively tuned to opposite sides of the mean frequency value of 15 kc., and the opposite mistuning is by equal frequency values. The junction of the resonant circuits 56 and 51 is connected to the midpoint of an output resistor 58, the lower end of which is grounded and the equal sections of which are shunted by high frequency bypass condensers. The diode rectifiers 59 and 60 are connected in opposed relation as shown. ,It will be recognized by those skilled in the art that the discriminator-rectifier circuit is one which is well known in the art. There will be developed across the resistor'58 the audio wave voltage which was originally applied to the grid 24 of tube 23. Indeed, this type of discriminator-rectifier network is so well known that it need not be described in any further detail.

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 organizationsshown and described, but that many modifications maybe made without departing from the scope of my invention, as set forth in the appended claims.

- What I claim is:

1. In a sound recording system of the type wherein the sound waves are employed to frequency modulate superaudible oscillations, the combination, which comprises a pair of electron discharge tubesarranged in an inductance-free regenerative feedback circuit to produce said 0 oscillations at a predetermined frequency and at constant amplitude, and means responsive to ap plied sound waves for deviating the frequency of said oscillations to a maximum value of the order of 2 to 3 kilocycles on either side of said predetermined frequency.

'2. In a sound recording system of the type wherein the sound wavesare employed to frequency modulate oscillations, the combination which comprises a pair of electron discharge devices arranged in resistance-capacity regenerative feedback circuit free of inductance to produce constant amplitude oscillations at a predetermined frequency of the order of kilocycles, means including a resistor element of said feedback circuit, responsive to applied sound waves, for deviating the frequency of said oscillations to a maximum value on either side of said predetermined frequency which is a relatively large percentage of the latter value.

3. In a sound recording system of the type wherein audio frequency waves are employed to frequency modulate superaudible oscillations, the

a combination which comprises a pairof electron discharge tubes arranged in a regenerative feedback circuit to produce said superaudible frequency oscillations of a predetermined frequency, said feedback circuit being free of inductance and consisting solely of series capacity and shunt resistance chosen to provide said frequency, means responsive to applied sound waves for deviating the frequency of said oscillations to a maximum value on,either side of said predetermined frequency which is a relatively large percentage of the latter value, and said last means comprising an electron discharge device in circuit with a resistance element of said feedback circuit.

4. In a solund recording system of the type wherein sound waves are employed to frequency modulate oscillations, the combination which comprises a pair of electron discharge tubes, a resistance-condenser network free of inductance providing a feedback circuit between the cathode of one tube and the input electrode of the other of 15 kilocycles, and means responsive to applied sound waves for deviating the frequency of said oscillations to a maximum value of the order of 3000 cycles on either side of'said predetermined frequency.

'5. In a sound recording system, a source of oscillations comprising a pair of electron discharge tubes, means coupling the output elec- 'trode of one of the tubes to an input electrode 6.'In a sound-recording system, a source of oscillations comprising a pair of electron discharge tubes, means coupling the output electrode of one of the tubes to an input electrode of the second tube, said second tube including a cathode load consisting of a resistor-condenser network free of inductance, said load havin an output connection to an input electrode of the first tube and functionin as a regenerative feedback path, a third electron discharge tube having its space current path in series with a resistor of said feedback path, a source of audio frequency wave energy connected to an electrode of the third tube a point on said cathode load. a fourth electron discharge device having its space current path in series with the space current path of the third tube, and a source of direct current for energizingsaid third tube and said fourth device.

CHESTER M. SINNETT.

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