US2379707A - Audio-frequency compensator circuit - Google Patents

Description

July 3, 1945. J. L.. HATHAWAY AUDIO FREQUENCY COMPENSATOR CIRCUIT Filed NOV. 12, 1943 WMV RW Y UM/OM T7 N4 R WH m4 .A mm 7 M J n Ww l 7.

Patented July 3, 1 945 UNITED STATES PATNT orrics AUDIO-FREQUENCY COMPENSATOR CIRCUIT Jarrett L. Hathaway, Cambridge, Mass., assignor to Radio Corporation of America, a corporation of Delaware Application November 12, 1943, Serial No. 510,008

13 Claims.

(Cl. 17h-100.4)

dicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 schematically shows a circuit diagram embodying the invention,

Fig. 2 graphically shows the functioning of the tone compensation network,

Fig. 3 shows a modication of the compensation circuit, adapted to be applied to a record play-back system wherein the pickup moves from ery. Such changes in frequency response also occur during the sound recording process, and the response characteristic, as a function of time, depends upon whether the cutter' travels from the periphery toward the center of thelrecord, or in the reverse direction. q

Accordingly, it may be stated `that it is one of the main objects of my present invention to provide a novel method of, and means for, automatically compensating for the frequency response characteristic of a record disc as a function of diameter, and the compensation being adapted for either the recording or play-back process to make up for the relative loss of high audio frequencies where the record velocity is low.

A more specific object of my invention is to provide a tone compensation network in the output circuit ofthe electric pickup of a phonograph record play-back system, such tone compensation network comprising essentially an elecu tronic impedance whose magnitude has a predetermined relationto the record speed.

Yet another object of my invention isto provide in an audio frequency transmission line an automatic tone compensation network consisting of an electron discharge device whose input impedance is related in a predetermined manner to the rate of disappearance of a negative blasdeveloped in the input circuit ofl the discharge device. l

Still other objects of my invention are to improve generally the efficiency and reliability of record reproducing circuits,v and more especially to provide automatic tone compensation networks for sound record systems which are not only reliable in operation, but are economically manufactured and assembled.

I'he novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; Ythe 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 I have inthe center of the record toward'the periphery.

Referring, now, to the accompanying drawing, wherein like reference characters in the dierent figures designate similar circuit elements, there is shown in Fig. 1 in a purely schematic manner aturntable i adapted to be driven by a conventional motor 2 energized from current source 3' through conductors 3. The numeral t denotes the spindle of the turntable'upon which is properly centered the record disc 5, which may be either a master `disc upon which a sound recording is to be cut or a record which is to be played back. The numeral t denotes the stylus in the case of a play-back system. or it may be the cutter in the case of sound recording.

The numeral l designates the usual inductance coil which is associated with the stylus,` there being generated across coil l the audio frequency currents corresponding to the modulation of the sound grooves of record 5. In the latter case, of course, we are dealing with a play-back system. The invention is not limited to the particular pickup shown. For'the sake of simplicity let it be assumed that record 5 has grooves corresponding to a desired audio frequency selection, while S-Jl is the usual electric pickup which is employed to convert the mechanical vibrations of the stylus 6 into corresponding audio frequency currents. As is well known to those skilled in the art, the electric pickup is mounted at the free end of a tone arm.

The record may be played back in either of two Ways. The stylus point may be positioned at the periphery of the record and travel toward the center-of the record in a radial fashion, as is generally the case. On the 'other hand the play# back path may be radially from the center of the record toward the periphery.` In either case the audio frequency current developed across coil l is transmitted over an audio frequency 'transmission line 8 to the input electrodes of an amplier tube. The latter may be followed by one or more separate audio frequency amplifier stages terminated by a reproducer. These are networks so well known to those skilled in the art that it vis not necessary to illustrate them in the drawing. In Fig. 1 the low `potential sides of the input and output terminals of the transmission line are shown at ground potential.

As is well known, there occurs a relative loss of high audio frequencies when the stylus 6 rides toward the center of record 5. This occurs because the relativerecord velocity at that point is low. The linear velocity of any rotating disc is greater at its p eriphery than close to its center. Hence, to the ear of the listener there occurs an apparent diminution of the higher audio frequencies. According to my invention, an automatic compensation is secured for this apparentloss of high audio frequency response. Compensation circuits elsewhere in the system, whether play-back or recording, provide sufflcient high frequency accentuation to attain the desired response characteristic over the entire record.

The automatic tone compensation action is' secured by providing across' the transmission line an electronic impedance which normally nected to ground through an unbypassed resistor 1l. The control grid I2 is connected by the direct current blocking condenser I3 to the junction of a pair of series-connected isolation resistors Il and I5. Condenser I3 is suillciently `large to offer substantially zero impedance at the lowest reproducible frequency. The isolation resistors are arranged4 in the high potential side of the audio transmission line. These isolation resistors may be dispensed with, if desired. 'Ihe plate I6 of tube 9 isiconnected to a point of positive potential of a direct current source (not shown). 'I'he screen grid of the tube is connected through a voltage reducing resistor I1 to the aforesaid positive potential point. 'I'he condenser I8 bypasses the screen grid to ground.

Control grid I2 is connected to a point of predetermined negative potential relative tol ground through a path which comprises a grid-return resistor I 9 and a switch 2I. The negative bias source is schematically represented. Any suitable source of negative bias may be used. Thev lower end of resistor I9 is connected tothe grounded end of cathode resistor I I through a resistor 22 which is shunted by condenser 23. Resistor 22 may be made adjustable for -a reason trol grid I2 is also directly connected to cathode I by condenser 30.

In my U. S.' Patent No. 2,323,598, granted July 1 6, 1943, I have disclosed and claimed an electronic impedance device of the type shownin Fig. l. In that patent there has been described a network having an electron discharge vdevice provided with at least -a cathode, control grid and plate, the cathode including in circuit therewith an'unbypassed resistor. An external impedance element is connectedbetween the con-- ses . which willbe hereinafter set forth. The con-y trol grid and cathode. It has been .shown in that patent that the eiective impedance between the control grid of the tube and the low potential side of the cathode resistor is a function of the gain of the tube.-

More specifically, it has been explained in my said patent that if the product of the impedance pedance between control grid I2 and ground is multiplied by the magnitude of the tube 'transconductance. Preferably, the impedance of-'condenser 30 should be'greater than the resistance of resistor II. In brief, the transconductance of tube 9 is used to reduce the eective impedance offered by an element connected across grid and cathode. Condenser 30 is the element whose impedance is controlled.

When the switch 2| is momentarily closed, the condenser 23 of the' time constant circuit Rza-C23 is charged up to a predetermined value. Preferably, the bias applied from the negative bias source has a magnitude such that when switch 2| is closed control grid I2 will be instantly biased to the plate current cut-olf point. In other words, the effective impedance between control grid I2- and ground will be a minimum if the transconductance of tube 9 is a minimum.

However, as the transconductance increases the effective impedance between control grid I2 and ground also increases. Since the eiective impedance of control grid to ground of tube 9 is connected across the audio transmission line it will be seen that the attenuationvof the higher audio frequency components depends upon the transconductance of tube 9. The constantsof Raz-C23 are chosen that the initial approxi'- mately cut-olf bias applied to grid I2 gradually disappears as the stylus 6 rides in the sound grooves toward the center of the record. In other words, the higher audio frequencies are attenuated to a maximum extent when the stylus pointjs atv the vperiphery of the record,

while the attenuation is a minimum when the stylus has traveled toward the center of the record. Another way of explaining theV action isto state that there is provided a form of equalizingaction by initially robbing the pickupl The motor switch 4o and the bias switch 2| may be conjointly actuated as suggested by the dotted line 4I, although in many cases the motor should be separately controlled. In otherwords, switch 2| may be adjusted to be momentarily closed when the motor 2 is energized thereby to start the record play-back. Immediately a negative charge isv given to Vcondenser 23. `Switch` so chosen that complete, or nearly complete, discharge will have occurred during the length of time required for playing the record back.

Where the record play-back is from the center of the record toward the periphery, then the biasing arrangement shown in Fig. 3 isemployed in the grid-cathode circuit of tube 9. In this case, the switch 2i functions to short out 'the negative bias from a suitable direct current source 5d. In other words, the attenuation of the higher audio frequencies is a minimum at t e beginning of record play-back, whereas the attenuation increases toward the-periphery of the disc by virtue of the fact that the bias from Vnegative voltage source 50 is built up as the stylus approaches the periphery of the disc. It isto be understood that the functions and operation described heretofore are equally applicable to the master disc. In that case, the audio frequency currents will be derived from theright-handside of the transmission line in Fig. l1. However, the circuit arrangement will otherwise be the same. It is to be understood, of course, that the R-C value of networki23-22 may readily be adjusted for different types of record diameters by ad` justing the relative magnitudes of resistor 22 and condenser 23 in a predetermined manner, the time constant of the'circuit becoming longer as the product of the resistance -and capacity is increased. Y 4

-Whi1e 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 organizations shown and described, but that many modifications may be made without V departing from the scope of my invention, as set forth inthe appended claims.

What I claim is: Y 1. In combination with a source oi' audio frey 3 the voltage charging of said time constant network -by said negative voltage source.

3. In combination with an audio frequency `transmission line having input and output terminals, an attenuation network comprising an electron discharge tube having at least an anode, a cathode and a control grid, an unbypassed resistive impedance arranged in the space current path of said tube between the cathode and ground, a condenser connected between the control grid and cathode, said condenser and resistive impedance being in series'relation, a resistor and second condenser connected in shunt relation between thecontrol grid'and the ground-` ed end of said resistive impedance, means adapted .to apply ,a negative voltage of a predetermined magnitude across said second condenser, said shunt condenser and resistor providing a time constant network adapted to dissipate said charge after` the passage of a predetermined period of time. j

4. In combination with an audio 4frequency transmission line having-input and 'output terminals, an attenuation network comprising an electron discharge tube having lat leastl an anode, a cathode and a control grid, an unbypassed resistor arranged in the space current path of said tube between the cathode and ground, a condenser connected between the control grid and cathode,.said condenser and resistor being in series relation, a second resistor and second condenser connected in shunt relation between the control grid and the grounded end of said resistive impedance, means adapted to apply a negative Avoltage of a predetermined magnitude across. said. last named condenser, said shunt condenser and resistor providing a time constantnetwork adapted to dissipate said charge after the passage of a predetermined period of time.

5. In a systemof the type employing a source of audio frequency currents. an audio current utiquency currents, anaudio frequency utilization i circuit, an audio transmission line therebetween, A anattenuation network for the higher audio frequency components of said currents, said network comprising an electron discharge' tulbe circuit having its' effective input impedance Iin .shunt across said line, said circuit having its elemen s so chosen and arranged'that the eilectivemagmtude of said input irripedancelsv proportional to the transconductance of the electron discharge tube. said input impedance normally,providing` lization circuit and an laudio transmission line therebetween; the improvement comprising an attenuation network -for the higher audio frequency'components of said currents, said network comprising an electron discharge tube having its effective input 'impedance in shunt across said line, the effective magnitude of said input impedance being proportional to the transconductance of the electron dischargegtube, said input impedance normally providing a maximum attenuation of thel higher audio frequency commonents of said currents, and means for providing a transconductance-reducing bias for said tube which is a function of the passage of a predetermined period of time.

6. In a phonograph systemof the type employing -aI record disc, a transducer device mechanical` ly associated with said disc" and an audio transmission line connected tsaid transducer; the improvement comprising means for automatically controlling the transmission of higher audio frequency components over said line, said means comprising an electron discharge tube having its the transmission of higher audio frequency components through. said line, said means comprising-an 'electron discharge tube having its linput impedance in shunt across said line Athereby to provide normal attenuation of the higher audio frequency components of audio currents flowing through said line, a. time constant network connected between the input electrodes of said tube. a source of negative potential connected to said time constant network, and means for controlling input 'impedance connected to said line to provide normal attenuation of the higher audio frequency components of audio currents owing over said line, a time constant network connected .between the input electrodes of' said tube, and means for controllingv the voltage charging of said time. constant network.

.7. In combination with-a phonograph record K electrical pickup, an audio frequency current utilization circuit,` and an audio frequency transmission line therebetween; an attenuation network for the higher audio frequency components of 'Jsaid currents, said network comprising an elec-.-

tron discharge tube circuit having its effective input impedance connected to said line, said circuit having its constants so chosen that the effective magnitude of said input impedance is proportional to the transconductance of the electron discharge tube, said input impedance normally providing a maximum attenlation of the higher I audio frequency components of said currents,

and means for controlling the transconductance of said tub@ as a function of the passage of a predetermined period of time.

8. In combination with a transducer device mechanically associated with'a record, an audio transmission line conneted to said transducer,

means for automatically controlling the trans- 'mission of higher audio frequency components over said line, said means comprising an electron discharge 'tube having lits input impedance -in shunt across said line thereby to provide normal attenuation of thehigher audio frequency components onfaudio currents vflowing over said line,-

a time constant network connected between the input electrodes of said tube, a source 'of negative potential connected to said time constant netl-work, and means for controlling the voltage charging of said time constant network by said negative voltage source. Y l i 9. In combination, a source of audiofrequency currents, an audio frequency utilization circuit,

an audiorirequency transmission line therebetween, and an improved attenuation network for the higher audio frequency components of said currents comprising an electron discharge tube circuit having itseiective input 'impedance in shunt across said line, saidjcircuit having its elementsso chosen and arranged thatthe eil'ective ,magnitude of said input limpedance is proportion-a1 to the transconductance of the electron discharge tube,.said input impedance being condensive and normally providing attenuation of the higher audio-frequency components of said' currents, a 'switch-controlled current source providing a transconductance-'reduclng bias for said tube, and an RC timelconstant circuit connected Jto vsaid source 'and said tbefor causing said bias tobe a function of the passage of n predetermined period of time.

10. In combination, a record disc, a transducer pence-mechanically associated with said disc, an audio transmission line connected to said transducer, and improved means for automatically .controlling the transmission oi' higher audio frequency componentsthrough said line which comprises an electron discharge tube having an input capacitance in shunt across said line therehy to provide attenuation of the higheraudio frequency components of audio currents flowing through said line, an RC time constant network tronic control over said attenuation which'has a decreasing eiect with the passage of time, and progressively decreasing the attenuation 'of said Vhigher` audio components in response to said decreasing effect of said electronic control as a function of the time 'during which complete trans- -lation takesplace. 1

12. In a. system of theftypeemploying a source utilization circuit and an audio transmission line therebetween; the improvement comprising anattenuation network for predetermined audio frequency components of said 4currents, said net .work comprising a reactive impedance in shunt across said line, a tube for controlling the eilect of said reactive impedance, the effective magnitude of said impedance being proportional to the transconductance of the tube, and means for providing a transconductance-reducing bias for said tube which is a function of the passage of a predetermined period of time.

Y 13. In a system of the type employing a record I disc, a motor-actuated tuntable supporting said disc, a transducer device mechanically associated 40` with said ldisc; andi an audio transmission line connected to saidv transducer; the improvementV comprising means for automatically controlling the transmission of higher audio frequency componentsover said line, said means .comprising anY electron discharge tube having its input impedance connected to" said line to provide normal attenuation of-the higher audio frequency com,- ponents of audio currents owing over said line, `a time constant network connected between the input electrodes of said tube, and means responsive to energization of said turntable motor for controlling the voltage charging of said time com' stant network.

` JARRE'I'I L. H ATHAWAY.

`of audio frequency currents, an audio current l

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