US2304633A

US2304633A - Electric recording and reproducing system

Info

Publication number: US2304633A
Application number: US319111A
Authority: US
Inventors: Philo T Farnsworth
Original assignee: Farnsworth Television and Radio Corp
Current assignee: Farnsworth Television and Radio Corp
Priority date: 1940-02-15
Filing date: 1940-02-15
Publication date: 1942-12-08
Anticipated expiration: 1959-12-08

Description

8, 1942. P. T. FARNSWORTH ELECTRIC RECORDING AND REPRODUCING SYSTEM Filed Feb. 15, 1940 FIG.]

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INVENTOR Patented Dec. 8, 1942 2,304,633 ELECTRIC RECORDING AND REPRODUCING SYSTEM Philo T. Farnsworth, Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation, a corporation of Delaware Application February 15, 1940, Serial No. 319,111

13 Claims.

This invention relates to systems for recording and reproducing signals.

Electric recording and reproducing apparatus such as is conventionally employed embodies various undesirable features. One of the inherent features of such apparatus is the reproduction of extraneous or background noises which substantially anatrromahe"quamyand fidelity of the reproduced recordings. Moreover, such apparatus has been subject to various other limitations effecting not only the quality and fidelity of the reproduction but the length of programs which may be recorded on a record of given size.

In conventional phonograph disc recording and reproducing apparatus, for example, it is customary to produce recordings of sound upon a wax record by inscribing a continuous trace thereon. This trace is produced by the motion of the wax record in conjunction with the movement of a recording stylus or cutter in accordance with electrical signals into which the sound is converted. In the reproduction of the recorded sound, a reproducing stylus or needle is caused to follow the trace of the recording stylus and the motion of the reproducing stylus is utilized to produce electrical signals which are then converted into sound. In such a system, the recording as well as the reproducing styli are at rest when no impressions or signals are 7 being recorded or reproduced. Only the presence of a signal produces motion of the stylus.

The grooves inscribed by a recording stylus upon a wax record, which serves as a master for the manufacture of other identical records, are not as smooth as would be desirable, and a certain background noise, which is commonly referred to as needle scratching is usually present, when reproducing the recorded impressions.

In recording signals upon a physical medium, such as a wax disc record, the recording stylus is usually deflected to either side of a predetermined trace in accordance with the amplitude and frequency of the signal to be recorded. This method of recording is commonly referred to as lateral recording whereby the width of the grooves inscribed by the recording stylus varies in accordance with the amplitude of the signal being recorded. The recording stylus is guided along a spiral path on a plane-wax record. The length of the recording is determined by the number of spirals which can be placed upon a record of a given diameter. The number of grooves, however, is determined by the Width of the individual groove which in turn is determined by the maximum deflection of the recording stylus due to the greatest intensity of the signal to be recorded. Hence, it is evident that for weaker signals a large amount of the recording area is not utilized.

The object of the present invention is to provide a new and improved method of and system for recording and reproducing impressions upon a physical medium, which system will be free from the limitations and undesirable character istics of the systems of the prior art above referred to.

In accordance with the present invention, there is provided a system for recording and reproducing impressions of audible and visible nature by the agency of electrical signals corresponding thereto. The system includes a means for developing a carrier frequency signal whose frequency is varied in accordance with a modulation signal representing the impressions to be recorded. Means are provided to vary the characteristics of a physical medium in accordance with the modulated carrier frequency signal to produce a record of these impressions. The recorded impressions are reproduced by a system comprising a means responsive to the varying characteristics which reproduces the frequency-modulated carrier signal. A detector is provided for detecting the carrier signal and the detected signal is applied to suitable means for reproducing the recorded impressions.

For a better understanding of the invention, together with other and further objects thereof, reference may be had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims,

In the accompanying drawing, Fig. 1 is a schematic diagram of a recording system, while Fig. 2 is a similar diagram of a reproducing system, both systems embodying the present invention and together representing a system of recording and reproducing impressions of audible nature.

Referring now more particularly to Fig. 1 of the drawing, there is here shown schematically a system for recording audible impressions comprising a microphone l of conventional construction adapted to convert audible impressions into electrical signals which are applied to the control grid of an electron discharge tube 2.

For the purpose of generating a carrier frequency signal, there is provided a generator of preferably asymmetrical electrical oscillations comprising an oscillator tube 3 having a grid circuit and an anode circuit as shown. An anode voltage supply battery 4 in series with an inductance element 5 is connected in the anode circuit of the tube 3. The grid circuit includes an inductance element 6 coupled to the inductance element 5 and a resistor 1 connected in series therewith. In order to effect frequency modulation of the oscillations generated with the aid of the tube 3, the tube 2 is connected in parallel with the inductance element 6 and the resistor I so as to provide an effective impedance in the oscillator circuit which is variable with variations in the voltage impressed upon the control grid of tube 2.

The anode of oscillator tube 3 is capacitively coupled to the control grid of a vacuum tube ID by means of a condenser 8 and a resistor 9. The tube I is operated as a limiting device for the purpose of providing an output signal of constant amplitude. A resistor connected in parallel with a condenser I2 is provided in the cathode circuit of tube I0, in order to produce the proper operating bias potential for this tube.

For the purpose of developing from the asymmetrical oscillations, whose frequency is modulated in accordance with the modulation signals which are supplied from the tube 3, sinusoidal oscillations similarly modulated, there is provided in the anode circuit of the tube Ill a parallel resonant circuit l3 comprising an inductance element l and a condenser |4 tuned to the fundamental frequency of the oscillations supplied by the oscillator tube 3.

In order to record the frequency-modulated carrier signal, there is provided a suitable recording mechanism which may be of any conventional construction. For example, as shown in Fig. 1, a physical medium such as a base in the shape of a wax disc may be disposed on a turntable 2| driven by a suitable motor 23, by Way of a shaft 22 and a suitable mechanical filter 24. The record 20 is thereby rotated about its axis at a substantially uniform speed. A cutting stylus 25 is supported in a cutting head 26 which is associated with a screw 21 by means of which the stylus is moved across the disc 20 in substantially a radial direction. The screw 21 is driven by the motor 23, being connected therewith by means of a gear train indicated generally at 28. The circular motion of the record 20 and the radial motion of the stylus 25 cause spiral grooves to be cut into the surface of record 20.

In order to obtain a lateral recording of the impressions to be recorded, there is provided in the cutting head 26 a driving system of any conventional type comprising, for example, a permanent magnet and a coil system, not shown, which is inductively coupled to the resonant circuit |3 by means of an inductance element II.

In Fig. 2, there is shown schematically a system for reproducing audible impressions recorded upon a disc 33, which, for example, may be a record made by the apparatus of Fig. 1. The reproducing mechanism comprises a turntable 3| rotatably supporting the disc 33 and driven by a motor 32 at a predetermined speed.

For the purpose of converting into electrical signals the frequency-modulation characterics of the groove cut in the disc, there is provided a stylus or needle 34, inserted in a conventional pickup device generally designated at 35 and preferably comprising a crystal arrangement housed within a ton arm 36. Such devices being well known in the art, it is deemed unnecessary here to provide a detailed explanation thereof. The electrical signals produced by the pickup device are conducted by means of conductors 3T, 31 to the primary winding of a matching transformer 38. One side of the secondary winding of the transformer 38 is grounded while the other side may be connected to a switch 39 having two poles A and B as shown. Pole A is capacitively coupled to a parallel resonant circuit 4| comprising an inductance element 42 and a condenser 43, tuned to a frequency differing by a predetermined amount from the frequency-modulated carrier frequency. Preferably the resonant circuit is sharply tuned to a frequency higher than the carrier frequency.

For the purpose of detecting the modulation signal, there is provided a vacuum tube detector 44 having its control grid circuit connected across the resonant circuit 4|. The cathode of the tube 44 is connected to ground by way of a resistor 45 connected in parallel with a condenser 46, across which arrangement the proper bias potential is developedfor operation of the tube as a detector. The anode circuit of tube 44 includes an output resistor 4'! across which the detected signal appears, and a source of operating anode voltage as represented by the battery 48.

For the purpose of amplifying the detected signal, there is provided an amplifier tube 5| capacitively coupled to the output resistor 41 by way of a coupling condenser 49 and a grid leak resistor 53. Proper biasing potential is obtained for the tube 5| by the provision of a suitable cathode resistor 52 by-passed by a condenser 53 as shown. The anode circuit of tube 5| includes the primary winding of a transformer 54, across which the amplified signal is developed, connected in series with a suitable source of anode operating voltage or battery 55. The secondary winding of the transformer 54 is connected to the voice coil 5'! of a loud speaker 58 comprising the voice coil and a membrane 59 by means of which the recorded audible impressions are reproduced.

In order to make the system just described also applicable to the reproduction of impressions recorded on discs in the conventional manner, there is provided a conductive connection between the pole B of switch 39 and the control grid of the amplifier tube 5|. With this arrangement, by simply throwing the switch from the position shown in the drawing to that indicated by broken lines, the tube 44 and its associated circuits are disconnected from the system and a substane tially conventional electrical phonograph is provided.

Referring now more particularly to the operation of the recording device shown in Fig. 1, audible signals are converted into electrical sig. nals by means of the microphone I, which signals are applied to the control grid of the tube 2. The oscillator tube 3 in conjunction with inductance elements 5 and 6 and resistor 1 disposed in its plate and grid' circuits respectively generates asymmetrical oscillations of a predetermined frequency. Since the discharge space of the tube 2 is connected in parallel with the series combination of the inductance elements 6 and 1 and, since the operating characteristics of the tube' 2 change in accordance with the signals applied to its control grid, the tube 2 and its associated elements function as an impedance in the oscillation circuit whose value is varied in accordance with the voltage applied to the grid of tube 2. The frequency of the oscillations developed by the generator 3, 5, 6, and I is thus varied in accordance with the signal supplied to the tube 2.

The frequency-modulated asymmetrical oscillations are supplied, by way of the coupling-condenser 8 and grid leak resistor 9, to the amplitude limiting tube I0. The operating bias developed across the parallel combination of the resistor II and the condenser I2 is of such value that the tube becomes saturated if the signals applied to its control grid exceed a certainpredetermined amplitude. The use of asymmetrical oscillations rather than sinusoidal oscillations is preferable since a sinusoidal output signal of constant amplitude and predetermined fundamental frequency can be more readily produced by amplitude limitation of an asymmetrical input signal having the same fundamental frequency, than by amplitude limitation of a sinusoidal input signal of the same frequency. Hence input signals comprising impulses of varying frequency and having undesirable fluctuations in amplitude applied to the limiting tube l0 produce output signals of constant amplitude and sinusoidal wave shape in the tuned circuit I3. This circuit is tuned sufficiently broadly to have a substantially uniform response over a range of frequency variations equal to the range over which the oscillations of the tube 3 are varied.

The signals developed across the tuned circuit |3 are applied to the cutting head 26 by means of the winding inductively coupled to the inductance element I5. The cutting head 26, being preferably of the conventional construction comprising a magnetic driving system, causes the cutting stylus 25 to be displaced in accordance with the signals applied to the cutting head 26. The wax disc is rotated and the cutting head 26 is simultaneously moved in radial direction as previously explained. In this manner, the cutter is guided along a predetermined spiralshaped trace and cuts a groove oscillating with respect to the trace with a constant amplitude and at a frequency which varies in accordance with the frequency-modulated carrier signal applied to the cutting head 26.

There is thus produced a record representing impressions to be recorded comprising a disc and a continuous spiral groove on its face, having an irregular inner surface varying in accordance with a carrier signal whose frequency is modulated in accordance with the impressions to be reproduced.

Due to the fact that with this type of signal the groove irregularities cut in the record are of uniform width and this width may be relatively small as compared with conventional records, it is possible to record programs of substantially greater length on a record of a given size than was heretofore possible.

In the operation of the reproducing device of Fig. 2, the disc 33, provided with spiral-shaped grooves having characteristics representing a frequency-modulated carrier signal, is rotated by means of the turntable 3| driven by the motor 32 at the same speed at which the recording was effected. The needle 34, supported by the tone arm 36, follows in lateral direction the path of the grooves in the well-known manner and its motion causes electrical signals to be produced by the pickup device 35. These signals are applied to the primary winding of the matching transformer 38 by means of conductors 3| guided inside the tone arm 36. The frequency-modulated carrier signal is then applied by way of pole A of switch 39 and coupling condenser 40 to the resonant circuit 4|, which, as explained above, is sharply tuned to a frequency higher than the carrier frequency. Due to the fact that the impedance presented by the resonant circuit 4| varies in accordance with the applied signal frequency, the frequency-modulated carrier signal is converted into an amplitude-modulated carrier signal which appears across the resonant circuit. Since the resonant circuit 4| is included in the control grid circuit of the detector tube 44, detection of the modulation signal is effected by tube 44 and the modulation signal appears across the output resistor 41. This signal is applied, as explained above, to the control grid of the amplifier tube 5| wherein it is amplified. The amplified signal appearing across the primary winding of the transformer 54 is induced in the secondary winding thereof and is applied to the voice coil 51 of the loud speaker 58 where it actuates the membrane 59 to reproduce the audible impressions recorded upon the disc 33.

If it is desired to reproduce recordings made in the conventional manner, the switch 39 is moved in the position to connect the secondary Winding of the matching transformer 38 to the pole B. The electrical signals derived from the pickup device 35 are then supplied directly to the control electrode of the amplifier tube 5| since such recorded signals do not require detection.

Preferably, the carrier frequency is chosen to be in the order of between 15 and 20 kilocycles for a range of modulation frequencies of between 20 and 5,000 cycles, and the frequencies to be transmitted may be either symmetrically or asymmetrically disposed about the carrier frequency in accordance with well-known principles of the art. All other frequencies can readily be prevented from actuating the loud speaker, by attenuating the same in the transmission path between the pickup device and the loud speaker. Attenuation of signals within the audible range is inherent in the use of the tuned circuit 4| as a means for converting frequency modulation into amplitude modulation, since the impedance of this tuned circuit is preferably negligible for frequencies lower than the lowest frequency which must be transmitted while this frequency is still above the audible range. If desired, a bandpass filter of conventional design may be inserted in the circuit between the pickup device 35 and the tuned circuit 4|. Since the background noise present in conventional reproductions is due to erratic vibrations of the needle 34 caused by roughness of the cut grooves and lies within the range of audible frequencies, and since such frequencies are not transmitted in the reproducing system in accordance with the present invention, all background noise is eliminated.

In the foregoing, a system for recording and reproducing impressions of oral nature has been described. It is evident, however, that impressions of visual and other nature can be likewise recorded and reproduced. Further, while the physical medium upon which characteristics representing the frequency-modulated carrier signal are recorded has been described as a disc of the conventional type used for phonographic recording, whereby the first recording is made on a wax disc serving as a master for the manufacture of other identical discs, it is obvious that other physical media can be used, such as, for example, photographic films, magnetic tape, or the like, whereby characteristics such as transparency, magnetization and the like are varied in accordance with a frequency-modulated carrier signal. It will be appreciated that all and any such arrangements are contemplated as a part of the present invention.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modi- 'fications as fall within the true spirit and scope of the invention.

I claim:

1. A system for recording and reproducing impressions comprising recording apparatus including means for developing a carrier-frequency signal, means for developing a modulation signal representing impressions to be recorded, means for varying the frequency of said carrier-frequency signal in accordance with said modulation signal, means for recording on a physical medium characteristics representing said frequency-modulated carrier signal, and reproducing apparatus including means responsive to said characteristics for deriving said frequency-modulated carrier signal, means for deriving said modulation signal from said carrier signal, and means for utilizing said derived signal to reproduce said impressions.

2'. An electric sound recording device comprising means for developing a carrier signal of superaudible frequency, means for developing an audible modulation signal, means for varying the frequency of said carrier signal in accordance with said audible modulation signal and means for recording on a physical medium a continuous groove having characteristics representing said superaudible frequency-modulated carrier signal.

3. An electric sound recording device comprising means for developing a carrier signal of superaudible frequency, means for developing an audible modulation signal, means for varying the frequency of said carrier signal in accordance with said audible modulation signal and means for producing on a disc record said superaudible frequency-modulated carrier signal.

rier signal of sinusoidal Wave shape and substantially constant amplitude, and means for recording on a physical medium characteristics representing saidfrequency-modulated sinusoidal carrier signal.

5. An electric recording device comprising an electrical oscillator for generating a carrier-frequency signal of asymmetrical wave shape, means for developing a modulation signal representing impressions to be recorded, means for varying the frequency of said carrier signal in accordance with said modulation signal, means for deriving from said frequency-modulated carrier signal of asymmetrical Wave shape a frequencymodulated carrier signal of sinusoidal wave shape and of substantially constant amplitude, means for rotating a disc record at a substantially uniform speed, a groove-cutting device, means for moving said groove-cutting device at a substantially uniform speed to cut spiral-shaped grooves into said disc record, and means for actuating said groove-cutting device in accordance with said sinusoidal frequency-modulated carrier signal.

6. A device for reproducing impressions from a record having characteristics representing a carrier signal having its frequency modulated in accordance with said impressions, comprising means responsive to said characteristics for deriving said frequency-modulated carrier signal, means for deriving said modulation signal from said carrier signal and means for utilizing said derived signal to reproduce said impressions.

'7. A device for reproducing impressions from a record having characteristics representing a carrier signal having its frequency modulated in accordance with said impressions, comprising means responsive to said characteristics for deriving said frequency-modulated carrier signal, means for converting said frequency-modulated carrier signal into an amplitude-modulated carrier signal, means for deriving said modulation signal from said amplitude-modulated carrier signal and means for utilizing said derived modulation signal to reproduce said impressions.

8. A device for reproducing impressions from a disc record having characteristics representing a carrier signal having its frequency modulated in accordance with said impressions comprising means responsive to said characteristics for deriving said frequency-modulated carrier signal, means for converting said frequency-modulated carrier signal into an amplitude-modulated carrier signal, means for deriving said modulation signal from said amplitude-modulated carrier signal, means for amplifying said derived signal, and means for utilizing said derived signal to reproduce said impressions.

9. A device for reproducing impressions from a disc record having characteristics representing a carrier signal having its frequency modulated in accordance with said impressions comprising means for rotating said disc record at a substantially uniform speed, pickup means including a needle for deriving from said characteristics a frequency-modulated carrier signal characteristic of said impressions, a tuned circuit for converting said frequency-modulated carrier signal into an amplitude-modulated carrier signal, means for deriving the modulation signal from said amplitude-modulated carrier signal, and means for utilizing said modulation signal to reproduce said impressions.

10. A phonograph record representing sound impressions to be reproduced comprising a base and a continuous spiral groove of wave form on said base varying in accordance with a carrier signal of superaudible frequency and having said frequency modulated in accordance with signal of audible frequency. I I

11. A phonograph record representing sound impressions to be reproduced comprising a base and a continuous groove of wave form and of uniform amplitude on said base having variations on its inner surface representing a carrier signal of superaudible frequency and having said frequency modulated in accordance with a signal of audible frequency.

12. A device for reproducing impressions from records of a first type having characteristics representing an electrical carrier signal having its frequency modulated in accordance with impressions and of a second type having characteristics representing an electrical signal of modulation frequencies representing said impressions, comprising means responsive to said record characteristics for deriving an electrical signal represented by said characteristics, converting means for developing from said derived frequency modulated carrier electrical signals, a modulation signal representing the impressions to be reproduced, means for reproducing said impressions and means for selectively connecting said record characteristic responsive means either to said converting means or directly to said reproducing means for reproducing a record of said first type or said second type respectively.

13. The method of recording and reproducing impressions which comprises developing a carrier-frequency signal, developing a modulation signal representing impressions to be recorded, varying the frequency of said carrier-frequency signal in accordance with said modulation signal, recording characteristics representing said frequency-modulated carrier signal, utilizing said recorded characteristics to reproduce said frequency-modulated carrier signal, deriving said modulation signa1 from said carrier signal, and utilizing said derived signal to reproduce said impressions.

PHILO T. FARNSWORTH.