US2388578A - Record reproducing system - Google Patents

Description

Nov. 6, 1945.

, c. M. SINNETT ET AL RECORD REPRODUCING SYSTEM Filed Jah. 5, 1944 Orr/41.470?

8 AT ToRNEY Patented Nov. 6, 1945 2,388,578 nncoim nnraonucmc SYSTEM Chester M. Sinnett, Westmont, and Rene Snepvangers, Haddon Heights, N. 1., assignors to Radio Corporation of America, a corporation of Delaware Application January 5, 1944, Serial No. 517,010

9 Claims.

reproduceraand more particularly to a phonograph system which does not employ a conventional form of tone arm. This application is a continuation-in-part of our application Serial No. 465,852, filed November 17, 1942.

Sound records have been reproduced in the past by placing the record on a motor-driven turntable which had operatively associated therewith an electrical pickup device supported by a so-called "tone" arm. The pickup device is usually provided with a finely-pointed stylus or needle which rides in the record grooves. Various types of pickup devices have been used, viz; magnetic, crystal, capacitative. Various electrical networks have been used to amplify the voltage output of the pickup. For example, in the co-pending application of C. M. Sinnett, Serial No. 459,375, filed September 23, 1942, there is shown a system wherein the pickup device is capacitative, and acts to frequency modulate a high frequency oscillator in accordance with pickup capacity variations. In the Sinnett system the frequency modulated oscillations are detectedby a simple form of mistuned diode rectifier.

The design of the pickup support arm involves many problems. Typical support arms are provided with two pivoting points. The entire mechanical system of the arm must be carefully designed, as the pickup must be supported without excessive pressure on the record. Furthermore, it is practically impossible to eliminate altogether swinging and torsional tone arm resonances in conventional tone arms. Avoidance of the use of the conventional form of pickup support arm obviates the need for solving such mechanical design problems. In our aforesaid parent application we disclose and claim a phonograph record playback system wherein a "tone arm, or pickup support arm, is completely eliminated, the pickup device being characterized by the fact that it is entirely supported on the record disc.

An important object of our present invention is to improve phonograph record playback systems by employing an ultra-high frequency oscillator in a circuit for converting small capacity changes, corresponding to motional displacements of a stylus, into audio frequency voltages.

Yet another object of our invention is to provide sound record reproducing equipment which consists essentially of a pickup and supporting means connected with a voltage utilizing net- Our present invention relates to sound record work by means of a transmission line, the utilizing network comprising an oscillator having a predetermined degree of coupling to the output of the transmission line, and a discriminatordetector being coupled to the oscillator and/0r transmission line.

Still other objects of our invention are to improve generally the simplicity and efliciency of sound record reproducing systems, and more especially to provide reliable and economical devices and circuits for use in such systems.

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

In the drawing:

Fig. 1 shows the pickup device in a random position on the record face while the turntable is at rest, and the reproducing circuit is connected to the pickup,

Fig. 2 shows the initial and final positions of the pickup device on the record face during play- I back,

Fig. 3 shows a modification of the reproducing circuit.

In Fig. 1 of the drawing. we have shown one embodiment of our invention wherein the usual and conventional motor-driven turntable is schematically represented. On the turntable is shown a phonograph record I of any customary type; the manner of cutting of the record grooves is not important to the present invention. The pickup itself may be of any well known type. Where the output voltage, provided by the me chanical motion of the finely-pointed stylus riding in the record groove, is to be amplified by the use of the aforementioned Sinnett system. the pickup device may be of the capacity type. Of course, the pickup may be of the magnetic or crystal types, if desired.

In the present application let it be assumed that the pickup unit is designated schematically by numeral 2, and that it is inserted within a circular opening provided in the support 3. The pickup unit 2 may be constructed in accordance with the teachings of C. M. Sinnett in his application Serial No. 414,305 filed October 9, 1941. In general, such a pickup unit consists of a finelypointed stylus or needle 4. which rides in, or scans.

the grooves provided in the face of .the record. In actual construction the stylus 4 may constitute one plate of a condenser of small size or may be connected for conjoint movement with the condenser plate. The fixed plate of the condenser is spaced within unit 2 from the stylus electrode, and the movable electrode may take the form of a metallic ribbon operated by the stylus 4. The numeral denotes the fixed or "hot plate of the condenser. It is not necessary to show the details of the pickup unit interior, since the last mentioned Sinnett application clearly discloses them. Furthermore, since the present invention is independent of the specific construction of the pickup unit, the details of the latter are omitted.

Instead of using a conventional tone arm, and its finely-balanced construction, the supporting means 3 is utilized in place thereof. This support 3 generally is a metallic mass which may have any desired configuration. Specifically, and merely by way of illustration, the shape of the support 3 is shown as being substantially L- shaped. The L-shaped support 3 is preferably thin relative to its width. The lower face of the section 6 is provided with a member I which is in direct contact with the record face, and which is preferably composed of a fibrous material, such as felt. The felt member i which may have the shape of section 6, functions to pull the entire pickup and support along to the extent permitted by wires 8 and 9, thus allowing the stylus 4 to trace the modulated grooves accurately. The felt pad will not tend to grip the record face, because the weight of the support is distributed over the large surface of the pad. Its thickness should be such that when the stylus point is positioned in a groove the lower face of the pad will contact the record face at all points. Element 1 is, therefore, in frictional and sliding contact with the record face.

The metallic support 3 can be of any metallic composition. We have found brass or copper to be most suitable. However, the mass of the support should be suflicient and so distributed as to exert proper pressure on the stylus 4. The support weight, in general, should be so chosen that its vertical inertia is low whereby it can follow an uneven record surface. Its horizontal inertia should be sufiiciently low to minimize pressure on the groove sidewall and to avoid groove jumping, but not so low as to allow low frequency vibrations to be set up. Of course, materials may be used which are different from those disclosed herein. The present system has the advantage of eliminating swinging and torsional tone arm resonances, which are practically impossible to eliminate with conventional tone arms,

In Fig. 2, which is a plan view of the record, there are shown the initial and final positions of the support 3 during playback of a record. Assuming that record 1 in Fig. 2 is being rotated by the turntable at proper speed, the support 3 is positioned as shown by the solid lines. In this position the stylus 4 will be located in the usual initial position. The support will travel radially towards the center of the record to the position depicted by the dotted lines of Fig. 2.

The tangential motion of the support 3 is restricted by a pair of twisted conductors 8 and 5 as shown in Fig. l. One-end of the conductor 8 is connected to the support 3, while the opposite end is connected to one terminal 8 of a tuned link circuit. The latter consists of coil ID in shunt with condenser II. The coil III has but a single loop, if, as is preferred, the operating frequency is chosen within a range of 100 to 500 megacycles (mc'.). The stylus 4 is to be understood as being in electrical contact with the support 3 whereby the stylus electrode is electrically connected to terminal 8'. The second condutor 9 has one end thereof connected to fixed electrode 5 of the pickup, while its opposite end is connected to terminal 9' of tuned circuit lO-l I. The conductors 8 and 9 may be a pair of twisted insulated wires. They are looped about a supporting post l2 so that tangential motion of the support 3-is restricted. Preferably, the transmission line 8-9 is composed of fine wires. The line a-9 is preferably less than one quarter of a wave length long at the operating frequency.

High frequency oscillations are provided by an oscillator circuit which comprises the tube It acting as an oscillator tube. The tube l3 may be, for example, a triode of the 6J5'type. The circuit shown operatively connected to tube 43 is well known, and is employed to generate ultrahigh frequency oscillations. The resonant tank circuit comprises an inductance I4 which preferably consists of a single loop of wire. One end of the loop is connected to the plate l5, while the other end of 'the loop is connected to grid l5 through the direct current blocking condenser ii. The midpoint of loop I4 is connected to the +3 terminal of a direct current energizing source, while the cathode I8 is grounded. The grid side of condenser I1 is connected to ground through grid leak resistor IS. The inductance of wire H is resonated by the distributed capacity thereof to the operating frequency of the oscillations. Other mechanical arrangements such as plates, cups, etc., may be used for the U. H. F. (ultra-high frequency) resonant circuits as may be common in U. H. F., or micro-wave work.

While a frequency of 200 mc. can be used as an example, it is to be clearly understood that any other frequency in the stated megacycle range may be employed. Moreover, frequencies as high as 2,000 mc., or frequencies lower than 100 mc., may be employed without departing from the scope of my invention. Oscillator circuits for generating frequencies of the order of 2,000 mc. are known. It will be understood that the oscillator circuit generates the ultra-high frequency oscillations with substantially constant amplitude.

A discriminator-detector circuit is provided, and the circuit comprises a loop conductor 20 which is shunted by a tuning condenser 21. Of course, the distributed capacity of conductor 20 may be employed to resonate conductor 20. The resonant frequency of circuit 20--2l is chosen so that it is located above or below the normal frequency of tank circuit H. For example, circuit Zll-Zl may be so tuned thatthe mean frequency of the oscillator tank circuit is approximately upwards on one side of the slope of the resonance curve of the circuit 2ll2l. In other words, the resonant circuit 20-2! is slightly mistuned with respect to the frequency of the oscillations generated by the oscillator circuit. The circuit Hl0, tuned to the frequency of oscillator tank circuit II, is coupled to circuit 20-2i, and will transfer to the latter any ultrahigh frequency energy which exists in the link circuit |0l I.

Since the capacity of the pickup is connected across the tuned link circuit Ill-l l, variations in the magnitude of the pickup capacitance will will be essentially frequency modulation.

close coupling.

lations developed therein by virtue of the couvary the frequency of the tuned link circuit in accordance with the modulation sound grooves of the record. In other words the recorded sound waves are converted into capacity variations, and the latter are transformed into frequency variations of circuit I -l|. However, circuits Ifl--H and 20-2l are in the field oi the oscillatory energy radiated by the oscillator. Hence, the ultra-high frequency energy absorbed by circuit lfl-ll is modulated in accordance with the frequency variations of that circuit.

It. will, therefore, be seen that there is produced in circuit l0ll modulated high frequency energy. and this modulation is in accordance with the sound waves which were recorded on the record. Since the circuit 2ll2| is mistuned relative to the mean frequency of the modulated high frequency energy, there is provided a discrimination action in the event frequency modulated oscillations are applied thereto.- In the latter case there is secured a transformation of the .frequency modulated signal energy into correspondingamplitude modulated signal energy. The diode rectifier 30 is provided to rectify the amplitude modulated signal energy, and thus furnish the audio voltage which is reproduced to produce the originally recorded sound waves.

The anode 3| of rectifier 30 is connected to one terminal of condenser 2|, while the cathode 32 is connected to ground. The opposite terminal of circuit 20-2! is connected to ground by a load resistor 40, which is bypassed by condenser 4| for radio frequencies. The audio voltage developed across resistor 40 may be transmitted to a following audio frequency amplifier through the coupling condenser 50.

In considering the operation of the circuit there are two possibilities of adjustments. In the case where the oscillator circuit l3-l9, link circuit l0l|, and discriminator including circuit 20-2l are all mounted on one chassis with some common couplin to the oscillator the operation In other words, let it be assumed that oscillator tank circuit I4 is relatively closely coupled to the link circuit l0l l. Under these conditions, and with the link circuit tuned approximately to the operating frequency of 'tank circuit l4, frequency changes of the link circuit will react on the oscillator circuit. It has been previously explained that capacity changes in the pickup unit will affect the resonant frequency of circuit l0ll. These frequency changes will in turn cause a frequency modulation of the oscillations produced in the oscillator tank circuit by virtue of the Link circuit IO-H affects the reactance of oscillator tank circuit l4 when the link circuit is sufficiently closely coupled to the tank circuit thereby causing the aforesaid frequency modulation effect.

At the same time if the tuning of circuit l0l I is to one side of the steady-state oscillator frequency some change in the amplitude of the tank circuit current induced in circuit Ill-4 I will take place. That is to say, the oscillator current flowing in circuit IlI-I l, and induced therein by virtue of the link circuit being in the field of the oscillator circuit, will be amplitude modulated by virtue of the frequency changes occurring in the link circuit. The discriminator circuit 20-21 has been assumed closely coupled both to the link circuit and to the oscillator tank circuit.

Accordingly circuit Zn-2|, by virtue of its mistuned condition, will first of all function as a discriminator for the frequency modulated oscilpling between circuit Ill-2| and tank circuit l4. These frequency modulated currents will, of course, be translated to a considerable extent into corresponding amplitude modulated currents. At the same time the amplitude modulated currents developed in the link circuit itself are transferred to circuit 20--2l. Hence,'there will be applied to the rectifier 30 the amplitude modulated currents from both actions. Even when rectifier input circuit 202l is mistuned, the rectifier is still perfectly capable of detecting amplitude modulated energy. As a consequence there will be produced considerable audio voltage for amplifica tion by the following audio amplifier.

The frequency modulation of the currents flowing in tank circuit M will predominate the closer the coupling between the link circuit and tank circuit l4, and the less the coupling between the link circuit and circuit 20-2fl. In Fig. 3 we have shown the ultimate case where the link circuit is very tightly coupled to the tank circuit l4, and the link circuit has substantially zero coupling to the detector input circuit. In that case there will be solely frequency modulated oscillations induced in the detector input circuit from tank circuit M.

Consider now the condition where the tank circuit M is removed to some distance, say two feet, from the link circuit. In such case no reaction will be apparent between the link circuit and the tank circuit. With circuit l0--Il tuned to one side of the normal oscillator frequency, and with circuit 2ll--2l'tuned to the frequency of the link circuit, audio output voltage is developed by virtue of changing the constants of the link circuit due to the pickup capacity changes. In this case the action of the link circuit and the detector input circuit appears to be largely one which is characteristic of amplitude modulation. This follows from the fact that oscillator current induced in the link circuit is amplitude modulated due to the frequency changes occuring in the link circuit in response to the record groove modulation. These amplitude modulated currents will be induced in the detector input circuit 20--2I by virtue of its close coupling to the link circuit.

' While we have indicated and described systems for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular circuits shown and described, but that many modifications may be made without departing from the scope of our-invention, as set forth in the appended claims.

What we claim is:

1. A method of reproducing sound records which includes generating high frequency oscillations, absorbing oscillatory energy in a resonant circuit located in the field of said oscillations, varying the frequency of the last named resonant circuit in accordance with the recorded sound waves thereby to provide amplitude modulated oscillator energy, absorbing the amplitude modulated energy from said resonant circuit in a second resonant circuit, and deriving from the sec- 0nd resonant circuit audio voltage corresponding to said recorded sound waves.

2. In a system wherein sound is reproduced by translation thereof into corresponding reactance variation; the improvement comprising means for generating high frequency oscillations, a first tuned network closely coupled to said means for generating high frequency oscillations, said first tuned network being adapted to be varied in resonant frequency in accordance with said reactance variations thereby to produce frequency modulation of said high frequency oscillations, a rectifier provided with a tuned input circuit substantially free of coupling to said first tuned network, said rectifier input circuit being constructed to function as a discriminator for frequency modulated oscillatory energy absorbed therein.

3. A method of reproducing sound records which includes generating high frequency oscillations, absorbing the oscillatory energy in a resonant circuit normally tuned to the frequency of said oscillations, varying the frequency of the last named resonant circuit in accordance with the recorded sound waves thereby to provide amplitude modulated oscillatory energy, also frequency modulating the high frequency oscillations in accordance with the recorded sound waves, absorbing the amplitude modulated energy from said resonant circuit together with frequency modulated energy in a second resonant circuit which is off-resonance relative to said oscillatory frequency, and deriving from the second resonant circuit audio voltage corresponding to said recorded sound waves. v

i. In combination, a transducer including a reactance having a mobile electrode, means for generating high frequency oscillations, a circuit tuned substantially to the frequency of said oscillatio'ns, said tuned circuit being located in space at a sufilcient distance from the oscillation generating means to prevent any reaction between the tuned circuit and said generating means, said tuned circuit being located in the field of said generating means so as to absorb oscillatory energy, means connecting said transducer reactance in circuit with said tuned circuit whereby said mobile electrode is adapted to vary the resonant frequency of the tuned circuit in accordance with motion thereof thereby to produce amplitude modulation of said absorbed oscillatory energy, a rectifier including a tuned input circuit resonant to the frequency of said first tuned circuit, and said rectifier input circuit being sufilciently coupled to said first tuned circuit to derive therefrom said amplitude modulated oscillatory energy.

5. A method of reproducing sound records which includes generating oscillations of a frequency chosen from a range of 100 to 500 megacycles, absorbing oscillatory energy in a resonant circuit located in the field of said oscillations, varying the frequency of the last named resonant circuit in accordance with the recorded sound waves thereby to provide amplitude modulated oscillatory energy of the frequency of said oscillations, absorbing the amplitude modulated energy from said resonant circuit in a mistuned second resonant circuit, and deriving from the second resonant circuit audio voltage corresponding to said recorded sound waves.

6. A method of translating recorded sound waves into electrical waves which includes the steps of translating the recorded waves into reactance variations, translating the reactance variations into corresponding frequency changes,

generating high frequency oscillations of a predetermined normal frequency, subjecting said oscillations to said frequency changes in a manner to produce a combination of frequency modulated oscillations and amplitude modulated oscillations, maintaining said frequency modulation of the oscillations predominant, deriving from the frequency modulated oscillations corresponding amplitude modulated oscillations, and rectifying the first named amplitude modulated oscillations and the derived oscillations thereby to produce said electrical waves.

7. A method of translating recorded sound waves into electrical waves which includes the steps of translating the recorded waves into reactance variations, translating the reactance variations into corresponding changes in the resonant frequency of a tuned circuit, generating high frequency oscillations of a predetermined 'normal frequency, subjecting said oscillations to frequency changes resulting from changes in the resonant frequency of the tuned circuit, absorbing the high frequency oscillations into the tuned circuit at amplitudes varying with changes in the resonant frequency of the tuned circuit to produce a combination of frequency modulated oscillations and amplitude modulated oscillations, deriving from the frequency modulated oscillations corresponding amplitude modulated oscillations, and rectifying the first named amplitude modulated oscillations and the derived amplitude modulated oscillations thereby to produce said electrical waves.

8. In combination, a first tuned circuit, means responsive to reactance variations of a predetermined pattern for varying the frequency of the tuned circuit, an oscillator adapted to produce oscillatory energy of a predetermined frequency, said oscillator being spatially related to the tuned circuit to have a characteristic thereof affected by said frequency variation, a second tuned circuit spatially related to said oscillator so as to be affected by the oscillatory energy, said second tuned circuit being 'sufiiciently different in frequency from the oscillator frequency to act as a discriminator, and means for rectifying the high frequency currents across'the second tuned circuit.

9. A method of translating recorded waves into electrical waves which includes the steps of translating the recorded waves into capacitance variations, translating the capacitance variations into corresponding frequency changes, generating high frequency oscillations of a desired frequency,

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