US2489379A

US2489379A - Modulated-oscillator type signal translating apparatus

Info

Publication number: US2489379A
Application number: US662044A
Authority: US
Inventors: Henry P Kalmus
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1946-04-13
Filing date: 1946-04-13
Publication date: 1949-11-29
Anticipated expiration: 1966-11-29

Description

H. P. KALMUS MODULATED-OSCILLATOR TYPE SIGNAL Nov. 29, 1949 TRANSLATING APPARATUS Filed April 13, 1946 3 Sheets-Sheet 1 Relative Voltagesln Utilization Me ans Fr; quancy IOM Fig.1

Amplifying And/Or Utilization w. ML L s w Y W M R P Z w m T N A E 4 Y H 8 Y Wu Wm ,MFUMM rm MW Z 5 emdwla muoh 9MP$NC High Resistance Magnetic Material Nov. 29, 1949 KALMUS 2,489,379

MODULATED-OSCILLATORTYPE SIGNAL TRANSLATING APPARATUS Filed April 13. 1946 3 Sheets-Sheet 3 Fig. I/

Fig. I2 44 I \llll/lllllllllll HENRY P KALMUS INVENTOR.

W HQ' m sw H/:s ATTORNEYS Patented Nov. 29, 1949 MODi'JLATED-OSCILLAT0R TYPE SIGNAL TRANSLATING APPARATUS Henry F. Kalmus, Oak Park, 'Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Application April 13, 1946, SerialNo. ,662,044

7 Claims.

The present invention pertains to signal translating apparatus of the type in which vibrations of a member at audio frequencies produce corresponding variations in current in an associated electrical circuit. The invention relates more especially to such arrangements as phonograph pickup devices and associated apparatus inw'hich vibrations of a member produce amplitude modulation of a high frequency wave.

In my co-pending application, Serial No. 585,- 826; filed March 31, 1945; and assigned to the same assignee as the present application; there is disclosed a novel type of phonograph transducer, or pickup, wherein a member is vibrated in accordance with the undulations of a sound track on a movable record medium to vary at least one electrical characteristic of a winding associated with the member to produce corresponding variations in the amplitude of oscillations generated in a tuned circuit including said windmg.

'It'is an object of the present invention to provide an improved signal translating apparatus of'the above-type wherein an optimum relation is obtained between the'compliance of the vibratory member and the inertia of the supporting means, or tone arm, to produce a minimum amount of record wear consistent with reasonably small amplitude distortion of the modulated oscillations.

A further object of the present invention is to provide an improved form of electrical circuit for generating oscillations-and for demodulating the modulated oscillations produced by the pickup, which circuit is arranged to provide a much higher output than the corresponding circuit disclosed in my prior application whereby a higher signal to noise ratio is obtained.

A still further object of the present invention is to provide an improved electrical circuit, in accordance with the preceding object, which is arranged to operate in a non-linear manner complementary to an inherent non-linear characteristic of the modulating means whereby residual amplitude distortion produced by eccentricity or warpage of the record medium is substantially reduced.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention itselfQboth as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by referenec to the following description taken in connection with accompanying drawings in which:

Figure 1 illustrates a preferred form of system embodying certain features of the invention,

Figure "2 illustrates a vibratory member particularlyuseful in the arrangement of Figure 1,

Figures 3 to5 illustratedetails of a phonograph pickup construction particularly useful in the arrangement ofFigure 1, Figure 3 being a section take-n substantially on line 3-3 of Figure Figure 6 illustrates certain frequency response characteristics obtained with the use of the apparatus described herein,

Figure? illustrates a modified pickup structure for use in the arrangement of Figure 1, the structure of Figure '7 being a modification of the embodiment of Figure 3.

Figure 8 is a graph illustrating certain characteristics of the pickup structure described herein,

Figure 9 is a graph illustrating the operation of the electrical circuit disclosed in Figure 1,

Figure 10 is a graph illustrating the effects of varying the stiifness, or compliance, of the vibratory member of the transducer,

Figure 11 illustrates a modified pickup structure for'usewith vertically cut records, and

Figure 12 illustrates a modified pickup structure for use in arecord changer adapted to play both sides of the records automatically.

'The apparatus described herein, set forth by Way of example of one kind of apparatus which may embody the present'invention, is arranged as shown in Figure 1 to transform intelligence recorded on a movable record medium I into'suitable current variations of audible'frequency in a utilization or amplifying'device 2.

The intelligence on record I is in the form of laterally cut impressions of undulating character in which a'suit-able stylus 3 travels asthe record I is rotated by conventional :means, whereby a vibratory member 4 attached to the stylus is'vibrated to produce corresponding changes in at least one electrical characteristic of an inductance element or coil 5, which is coupled to the vibratory member.

Inductance element '5, whose effective resistance or inductance value may be cyclically altered'by correspondingmovement of stylus 3, constitutes one element of a single tuned circuit 5, ii, 'i, in an oscillatorycircuit including discharge device .8, which is shown in Figure '1 as a twin triode. The first triode section of discharge device'li with its associated circuits functions not only as an oscillation generatorbut also as a demodulator or detector.

The circuit formed by the serially connected inductancesfa and'B is connected in parallel with a condenser i to form the frequency determining circuit 5, 3,1. One terminal of the tuned circuit is connected through condenserl to the anode l t of the first triode section of devices. The other terminal of the tuned circuitis connected through condenser ll to the control grid or electrode 12 of'the first triode section of device 8. The junction point of inductance elements 5 and 6 is connected to the common grounded cathode I3. Grid-leak resistance I4 connected between grid I2 and cathode 13 serves to impress a suitable biasing potential on the control grid I2. Space current for anode I is supplied from voltage source I which has its positive terminal connected through resistance IE to anode I0 and its negative terminal connected to the grounded cathode I3.

The operation of the electrical circuit thus far described in connection with Figure l is as follows. Device 8, including anode I0, control electrode I2 and cathode I3, produces a sustained oscillatory current of constant amplitude in the tuned circuit 5, 5 and I in well known fashion when the vibratory member 4 does not move. Movement of vibratory member 4, for example, by stylus 3 traveling in a conventional phonograph record sound track, produces cyclical changes in the impedance of inductance element 5 to change the amplitude of the oscillatory current flowing in the tuned circuit 5, 6, and 1, whereby amplitude modulation of such oscillatory current is produced. The values of condenser II and grid-leak resistor I4 are chosen so that the bias voltage developed thereacross causes device I0, I2, I3 to function as a plate-bend detector to demodulate the amplitude modulated oscillations impressed between control grid I2 and cathode I3. The modulation components resulting from such plate-bend detection appear across load resistance IS in the anode circuit of device Ill, I2, I3. One important feature of the present invention, to be more fully described hereinafter, is that the device I0, I2, I3 serves to produce oscillations which are modulated in accordance with the sound track undulations on the phonograph record and simultaneously to demodulate the modulated oscillations in a nonlinear manner which is complementary to a nonlinear characteristic of the transducer, whereby distortion produced by the transducer is balanced out of the audio frequency signals developed in the anode circuit of device [0, I2, I3.

Anode I8 is supplied with operating potential from the positive terminal of source I5 through resistance IS. The control grid IT is maintained at a suitable continuous potential with respect to cathode I3 by connecting resistance 20 therebetween. The modulation components appearing at anode ID are transferred to the control grid-cathode circuit of device I8, I1, I3 through the series circuit comprising filter resistance 2I, isolating and coupling condenser 22 and filter condenser 23, grid I! being connected to the junction point of condensers 22 and 23 and the other terminal of condenser 23 being grounded. Filter condenser 23 has a relatively small reactance at the frequency of oscillation of device I0, l2, I3 so that substantially all of the amplified voltage appearing at anode I8 is of a frequency corresponding to the frequency of vibration of the vibratory member 4. The amplified voltage is applied to device 2.

The vibratory member 4 of Figure 1, which is coupled to coil 5, is preferably of high resistance material and in one form of the present invention is .004 thick stainless steel known in the trade as American Iron and Steel Institute Type No. 302. Stainless steel Type 302 contains about 18% chromium, about 8 or 9% nickel, about 0.08 to 0.2% carbon, traces of manganese, silicon, phosphorus, sulphur and copper and the remainder iron. Stainless steel Type 302 is preferred because it is substantially non-magnetic whereby a minimum amount of cycle hum is introduced into the pickup from nearby motors, transformers, etc. It has been found that stray 60 cycle fields shift the working point on the magnetization curve of the vibratory member, when it is made of magnetic material, with the result that the inductance and effective resistance of the coil 5 coupled thereto change. Such changes produce amplitude modulation of the oscillating current. For these reasons, where stray alternating magnetic fields are present, the material preferably possesses high resistance and low permeability. When such material is used the desired modulation is produced substantially entirely by eddy current loss in the high resistance material, and the modulation is substantially unaffected by variations of permeability of member 4 in the presence of such stray fields.

Vibration of a member of such material in the vicinity of inductance element 5 causes amplitude modulation of the oscillatory current because (1) the quality factor or Q of the inductance element is changed cyclically and (2) the amount of feedback voltage from the anode ID to grid I2 is changed cyclically due to cyclical changes in the inductance of element 5. The inductance of coil 5 is preferably less than the inductance of coil 6 so that when the impedance of coil 5 is altered the feedback factor is altered in such a way that amplitude modulation produced by inductance change is in phase with the amplitude modulation produced by the change in Q, the quality factor.

Viewed in another Way, the inductances 5 and 6 and condenser 7 form a pi network through which oscillation voltage is transferred from anode I0 to control electrode I2. In usual forms of electron discharge oscillation generators, and in the type illustrated in Figure 1, there need be only a part of the voltage on the anode fed back to the control electrode, and, to maintain the generator in operation, that voltage so fed back must be maintained within certain limits. That is, if the amount of voltage fed back to the control electrode is too little, the generator ceases the production of oscillations. The pi network 5, 6, I is effective to alter in substantial degree the amount of voltage fed back from anode ID to control electrode I2 as the impedance of inductance 5 changes.

In the fundamental form of the present invention, member 4 is conductive. To make it magnetic as well as conductive is useful when it is desired to reduce or eliminate inductance change of coil 5 and corresponding change of oscillation frequency. Also, to make member 4 magnetic introduces hysteresis loss which aids the eddy current effect in member 4 to change the effective resistance of coil 5 and consequent change of oscillation intensity. Cyclical changes in the inductance of element 5 cause a certain amount of frequency modulation of the generated wave in tuned circuit 5, B, I, but substantially only those components which produce amplitude modulation are detected in device I0, I2, I3.

When the movable element 4 is constructed of magnetic and conductive material, movement of such element closer to coil 5 has a twofold effect on the inductance of that coil. Since element 4 has a greater permeability than that of air the inductance tends to increase; moreover since element 4 is also conductive, the inductance tends to decrease. The net change in the inductance of coil '5 is equal to thefiifiefinCe between these twoopposing tendencies. Attire-same ti'mefia's element '4 approaches-thecoilboth the increased hysteresis loss and the increasededdy-current loss operate 'to decrease the damping *re'sistance reflected in coil 5. "In one arrangement incorporating an embodirnent' o'f the present invention, the permeability and specific "resistance of element 4A (Figure 2) are 'so proportioned that substantiallyno net inductance "change results frommovement of *theelemen-t "with "respect to the --coil. Thus amplitiide modulationof the oscillations generated in the-"tuned-bircuit 5, "ii, l! is effected with -substantial1y+no =accom any1ng frequency modulation.

Figures 3-5 show'apreferred =construction *f or mounting the vibratory membe having thegeneral reference numeral '4. -In 'Figures 3-5 the vibratory member is oi -the stainless steel type previously mentioned,-forreducing to a-rninirn'um the amount of "extraneous 120cycle hum introduced into the system,- and is of a thickness of approximately .004". The upper end thereof 'is embedded in 'a hard plasticbas'e 24 which in turn is clamped between two halves "2-5, -26 of a plastic casing '21, the 'two halve's being held together by cement or the 'l ike'orby "bonding with heat and pressure. Inductance coi1 5 is mounted on-coil form 2'8 whioh' i-sfas'tened 'for example by cement or the likefto thein'sid'e "face of easing half 26, in close proximity to member 4 with circular -coil "form-ldand circularly shaped member 4 substantially coaxial.

The free'end of vibratory-member ha-s a resilient L-shaped recordeontact stylus 'orheedle 29 aflixed thereon, for example by 'spot-welding or the like, to give a des'irable' vertical compliance for the purpose or reducing needle 'chatten'arid record weanand for produoing a desirable response curve which is substantiallyfiatbetween 2000 and 4000 cycles per se'cond with a sharp cutoff after 4000 'cyc lesper second.

The phonograph piokup i arrangement -"of Figures 3-5 incorporated in =Fig-ure-lepreferably has a frequency response characteris'tic 'of the type shown in Figure 6, wherem odt ut voltage in the utilization device 2 is plotted as oi dlriate'and frequency of the corresponding voltagesis plotted as abscissa. The sharp -='cutoff ait'e'r '4000 cycles is due mainly to the'fact that the' vibi atory memher 4 in Figures 3 5'isinherently#mechanioally resonant in its second modeat 'a frequency between 3000 and'4000cyclesfper seoond. At' this frequency the mode of vibration is such that the ends of thevibratoryfininbr' terid to remain stationary andthe middle portion adj-acent the inductance :element 5-*vibrates withielatively large amplitude. 'Inor'der that the amplitud'e of vibration at the resoriant freduericy be not too large a damping "element "'30, such as *a. block of plasticized "cellulose riittate, is fixed, as by heat and pressure, on the vibratory"memberat the upper end thereon i'n' -abuttin' relationship with the bottom" surface of the basemember' =24. This plastic material is "preferred "over 1 ordinary rubber since it possesses P more internal friction than rubber. Without the presenoe'of a damping element 30 the characteristic oilrve fl in "full line in Figure 6 ispeaked in the inanner indicated by the dotted line' portion fl'thereoL' but with a suitably proportioned vibratory member 4 and damping element? 'the characteristic curve 3| is substantially flat in the range of 2000 to 4000 cycles :l'per second. -W=ithout resonant efiects in the "mechanical "-vibratory member 4 6 the 'characteristi'c curve 'fis substantiaHy as indicat'ed by the dottedline33.

-In another form of the invention, shown in Figure *7, the-stylus'or record'en'gaging portion 3 at the end of "vibratory member 4 may be straight instead of L-shapedas shown in the preferred embodiment of Figure 2. Furthermore, instead of the upper end of the' vibratory member 4 being embedded in'a hard plastic base 24, as shown in the preferred embodiment in Figure 4, the upper' end of' member 4 may be embedded in' 'a'n elastic base 34 such as rubber. In such case the elastic body 34 is held between the two halves-'25, 26 of casing 2! in the same-manner as is'shown in Figure3.

Inductance coil 5 has its terminals 35, 36 permanently connected to metallic contact elements 31, '38, respectively, which are embedded ina'nd eXtend from the inside to the outside of casing 21, the outer ends "of contact elements 31, 38 being adapted to make good electricaland mechanical contact with corresponding contact elements 39, -40 embedded in and extending through the casin holder 4|. Conductors 42, 43, which are permanently fastened to the upper ends 'of contact elements 39,40, respectively, extend through at least part of the tone arm 44 to the appropriate terminals 45, '46 in the oscillator circuit of Figure 1.

The replaceable casing 21, 'which may be sepa'rately sold as an article of commerce, contains in-a unitary-structure the vibratory member 4 and coil -5 coupled thereto in predetermined spaced relationship, which unitary structure is held snugly in the casing holder'M by projections 41, 48- engaging correspondingly shaped depressions (not shown) in the wal-ls of easing holder 4| and by contact projections 49, 50 engaging correspondingly shaped depressions in contact elements 39,40.

The casing holder M is pivotally mountedabout the axis of fixed pin 5| ina pair of arms 52 disposed-on opposite sides of the casing holder 4|, each "arm having one "end thereof fastened to th'e tone arm, for example byscrew'53, and the other end thereof circularly formed to provide a bearing member with pin 5|. Arms 52 may be flexible but arepreferably non-flexible. The cas ing holder 4| is normally spring biased downwardly for rotation about the-axis of pivot pin 5| bycoil spring 54 having its ends fastened to tone arm 44, forexample-by screw 55, and having an intermediate portion disengaging a shoulder 51 on the-holder 4| to press'the'holder downwardly in the direction of counterclockwise rotation about pin 5|. The coil spring 54 is made sufficiently flexible to protect'theipickup unit against injury incase the tone arm is dropped on a record. In such case'the pickup unit including the casing holder is deflected upwardly against the action of weak spring 54 and'a piece of felt'58 onthe free end of arm 59 "engages the casing holderll, arm 59 being fastened to the tone arm, for'example by screw 53,'to serve as a stopfor limiting the movement of easing holder 4|; the heel of thereplaceable casing 21 is brought to bear on the surface of therecord'disc and the stylus 3 is retracted. Protection is also'afforded to the L-shaped stylus'by extending the casing halves 25, 26 sufliciently far downwardly so that, when=more than a predetermined force is applied to the 'stylus, it moves upwardly within the space defined i by the "casing ihalves 25, 26. Since the ends of arms 52 and' 59 are "both heldby the same-=screw53, itis obvious thatthe arm il may form an integral extension of arm 59. The tone arm 44 is pivotally secured at one end in any suitable manner, for example as illustrated in Figure 12 and fully described in the co-pending application of Ralph A. Mullaney et al.; Serial No. 649,146; filed February 21, 1946, and assigned to the same assignee as the present invention.

The oscillator detector arrangement shown in Figure 1 is identical to that shown in my prior application except for the values of condenser II and resistor Hi. In the present case these components have values which result in an entirely different manner of operation than that described in my prior application. In my prior application both grid detection and plate-bend detection took place in device ill, [2, l3. As these two types of detection act differentially in the anode circuit, the output which can be obtained is limited. It has been found that if the values of condenser H and resistor M are properly chosen, so as to prevent the bias voltage impressed on grid I2 from fluctuating substantially at an audio frequency rate, no grid detection occurs. A much larger audio frequency signal is then developed in the anode circuit of device l0, l2, l3 resulting in a. greatly improved ratio between the desired signal and tube noise due to thermal agitation effects. It has also been found that if the values of condenser II and resistor M are properly chosen, so as to enable the bias voltage impressed on grid [.2 to fluctuate at a rate corresponding to the rotational speed of the record disc I, the amplitude distortion of the modulated oscillations due to eccentricity of the record disc may be balanced out through variation of the detecting efliciency in a manner complementary to the nonlinear characteristic of the modulator and transducer.

In other words, in accordance with this invention, a resistance-capacitance biasing network is used to bias device [0, l2, E3 to a region of relatively high curvature of its anode characteristic to cause the device to operate as a plate-bend detector for amplitude modulation at audio frequency rates. The biasing network, comprising condenser II and resistor 14, has a time constant long for audio frequencies but short for subaudio frequencies to provide a bias potential, within the non-linear portion of the anode char acteristic, which varies substantially only in response to sub-audio frequency amplitude modulation of the oscillatory current in circuit 5, 6, I. In this way the detection efficiency of the platebend detector is varied in such manner as automatically to compensate for undesired slow changes in oscillation amplitude due to slow variation in the mean position of the vibratory member 4 which may result, for example, from eccentricity or warpage of the record disc.

It is preferred to construct the transducer and tone arm so as to result in a minimum of record wear consistent with low distortion of the modulated oscillations and to eliminate the residual distortion in the detector. Quite often in com mercial records the hole, or the sound track, is not accurately centered, and the tone arm head is driven back and forth in a radial direction during each revolution of the record disc. Due to the inertia of the arm, the mean position of the stylus then varies at a frequency of 1.3 cycles per second, corresponding to 78 revolutions per minute of the record disc. The deflection due to this motion is superimposed upon the deflection due to undulations in the sound track on the record disc. In pickups with large linear ranges (e. g. crystal or dynamic systems) eccentricity produces neither distortion nor amplitude changes. In electronic pickups, having only a limited linear range, eccentricity results in cyclical variations in amplitude which may be considered as amplitude distortion.

If the mass of the tone arm is small and the moving element has a low compliance, or in other words is stiff, the arm follows the stylus tip and the vane is not deformed and displaced relative to the tone arm. In this case eccentricity does not produce any amplitude change but the record wear is considerable due to the large forces between the stylus and the sides of the record grooves necessary to overcome the inertia of the tone arm. If the mass of the tone arm is large and the moving element has a high value of compliance, the arm does not follow the needle tip and the vane is greatly deformed and displaced relative to the tone arm. In this case the record wear is reduced to a minimum but the distortion due to non-linearity of the pickup is high.

The present invention provides a phonograph pickup in which the compliance of the vibratory member is made large, in order to minimize record wear and needle talk; the amplitude distortion due to non-linearity of the pickup is substantially compensated in the plate-bend detector by corresponding variation of the detection efficiency, as explained above.

The effect of varying the stiffness of the vibratory member and stylus of the transducer is illustrated by curves 1 and 2 in Figure 10 in which the ordinates are drawn to a logarithmic decibel scale and the abscissae are drawn to a linear scale. Curve 1 shows record wear as ordinate plotted against stiffness as abscissa. The record wear is inappreciable if the stiffness is lower than the value corresponding to the knee of the curve. If the stiffness is lowered too far the amplitude distortion due to record eccentricity becomes appreciable. Curve 2 shows such amplitude distortion as ordinate plotted against stiffness as abscissa. The position of the knee of this curve depends on the tone arm inertia, which should be low enough to make the knee of curve 2 occur at a lower value of stiffness than the knee of curve 1. At any value of stiffness between the knees of these two curves, which knees are indicated by the dotted abacissae, an optimum relation between record wear and amplitude distortion exists. The compliance of the vibratory member and stylus of the transducer described herein is preferably made in the order of 9.6 lilcentimeters per dyne, corresponding to a stiffness well below the knee of curve 1. The compliance is measured at the tip of the stylus in the direction of movement thereof by the undulations in the sound track. With a tone arm inertia of 20 grams the knee of curve 2 is located below the value of stiffness corresponding to a compliance of 9.6 10- centimeters per dyne.

Let the tone arm inertia or mass, assumed to be concentrated at the end of the arm, be represented by M, and let the compliance of the vane be represented by Cm. When the groove displacement at the needle tip due to eccentricity of the record is an, and $2 is the deviation of the vane from its rest position, then 9. where f is the frequency corresponding to the rotational speed of the record disc.

Using a value of Cm of 9.6x 10- centimeters per dyne, determined by curves 1 and 2 of Figure 10 as. described above, assuming a mass M of 20 grams and an eccentricity of s, and also assuming that f=1.3 c. p. s., we obtain 0.8 mil for 9:2 by substitution in the above equation.

Figure 8 is a graphical representation of the vane displacement characteristics of a large scale model of a pickup constructed in accordance with the-present invention, the linear dimensions of the: model being larger than those of a normal size pickup by a factor of 10. The characteristics for a normal size pickup are similar, if the abscissae are corrected by a factor of Curve 1 represents the relationship between the oscillation amplitude and the displacement or distance between the vane and its associated coil; point A corresponds to the minimum gap width and points B and C correspond to the limits of the normal working range in the absence of record disc: eccentricity. The normal working range covers 10 mils in the large scale model, or 1 mil in the actual pickup.

In the absence of record disc eccentricity, the average vane displacement is contant (at a value midway between points B and C). If it is first assumed, for purposes of analysis, that the detection is linear, the audio output is proportional to the sloperof curve 1 at the operating. point. Curve 2 represents the first derivative of curve 1 and is indicative of that. slope; thus curve 2 is representative of the pickup sensitivity at any" given average gap width.

The normal vane displacement in the actual pickup is approximately 12 mils. In this region the, slope of curve 2 in Figure 8 is equivalent to arr amplitude change of 0.5 db per mil. When the stylus deflection due to eccentricity is 0.8 mil, the resulting variation in amplitude is 0.4: db. This'change is small enough so as not to be particularly objectionable and yet the compliance of the'movingvane is so high as to result invery little recordwear and also very little noise radiation from the pickup and record. If now, the platebenda detection arrangement of the present invention is substituted for the linear detection arrangement assumed above, even the small remaining amplitude distortion may be eliminated. As. the.- average displacement of the vane decreases. the average amplitude of the oscillations produced by device l; l2, [3 also decreases, thereby causing the peak value of the alternating voltage appearing between the grid l2 and cathode l3: to decrease. Fluctuations in the ave.- rage :gap width may be caused by eccentricity of the recorddisc and may occur at a frequency of 1.3 0.. p.. s. when the record disc rotates at 78 RP. M. When the values of resistor Hand condenser Marachosen. so as to-have a time constant which will permit. the bias voltage impressed on the. grid l2 to vary with the slow fluctuations. in the magnitude of the high frequency oscillations, buttoprevent fluctuations in: the bias voltage. at an-audiofrequency rate, the distortion can be eliminated from the demodulated signals developed in the. anode;- circuit of the detector, as has been illustrated: in Figure. 9.

Im Figure 9,. curves, A and 3 represent the voltages impressed on thegrid 12 when. the'tonearmis swinging away from and towards, respectively, the center of the record disc. When the tone-arm is swinging away from the center of the record disc the mean. value of the gap between the vane and the associated coil increases, resulting in a high oscillation voltage amplitude on the grid l2. Conversely, when the tone-arm is swinging towards the center of the record disc the average displacement between the vane and the associated coil decreases, resulting in a smaller oscillation voltage amplitude on the grid 42. Due to the change in the average value of the high frequency voltage impressed on the grid 12, the bias voltage impressed thereon varies from E1 to E2 thereby causing the detecting action to take place over a different portion of the plate characteristic curve C of the detector. The action of the plate-bend detector in compensating for slow changes in average vane displacement may be explained as follows. For a large average vane displacement, the oscillation voltage amplitude is high and the modulation depth for a given vane vibration amplitude is small. With a large oscillation voltage amplitude, the bias adjusts itself to an operating point whereat the curvature of the anode characteristic C is high, and at the same time the peak to peak swing of the control grid voltage extends over a large portion of that characteristic; consequently the detection efficiency is high.

Now, if the average vane displacement is decreased, the oscillation voltage amplitude decreases due to the loading effect of the vane on the tuned circuit 5, 6, I (Figure 1) but the modulationdepth for a given vane vibration amplitude increases. The decreased oscillation voltage amplitude results in a decrease in the negative bias on grid l2, and the device i0, l2 [3 (Figure 1) is biased to a region of slighter curvature of the anode characteristic C, and the peak. to peak swing of the control grid voltage extends over a narrower portion of that characteristic; therefore the detection efficiency is decreased. By proper design of the circuit the resulting audio-frequency signals, represented by the dotted curves A and B, in the anode circuit can be made equal in magnitude. It has been found that a substantially perfect balance can be obtained when tube 8 is a Type '7F7, resistor i4 is .15 megohm, condenser II is. 200,- 000 micromicrofarads, and the normal value of the highfrequency voltage impressed on the grid I2 is in the order of 1.8 volts.

Although the pickup construction thus far described is intended for use with laterally cut records, the same considerations apply to records of the vertically cut type. When vertically cut records are employed eccentricity produces no ill. effects, but warpage of the record disc results in vertical. displacement of the tone-arm which causes deviations in the mean position of the moving vane due to the force necessary to overcome. the. inertia of the tone-arm. A pickup structurev suitable for use with vertically cut records has been illustrated in Figure 11. The construction of the arm 44 is identical to that shown in Figure 5,. but. the transducer BI is mounted in the tone arm with the moving vane 63.1ying. in a plane which is. substantially parallel to the plane of the record disc. The coil spring 54 biases a casing holder 60 against the felt tipped arm. 59. The tension of this spring is just sufficient to maintain the casing holder fill in contact with the felt tipped'arm 59 during normaloperation. of the apparatus, but is sufficiently weak. sov thata sudden jar, such. as that produced by dropping the tone-arm on a record, allows the casing holder 60 to be deflected upwardly to prevent damage to the moving vane of the transducer 6|, which is constructed similarly to the transducer 21 disclosed in Figures 3, 4 and '7 and is removably secured in the casing holder 60. The transducer differs from that previously described principally in the fact that stylus 62 is formed perpendicular to the plane of the moving vane 63 to adapt the transducer for use with vertically cut records. The winding of transducer BI is connected to terminals 45 and 46 of the circuit of Figure 1 by conductors 42 and 43.

The modified form of pickup disclosed in Figure 12 is similar to that disclosed in Figure 5 except that there is provided a casing holder 54, adapted to receive a pair of transducers 21, each identical to that disclosed in Figures 3, 4 and 7. An opening is provided in the top of the tone-arm 44, through which one of the transducers 21 projects to enable it to engage the under side of a record disc in the manner illustrated in the afore-mentloned copending application of Ralph A. Muilaney et al. The windings of the two transducers are connected in series, as illustrated, or in par allel to terminals 45 and 45 of the circuit of Figure 1 by conductors 42 and 43 in the same manner as for a single transducer.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects,

and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. Signal translating apparatus comprising: an oscillating plate-bend detector having an oscillation generator portion for producing oscillations of superaudible frequency and including an electron discharge device having a grid voltage-- anode current characteristic which is non-linear throughout a predetermined range of grid potentials; a transducer having a modulating element coupled to said generator portion to amplitude modulate said oscillations in response to relative movement of said element and a component of said generator portion; a biasing network for said device included in said oscillating detector and comprising a resistor and a condenser having a time constant long for audio frequencies and short for sub-audio frequencies to provide a grid bias potential within said range which varies substantially only in response to sub-audio frequency amplitude modulation of said oscillations; and an anode load impedance for said device for deriving audio-frequency modulation components of said modulated oscillations.

2. Signal translating apparatus comprising: an oscillation generator and plate-bend detector system in which the generator portion produces oscillations of superaudible frequency, said system including a frequency determining circuit for determining the frequency of said oscillations and further including an electron discharge device having a grid vo1tage-anode current characteristic which is non-linear throughout a predetermined range of grid potentials; a transducer having a modulating clement coupled to said circuit to amplitude modulate said oscillations in response to relative movement of said element and said circuit; a biasing network for said device included in said generator detector system and comprising a resistor and a condenser having a time constant long for audio frequencies and short for sub-audio frequencies to provide a grid bias potential within said range which varies substantially only in response to sub-audio frequency amplitude modulation of said oscillations; and an anode load impedance for said device for deriving audio-frequency modulation components of said modulated oscillations.

3. Signal translating apparatus comprising: an oscillating plate-bend detector having an 05- cillation generator portion for producing oscillations of superaudible frequency, said oscillating detector including a frequency determining circuit, comprising an inductance coil, for determining the frequency of said oscillations and further including an electron discharge device having a grid voltage-anode current characteristic which is non-linear throughout a predetermined range of grid potentials; a transducer having a movable conductive element inductively coupled to said coil to vary the damping resistance thereof thereby to amplitude modulate said oscillations in response to movement of said element; a biasing network for said device included in said oscillating detector and comprising a resistor and a condenser having a time constant long for audio frequencies and short for sub-audio frequencies to provide a grid bias potential within said range which varies substantially only in response to sub-audio frequency amplitude modulation of said oscillations; and an anode load impedance for said device for deriving audio-frequency modulation components of said modulated oscillations.

4. A record player for reproducing sound from a record rotating at a sub-audible frequency comprising: an oscillation generator and platebend detector system in which the generator portion produces oscillations of superaudible frequency, said system including an electron discharge device having a grid voltage-anode current characteristic which is non-linear throughout a predetermined range of grid potentials; a transducer having a movable element, carrying a compliant stylus adapted to track the undulations of a movable record medium, coupled to said generator portion to amplitude modulate said oscillations in response to vibration of said stylus; a self-biasing network for said device in-, cluded in said system and including a resistor and a condenser having a time constant long for audio frequencies and short with respect to said sub-audible rotational frequency to provide a. grid bias potential within said range which varies substantially only in response to sub-audible frequency amplitude modulation of said oscillations; and an anode load impedance for said device for deriving audio-frequency modulation components of said modulated oscillations.

5. A record player comprising: an oscillating plate-bend detector having an oscillation generator portion for producing oscillations of superaudible frequency, said oscillating detector including a frequency determining circuit, comprising an inductance coil, for determining the frequency of said oscillations and further including a triode having a grid voltage-anode current characteristic which is non-linear throughout a predetermined range of grid potentials; a transducer having a movable conductive element, carrying a compliant stylus adapted to track the undulations of a rotatable record disc, coupled to said coil to vary the damping resistance reflected therein thereby to amplitude modulate said oscillations in response to vibration of said stylus; a self-biasing network for said triode included in said oscillating detector and comprising a resistor and a condenser having a time constant long for audio frequencies and short for sub-audio frequencies to provide an operating grid bias potential within said range which varies substantially only in response to sub-audio frequency amplitude modulation of said oscillations; and an anode load impedance for said triode for deriving audio-frequency modulation components of said modulated oscillations.

6. A record player for reproducing sound re corded on a rotatable record disc comprising: an oscillating plate-bend detector having an oscillation generator portion for producing oscillations of superaudible frequency, said oscillating detector including a triode having a cathode, a control grid, and an anode and having a grid voltage-anode current characteristic Which is non-linear throughout a predetermined range of grid potentials; a frequency determining circuit included in said generator portion comprising the parallel combination of a condenser and a pair of serially connected inductance coils coupled between said anode and said control grid; a connection between the junction of said coils and said cathode; a transducer including one of said coils and having a movable conductive element carrying a compliant stylus and coupled to said one coil to vary the damping resistance reflected therein in response to vibration of said stylus for amplitude modulating said oscillations, said element having a compliance, measured at the tip of said stylus in the direction of movement thereof by the undulations of said record disc, greater than 10-' centimeter per dyne; a self-biasing network for said triode included in said oscillating detector and comprising a resistor and a condenser having a time constant long for audio frequencies but short for subaudio frequencies to provide an operating grid bias potential within said range which varies substantially only in response to sub-audio frequency amplitude modulation of said oscillations; and an anode load impedance for said device for deriving audio-frequency modulation components of said modulated oscillations.

7. A record player for reproducing sound recorded on a rotatable record disc comprising: an oscillating plate-bend detector having an oscillation generator portion for producing oscillations of superaudible frequency, said oscillating detector including a triode having a cathode, a control grid, and an anode and having a grid voltage-anode current characteristic which is non-linear throughout a predetermined range of grid potentials; a frequency determining circuit included in said generator portion comprising the parallel combination of a condenser and a pair of serially connected inductance coils coupled between said anode and said control grid; a connection between the junction of said coils and said cathode; a transducer including one of said coils and having a movable conductive element carrying a stylus and coupled to said one coil to vary the damping resistance reflected therein in response to vibration of said stylus for amplitude modulating said oscillations; a selfbiasing network for said triode included in said generator oscillating detector and comprising a resistor and a condenser having a time constant long for audio frequencies but short for subaudio frequencies to provide an operating grid bias potential within said range which varies substantially only in response to sub-audio frequency amplitude modulation of said oscillations; and an anode load impedance for said device for deriving audio-frequency modulation components of said modulated oscillations.

HENRY P. KALMUS.

REFERENCES CITED The following references are of record in the file of this patent;

UNITED STATES PATENTS Number Name Date 1,687,231 Speed Oct. 9, 1928 1,724,099 Marshall Aug. 13, 1929 1,830,800 McClatchie Nov. 10, 1931 2,288,000 Kelly June 30, 1942 2,441,464 Albright May 11, 1938 FOREIGN PATENTS Number Country Date 258,643 Great Britain Sept. 22, 1926 373,973 Great Britain Dec. 1'7, 1930 558,680 Great Britain Jan. 17, 1944 Certificate of Correction Patent No. 2,489,379 November 29, 1949 HENRY P. KALMUS It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 14, line 22, strike out the word generator;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 18th day of April, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,489,379 November 29, 1949 HENRY P. KALMUS It is hereby certified that error appears in the printed specification of the above numbered patent requiring correct-ion as follows:

Column 14, line 22, strike out the Word generator;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 18th day of April, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.