US2400953A - Method of and system for recording audio-frequency waves - Google Patents

Description

Patented May 28, 1946 UNITED STATE METHOD OF AND SY AUDIO-FREQ Indianapolis, In sil Radio Corporation of America, a corporation of Henry E. Roy! Delaware s'rssi Foa nucoanmo UnNcY waves Application September 1:, 1m, Serial No. scams its-100,0

19 Claims.

My present invention is embodied in a method of and system for cutting disc records in accordance with sound waves, and more particularly to recording systems employing negative or degenerative audio frequency feedback.

In the past, audio frequency recording systems have utilized negative or degenerative audio frequency feedback. In all such prior systems the degenerative audio frequency voltage was derived from the velocity of the cutting stylus. The derived audio frequency voltage was then applied to the input terminals of the amplifier driving the recording unit. In this way a reduction of distortion and a more uniform or flat frequency response curve was secured. Moreover, the prior feedback systems have generally employed a coil to translate the vibrations of the cutting stylus into an audio frequency voltage for the purpose of providing the degenerative feedback voltage. It has been found that undesired coupling reactions tend to arise between the cutter driving coils and the feedback coils and create diiilculties in the design of such devices. In particular, such undesired coupling reactions necessitate shielding elements thereby increasing the expense of the cutting unit and also the weight thereof.

It may, therefore, be stated that it is one of the main objects of the present invention to provide a negative feedback circuit for a recording system, wherein there is no electromagnetic or electrostatic coupling between the cutter driving coils and the audio frequency feedback elements, such prevention of coupling being secured by the provision adjacent the cutting stylus of high frequency elements from which theaudio frequency feedback voltage is derived, while the cutter driving coils operate at audio frequency.

Another important obiect of the invention is to provide a novel method of using frequency modulation to provide degenerative audio frequency voltage for improvements! sound recording; the frequency modulationcircuit employed utilizing a capacitative pickup element which may be located very near the cutting stylus or other element movable with it thereby making possible a highly accurate translation of stylus motion into audio frequency feedback voltage;

Another obiect of my invention may be stated to reside in the provision of a method of deriving audio frequency voltage, necessary for negative feedback operation of a disc recording head, from a high frequency voltage produced by a combination of a high frequency oscillator and a discriminator-rectifier, the tuning of one of which is controlled by a condenser; at least one electhe other electrode of a condenser;

of the moving system, and

connection with the trade of the condenser being preferably located near the cutting stylus without any shielding being employed and the other electrode of the condenser being constituted by the stylus holder or a member movable therewith; the mass of the moving system of the cutting head being preferably unaifected by the addition of the condenser electrode, and the discriminator-rectifier being of suitable form to convert the frequency modulated high frequency voltage to an audio frequency voltage for application to the audio ampllfler driving the recorder.

Yet another object of the invention is to improve the operation of a conventional magnetic type of record cutter, wherein the armature has a stylus holder attached to it and the armature is part of an electro-magnetio system consisting of coils. pole pieces and a permanent magnet; my improvement consisting in employing at least one condenser electrode disposed closely adjacent to the stylus holder or other member movable with the stylus holder which then functions as this condenser frequency modulating an oscillator whose output is detected and employed to provide negative feedback voltage for the input terminals of the driving audio amplifier; the condenser electrode being very email, not adding to the mass being capable of location at almost any point adjacent to the stylus holder; a a v 7 Still other objects of my invention are to improve generally the recording of audio frequency waves of any type, and more, especially to provide recordings which are substantially distortionless, free of harmonics, of a low noise level, and of a hi h fidelltm- The novel features which I believe to be characteristic of my 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 vthe following description, taken in drawin s. in which I have indicated diagrammatically several circuit organisations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 schematically represents one possible embodiment of my novel recording system,

Fig, 2 illustrates a modification of the frequency modulating condenser,

Pie. 3 shows a further modification of a frequency modulating condenser,

Fig, 4 shows a still further modification,

pieces a are spaced from the Fig. 4a is a detail taken along line 4-4 of Fig. 4, looking in the direction of the arrows,

Fig. 5 schematically represents another embodiment of the invention which utilizes so-called balanced" frequency modulation,

Fig. 5a is a detail taken along line 5-5 of Fig. 5 looklngin the directionof the arrows. V V

Referrlngnow to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 a schematic representation of one embodiment of my invention. Various wellknown units and elements employed in my im-- proved method and system may be suiiiciently described by generalized disclosures. v For example, the cutter unit per se may assume a conventional and known form. In the specific figure the cutter is shown as oi the electromagnetic ty the cutting stylus I being adapted to cut lateral recordings on a record disc 2 of any well-known form. The disc 2 will, of coursa'be rotated in the usual fashion, and the stylus I is vibrated laterally in response to audio frequency currents flowing through thecolls I and 4. a

The holder I is providedwith a sleeve 8 for securing the stylus I in position. The holder is metallic, and may be in the form of a vertical arm rigidly secured to a horizontal metallic shaft. The latter may be considered asbeing at ground potential. The shaft 1 will be supported at each end thereof by bearings, and will also be properly centered for limited motion onthe bearings. These mechanical details are too well-known to show in the figure, and are, therefore, omitted.

The driving coils ii and 4 are connected in series, or parallel. between the output terminals of the audio frequency amplifier I. The armature 9 may be of any construction desired. In Fig. 1 it is represented as being mounted on the shaft iin spaced relation to stylus arm 5. -The armature may be laminated, if desired. Coil 3 surrounds the upper tapered section of. the armature, while coil 8 surrounds the lower section of the latter. The coils are located between the pole pieces'contacting a permanent magnet a. The coils are not wound directly on the armature, but are separate fixed coils which surround the respective endsiof the armature and are spaced from it. The coils are not in the air gap field. The magnet is shown in part, since it is of emmetrical construction. The N (North) pole pieces areshown' in alignment with the armature 9. It will be understood that the "8 (South) pole armature l in. the

cemented into place, or it may be secured by a The construction thus far described is that of a conventional'form'oi'cutis a reduction in distortion accompanied by a more uniform or flat frequency responsive curve. According to my present invention. audio frequency feedback voltage is derived from the vibratory displacement of the stylus by a method employing frequency modulation.

Let it be assumed that the numeral II denotes a usual form of high frequency oscillator which is provided with asuitable" resonant tank circuit II. The low potential side of the tank circuit is indicated as being grounded, while its high potential sideisconductively connected by a lead It to a relatively fixed metallic electrode l4. This electrode I4 is insulated from ground and may be located in close juxtaposition to the arm I. It is to be clearly understood that the capacitor plate or electrode It is schematically represented. Actually it will. be very small in comparison to the dimensions of the arm 5. Since the arm I is effectively at ground potential, it will be appreciated that the arm 5 and the'electrode I4 tank circuit I2.

Merely by way of illustration, and in no way restrictive on the scope of thi invention, let it be assumed that the oscillator has a normal oscillation frequency of 30 megacycles (mc.). This means that the tank circuit I2 is tuned to a normal frequency Fe of 30 megacycles. In addition, the condenser 5- has its normal'eapaclty value when the stylus is at rest, andis in non-vibrating condition. This modulating condenser 8I4 is partof the total tank circuit capacity which with 5--l4 in its normal position determines the mean oscillatorfrequency Fe. However, upon lateral vibratory displacement of the stylus arm I, the capacity value of condenser 5-4 4 will vary in accordance with the displacement of the stylus arm. The capacity variation of the modulating condenser will, in turn, affect the frequency of circuit 12 in accordance with the instantaneous displacement of the stylus arm.

The frequency II will be deviated relative to frequency Fe in accordance with the capacity variation of the modwith respect to the total tank circuit capacitance. The apparatus designer may work out thamaxithe requirements of'the system. By wayof illustration, it may be assumed that an over-all f quency swing of kiloeycles (kc.) is permissible. The oscillator itself can be of conventional" construction.

The irequency modulated oscillation energy is applied to a subsequent network II which is a discriminator-rectifier network; J-Buch discriminator-rectiflers arewell-known in the art of frequency modulation. known forms of discrlminator-rectifiers may be used, there is shown above the rectangle II a characteristic idealized 's-shaped response curve which relates the rectified output of network I! to the frequency deviation of its input with respect to Fe. For example, there may be employed for network ii a discriminator-rectifier'oi'the Since any of many well type disclosed and claimed by S. W. Seeley in his U. S. Patent No. 2,121,103, granted June 21, 1938.

The rectified output voltage of network l5. will be or audio frequency, and will be an accurate representation of the movements of the stylus bar. These ,audio frequency currents flowing in the output circuit of network 15 are applied over the transmisnon path it, denoted as "Degenerative feedback," to the input terminals of the audio amplifier ll in a manner known to those skilled in the art, one of the known ways being shown in the U. S. patents to Wiebusch, 2,162,986, and Vieth et al., 2,161,489. It is to be understood that the feedback of the audio frequency voltagemay be made to any desired point of the audio frequency amplifier 8 preceding its output terminals.

It will now be appreciated that accordingto my present system there has been provided a method of deriving audio frequency voltage, necessary for negative feedback operation of a disc recording head, by means of a frequency modulated oscillator ll controlled by a condenser. One plate of this condenser is relatively fixed, and its other electrode is a part of the vibratory system itself. One advantage of the frequency modulation system for securing'the degenerative audio frequency feedback is that there is no electromagnetic or electrostatic coupling between the driving coils 3-4 and the modulating condenser 5-. Hence, the usual trouble arising from such coupling, where it exists, is avoided. It is emphasized that the driving coils 3 l operate at audio frequency, while the modulating condenser 5-H operates at a relatively high radio frequency. Due to the lack of magnetic or electrostatic coupling between the audio system driving the recorder and the frequency modulation system for deriving the feedback voltage, no shielding or wide separation of these parts is needed. This is a definite advantage, since the modulation capacitor plates may be located alongside of the stylus bar and near the stylus tip if desired. Such a location means, of course, that the feedback voltage is more truly representative of the stylus motion than if the parts had to be widely separated due to electrical coupling.

Another advantage the frequency modulation system is that the modulation condenser may be made very small. The inertia of the moving, i. e. vibratory, system (i. e. for example armature, shaft, and stylus bar) need not be increased by additonal parts necessary to derive the feedback voltage. The over-all mass or weight of the entire recording head is not so important, but the mass of the moving system which affects the frequency range and power required to drive the moving system is important, and in all cases should be minimized. Hence, frequency modulation as used in my invention has an advantage in that no mass need be added to the moving system. This method of deriving the feedback voltage is not limited to a disc recording mechanism, but may be applied to any transducer, such as a loud speaker, for example. Moreover, my invention may be employed to produce, if desired feedback other than negative or degenerative.

The network consisting of the oscillator H and discriminator-rectifier l may be provided by the network disclosed and claimed by C. M. Sinnett in his application, Serial No. 459,375, filed September 23, 1942, particularly Fig. l of that application. Furthermore, similar results can be obtained by using a fixed-frequency oscillator, and varying the tuning of the discriminator circuit in accordance with the stylus motion so as to produce the required amplitude modulated voltage across the discriminator, In either case, whether the oscillator frequency is modulated or whether the oscillator frequency is fixed and the tuning of the discriminator circuit varied, a high frequency voltage of varying amplitude appears across the discriminator circuit and is applied to the rectiher. It is this voltage that, when rectified, is representative of'the stylus motion. It is noted that the frequency modulation system including condenser l--ll, oscillator I I, and discriminatorrectiiier 15 produces an audio frequency output voltage which is proportional to the amplitude of displacement of'the capacitor electrodes, in contrast to electromagnetic pickup negative feedback systems in which the feedback voltage is proportional to the rate of change of flux, and hence velocity, of the armature or moving coil.

The location of the fixed capacitor plate II is subject to variation. 7 For example, in Fig. 2 there is shown a modification in which the fixed metal capacitor plate ll may be located adjacent the upper end of the stylus arm 5. In this case the upper end of the stylus arm is given an inclined face, and the inclined face then provides the 0pposite electrode of the modulating condenser.

In Fig. 3 there is shown still another method of providing the modulating condenser. 1,, this case the fixed metal plate It is located in spaced relation to the flat face of the stylus arm 5'. It will be noted that the laminated armature 2|] in this case has the stylus arm bolted directly to one end thereof. In other words, the armature 2i] and the stylus arm I are provided as a single unit on the grounded shaft 1. The coils 3 and 4 surround, as previously explained, the opposite sections of the unit as shown in Fig. 3. This form'of construction has advantages insofar as economy of production is concerned. However, the mode of operation is the same as in the case of the construction shown in Fig. 1. The fixed plate l4 wil1 be always at a constant spacing from the face of arm 5". However, the capacity variation here is due to area variation. This is accomplished by giving the lower edge of plate ll an incline. As the stylus arm vibrates the effective are of plate ll cooperating with the adjoining face of arm 5' will vary, and hence capacitative change of the modulating condenser 5'-l4' will result. This capacitance change will be employed in the same manner as described in connection with Fig. 1.

In Fig. 4 there is shown still another method of providing a modulating condenser whose capacitance variation will be a very accurate representation of the motion of the stylus bar and the stylus which is rigidly attached thereto. In Fig. 4 the pole pieces 2! and 22 are shown, but a portion of the permanent magnet 9 is broken away in order to preserve simplicity of disclosure. The stylus arm 5' and the armature III are of the type shown in Fig. 3. That is to say. the arm 5' is secured directly to the end of the laminated armature 20. The fixed driving coils 3 and 4 surround the opposite sections of the entire unit on opposite sides of the grounded shaft I. The coils 3 and 4 are not in the air gap field. The capacitor plate 30 is here provided by a metallic element rigidly embedded in a block of insulation material 3 l. The block of insulation material is. in turn, rigidly secured by bolts 32 to the front face of pole piece 2!. As shown in Fig. 4, the lower "N pole of pole piece II is largely obsoured by block 3|. It bears the same symmetrical relation to the lower "8" pole which the upp r "it" pole bears to the upper "8"pole.

The lead II to the high potential side of the osciilatortankcircuitlsrunthroughtheblock II tothemetalstriplil. ltwllibeohscrvedthat the capacitor plate It on the insulation block II prolects from the pole piece Ii in line with the lower sectionoftheetylusarm I. Thisismore clearly shown in detail in Fig. do. However, it late be clearly understood thatthe motion of the stylus armrwill not be such during its greatest excursions as to permit contact between the stylus arm and plate II. The lateral displacement of the stylus arm is highly restricted, and those skilled in the art will clearly understand that the spacing between plate I and the adjoining side ofstylus arm I' will b suniciently great to prevent any contact whatever between the two. The advantage of locating the electrode II as close as'poesible to the Mills Mills that there is derived ultimately an audio frequency voltage which is exactly proportional to the stylus motion.

It will be appreciated that the insulation block II and its associatedcapacitcr plate 'Ili. being on the fixed portion of the'cutter head. do not add m re. to the movies system, and thus do not adversely affect the recording itself. Not only is there no addition of mass to the moving system, but the mechanical phase shift distortion is minimized with this form of feedback system due to the proximity of said capacitor, to the stylus tip. with a 0.004 to 0.005 inch spacing between the arm I and the plate II, suiilcient capacity variation for the purposes of the feedbacksystemmaybesecured. Thenominaldisplacement of the stylus tip during recording in present practice is about :0.001 of an inch.

In Fig. 5 I have shown a schematic form of the system wherein so -called "balanced" frequency modulation is utilized. In this form of the sys- .tem a pair of modulation condensers is employed. One of these modulation condensers comprises a fixed capacitor plate 40 and an adjoining face of the stylus arm I. The second modulation condenaerconsists oi the capacitor-plate Ii and its respective adjoining face of stylus arm I. It will be noted that the condenser electrodes ill and ll are the projecting tips of a pair of metallic strips which are risidiy embedded in an insult.- tlon block I0. These metallic strips 40 and Ii are located on opposite sides of the upper end of the stylus arm I in front of the respective air gaps of the magnetic system. The stylus arm construction may be precisely as, shown in Fig. 4, except for the fact that there is omitted the capacitor plate III of Fig. 4. Fig. 5a shows more clearly the relation between the strips I, II, and the stylus arm 6. In this embodiment the oscillator it has its tank circuit I'I shunted by a'flrst condenser consisting of electrode 40 and the stylus arm I. and the normal frequency Fa of the oscillator will therefore be varied in accordance with the capacity variation of modulation condenser lI-I. The second modulation condenser ll-l' in this case varies the tuning of the tuned input circuit I2 oi the discriminator-rectifier. The specific discriminator-rectifier shown comprises a diode rectifier 00 whose output load Ii, bypassed for high frequency currents in the usual manner, is arranged in series with the resonant circuit If,

The resonant circuit I: is mistuned with respect to Fe; 1. e., its resonant frequency is always on the same side of the oscillator frequency and 7s suillcientiy close to the oscillator frequmcy so that'the latter falls on theresonanee curve of circuit. The frequency modulated output of the Ills applied to'themisnmedlnput circuit II. It will be recognlaedthat this provides a very simple form of discriminator circuit, and that the frequency modulated oscillations are readily converted into. amplitude modulated osemoti n by such a mistuned input circuit. Concurrently with the 'frequency modulation of oil'- cult if, there is provided a tuning variation of the discriminator circuit II in acordance with the vibration of the stylus bar I. This concurrent tuning modulation of the discriminator circiut II is in'oppoeite phase to the effect of the frequency modulation'of circuit 12 in that when the frequency of the oscillator output is increased, the resonance frequency of circuit II is decreased and vice verse. gin this connection it is pointed out that this balanced, or eiiective push-pull, form of frequency modulation of the oscillator and discriminator has been described and claimed hyA. Badmaiel! in application Serial No. 490.614 iiled June' 12, 1943, now Patent No. 2,371,373, granted March 13. 1945. For this reason, it is not believed necessary to describe the functioning. of the balanced frequency modulation circuits in detail.

It is suihcient to point out that the audio frequency voltage developed across resistor Ii, and applied through the audio frequency coupling condenser III tothe degenerative feedback path, is substantially double the magnitude which would be secured when employing a single modulation condenser asshown in Fig. 1. Furthermore, the active electrode area of strips ll and II may be very small, since the resultant audio frequency voltage produced by this balanced frequency modulation is double that produced by frequency modulation of solely the oscillator tank circuit II. No claim is made in this application to the balanced or push-pull form of frequency modulation when used broadly in conjunction with transducers. This application, however, contains claims for the particular application of the push-pull frequency modulation circuit in providing degenerative audio frequency voltage in a recording system, as illustrated, for example, in Fla. 5.

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 organisations 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: 7

i. In a. method of recording sound waves wherein the sound waves are translated into audio currents which are amplified, and the ampiifled audio currents are translated into dis-- placements of a record-cutting stylus, the improvement which includes translating the cutting stylus displacements into corresponding capacity variations, varying the frequency of a high frequency oscillator in accordance with said capacity variations, translating the frequencyvariable oscillations into amplitude V variations, rectifying and translating said amplitude variations into audio currents which are a substantially accurate representation of the displacements of said stylus, deriving an audio frequency voltage from said audio currents, degeneratively combining said audio voltage with a voltage derived from said sound wave currents. and amplifying translitio'xiint sliiiiailspiacefiiehtsi e mcz. t

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tween the polepiec'es totprovide vibration or the stylus holder; theimprovementtwhich: comprises at least, one metallic-electrode located in spaced relation'close to atleast awportionot -said stylus holder thereby to provide: a condenser which consistsof the electrode and the adjacent portion of the stylus holder, an audio frequency amplifier having output terminals connected to said coils, a network activated by high frequency power responsive to variations in capacitance of said condenser, and means for deriving audio frequency voltage from said network and for applying such derived voltage to aid amplifier in degenerative phase with the audio frequency voltage at the amplifier input terminals.

6. In a method of recording sound waves wherein the sound waves are translated into audio currents which are amplified, and the amplified audio currents are translated into displacements of a record-cutting stylus; the improvement which includes translating the cutting stylus displacement into corresponding capacity variations varying the tuning of a tuned circuit which is being acted upon by a high frequency translating the audio currents into vibratory displacements ,oLa recording element, providing reaota-nce. variations proportional to the vibratory displacements, translating the reactance variati0ll,s-int0 trequencywarlable oscillations, deriving audio currents from said last oscillations which correspondsto said original audio frequency currents, and combining the derived and original. audio currents in degenerative phase.

10. In a record cutter unit of the type employing a magnet provided with pole pieces, a vibratory metallic stylus holder, coils associated with the stylus holder and located between the pole pieces to provide vibration of the stylu holder: the improvement which comprises a pair of metallic electrodes located in spaced relation close to said stylus holder thereby to provide a pair of condensers each of which consists of each electrode and the adjacent portion of the stylus holder 11. In a record cutter unit of the type employin spaced pole pieces, a vibratory stylus holder, coils associated with the stylus holder and located between the pole pieces to provide vibraontrol volta es.

'c n e arie iansnr pr qm i iie pm displacements; tIQ QMii lg-Jhfifliifl-QEQEQQ 843 tionsl into variations-m amn iwsiqlol slins it i iiliir f equ ncy c nrentswq ivill glusiiqi smenonol t m rents from "the said sllnereiu iblqlzireguep oy our rents whiflh correspond tos al filibmteli l d eillifiqiz'w 17. In ameeord: miter emit loiltiierliyilo mow: ing a. permanent magne mmvidedwiighpgamd armature moving said bowe empted.- moxie, side therieoil; CQHSiisurP undi gfl herhQ d and 5 3? 55' 1 lure and ilooateribetween-pthevpolemienea wowii videviiaration of thessiyiusholderztheimprove merit whichl oompmiseset least;one;meggzilli hen trode :located inespaced-g relation ;O10$BL$Q:'&$ dens? a portion of said stylus holder thereby to providem a condenser which consists of the electroder 'and the adiacent portion of the stylus holder. 18. In a. reoordeutter unit of. the-typelemploy ing a magnet provided with pole pieces, evibratory metallic ,siiylus hoider, provided with an armature coil surrounding the stylus holder and armature, located between'the pole pieces to pro vide vibration of the stylus holder: the improve-- ment which comprises a. pair of metallic electrodes located close to the upper endmf said stylus holder and-1n spaced relation to opposite edges thereof therebyiolprovide a pair: oiucflndensers each of which consists of each eiectrodezand its respective adjacent edge of-bhe stylus holder. l

19. In a system provided withmtransdueer coupled to the output terminals ot-an amplifier of audio frequency volta e-:- the impr e comprising means for translatingamplifier currents fed to the transducer into frequency modulate-d high frequency waves, derivingifrom said waves audio frequency modulation currents, and applying to said amplifier in predetermined phase relation a voltage derived from said audio :frequency modulation currents.

HENRY E. ROYS.

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