US2304331A

US2304331A - Sound recording apparatus

Info

Publication number: US2304331A
Application number: US372199A
Authority: US
Inventors: Belar Herbert
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1940-12-28
Filing date: 1940-12-28
Publication date: 1942-12-08
Anticipated expiration: 1959-12-08

Description

Dec. 8, 1942. H. BELAR I SOUND RECORDING APPARATUS Filed Dec. 28, 1940 *PpteubdDeaSJMt UNITED STATES TENT 'T SOUND RECORDING APPARATUS Herbert Belar, Philadelphia, Pa., assignor to Radio Corporation Delaware ApplicationDecember 2s,

5 Claims. (Cl.

This invention relates to sound recording apparatus and more particularly to a ground noise reduction amplifier for "use in conjunction with such apparatus. T 5 T T In photographic sound recording apparatus as now used commercially, the clear portion of the filmiaround the sound track is usually obscured byl Tan appropriate masking or envelop irecordT which prevents the transmissionof unnecessary T light through the film, while with variable density records the average exposure is varied to accomplish the'same result. T T T the production of such a record, an amplifieris used which amplifies and rectifies the voice currents so as to produce a direct current of variable amplitude corresponding to the vol- "ume: of the sound to be recorded and thisen- T velope current is used in'turn to controlthe shutters, T galvanometer maybe. T

or light valve, as'the case In such amplifiers, as hertofore constructed,

there has been utilized an audiofrequency amplifier which functions tobring the audio frequency input to the proper level andis followed bya rectifier and a filter. .The rectified and filtered current is then usedto control a D. C. ampllfier, the output from which actuated the ground noise reduction apparatus.

The filter whichwas necessary to eliminate the audiofrequency ripple from the output of the rectifier necessarily produces a time delay commensurate with the lowest frequency which it is designed to filter out and this time delay produces a corresponding retardation of the response of the ground noise reduction mechanism.

This necessarily produces a certain amount of clipping of the peaks of the waveswhen there is any rapid increase in signal amplitude.

In the present apparatus TIdecrease the adverse effects of the filter circuit and improve the operation of the apparatus by adding a volume compressor to the amplifier. The amplifier itself includes a tube of thevariable gain type of America, a corporation of 1940, Serial No 372,199 179-1003 which minimizes the clipping of peaks. The compressor then comes into-operation and decreases the amplification so as to provide only an appropriate margin during the steady state condition. Conversely, if there is a sudden decrease in the input signal strength, the compressor does not immediately change the amplification and if the signal then returnsto its original strength no over-shooting or clipping of peaks occurs. This feature is particularly advantageous in the case of complex waves where the fundamental may have a very low frequency which the ordinary groundnoise reduction amplifier would tend to follow while it may be quite rich in over tones which would normally actuate the usual ground noise reduction amplifier. In

T such a case the ordinary ground noise reduction amplifier \tends to reduce the apparent amplitude of the fundamental while my improved amplifier permits the transmission of the signal substantially unchanged.

One object of the invention is to provide an improved ground noise reduction amplifier.

Another object of the invention is to provide an improved soundrecording apparatus.

Another object of the invention is to provide I a ground noise reduction amplifier with compression.

Another object is to provide an improved compressor circuit for a ground noise reduction amplifier.

Another object is to provide a ground noise reduction amplifier in which the time delay of i the filter circuit is substantiall compensated.

Other and incidental objects of the invention will be apparent to those skilled in the art from a readingof the following specification and an inspection of the accompanying drawing in which the singlefigure illustrates a commercial and the amplification is set at such a point that i the application of compression is necessary to prevent over-amplification of the input signal. The compressor circuit itself is so designed that it has a time delay commensurate with, although not necessarily the same as, the filter eircuit. Thefresult of this arrangement is that when there is a sudden increase in signal it is trans mitted by the amplifier at the gain for which the amplifier is setfo r the signalTof lower amplitude thereby causing a rapid opening of the ground noise reduction shutter or other device type of ground noise reduction amplifier with my improved compressor circuit applied thereto.

The amplifier itself is, in general, of. commercial form and inasmuch as the resistor and capacitor values and tube types are indicated in the drawing no detailed description is necessary except insofar as the design is not commercial.

The audio frequency input is fed through the transformer ID to the amplifier H whichis of the variable gain type, and is providedwith appropriate volumecontrol devices. The output from the tube H is, in turn. fed to the amplifier l2, the output-of which is fed througha transformer and remote control switch I5 to the rectifier 13. This rectifier i3 is slightly different from that usually used in that it is constructed OFFICE as described and claimed in .Kellogg application Serial No. 237,603, filed October 29, 1938, Paten No. 2,227,906, issued Jan. 7, 1941.

The rectifier l3 charges the capacitors l1 and I8 simultaneously but these capacitors have different discharging times. The capacitor l1 discharges directly across the resistor I9 while the capacitor l8 discharges across the resistor IS in series with the resistor l6. Therefore, the opening time of the ground noise reduction shutter is determined by the charging rate of the capacitor 18 while the closing time is determined by the discharge rate of the two capacitors in combination. The capacitor 8 obviously cannot discharge through the resistor |6 until the capacitor H has reached a lower voltage than the capacitor l8.

The output from this rectifier circuit is amplified by the D. C. amplifier tube H, the plate current of which passes to the output circuit indicated, whence it is conducted to the ground noise reduction shutter or equivalent device.

A portion of the output of the audio amplifier 2 is directed through the lead 20 and the resistor 2| to the potentiometers 22v and 23. Appropriate amounts of the audio frequency current are taken off of the potentiometers 22 and 23 and applied through the capacitors 26 and 21 to the two rectifiers in the tube 40, the rectifier circuit being completed through the resistors 24 and 25. This rectifier tube 4|] provides the bias for volume compression applied to the tube The cathodes of the rectifier 40 are connected to the biasing potentiometers 33 and 34 which providean opposing bias in a manner and for a purpose described hereinafter, and the current passing through, the portion of these potentiometers above the contact passes through the resistor 35 and is returned to the opposite end of the resistors 24 and 25. The current passing through the resistor 35 I develops a potential across that resistor and across the capacitor 36 which potential is applied through the resistor 31 to the control grid 38 of the tube where it serves to decrease the output of the tube on an increase in signal strength. p

The current from the rectifier 40 also charges the capacitor 4| through the resistor 31 and through the upper portions of the potentiometers 33 and 34. The timing of the compressor circuit is therefore determined primarily by the values of the capacitors 36 and 4|, the resistors 31 and 35, the upper portions of the potentiometers 33 and,34 and the resistors 24 and 25 in the order'named. The upper portions of the potentiometers 33 and 34 and the resistor 3! affect principally the charging time of the capacitors 36 and 4|, while the resistor 35 affects principally the discharge time thereof,

A positive potential is derived from the plate supply of the apparatus throughv the resistor 3|), the resistor 3|, and the resistor 32, and this positive potential is applied across the potentiometers 33 and 34. These potentiometers may i be so adjusted as to develop any appropriate bucking potential opposing the passage of current through the two elements of the rectifier 40. No current will pass and, therefore, there will be no volume compression when the poten-- tial across the rectifier 40 is not greater than the minimum potential applied byeither the potentiometers 33 and 34. As the potential applied to one of the rectifying elements reaches the miniratus increases, the current through this portion. 3

of the rectifier will increase and at a predetermined point determined by the other potentiometer the other half of the rectifier will also start to pass current, thereby increasing the rate of increase of voltage drop across the resistor 35 with increase of inputv and determining the.

shape of the compression curve. Since the two potentiometers 33 and 34 are identical, either may be used to determine the minimum input at which compression will start and the other may be'used to determine the shape of the compression curve.

In the normal operation of the amplifier the switch 50 remains in its upper; position, as shown in the drawing. If it is desired to test'the amplifier or to operate it for any other reason without having it connected to the normal output circuit, the switch 50 should be moved to the lower position thereby connecting the 760 ohm resistor 5| across the output. The resistor 5| has substantially the same resistance as the usual output load and therefore if this resistor is substituted for the output circuit the amplifier may be operated in its customary manner for the purpose of adjustment.

It will be apparent that there will be a certain amount of time delay not only in the input circuit to the rectifier 40 but also in the input through the potentiometers 33 and 34 to the capacitor36 and the various delays in this compressor circuit may be 50 adjusted as to produce the most desirable timing efiect in conjunction with the filter circuit in the output of the rectifier l3. The delay in the compressor circuit may be'either greater or less than that in the filter circuit, and due consideration should also be given to the delays which occur between the amplifier tube and the rectifier in the audioi'requency circuit. Insofar as I am now aware, the values of the several resistors and capacitors should be substantially as indicated in the drawtion of the ground noise reduction apparatus just prior to the efiect of the compression circuit on the first input tube.

Having now described my invention, I claim:

1. An amplifier including an A. C, amplifier, a rectifier, and a filter circuit, and a compressor circuit connected between the output and input of said A. C. amplifier, said compressor circuit having a time constant of the same order of magnitude as the said filter circuit.

2. A ground noise reduction amplifier including an audio frequency input circuit A. C. amplifier, a rectifier, a filter circuit, a D. C. amplifier and an output circuit from said -D. C. amplifier for connection to ground noise reduction appar'atus, and a compressor circuit connected between the output and input of said A. C. amplifier, said compressor circuit having a time constant of the same order of magnitude as the said filter circuit,

3. An A. C. to DC. amplifier including an A. C. amplifier, a rectifier, a filter circuit, a D. C. amplifier, and a compressor circuit connected between the output and the input of said A. C. amplifiensaid compressor circuit having a time constant of the same order of magnitude as the said filter circuit.

4. In combination with a signal input circuit,

a filter provided with input and output circuits and a rectifier connected between said input circui ts, a compressor circuit connected to said signal input circuit and having a. time constant 01 5 n the same order of magnitude as that of said filter and means for adjusting. both the dynamic characteristics and the time constant of said amplifier.

5. An amplifier including an A; C. amplifier, a rectifier, and a filter circuit, and a compressor circuit connected betweenthe output and input of said A. C. amplifier, said compressor circuit including a rectifier circuit having a time constant or the same order of masnitudeas the said filter circuit.

HERBERT BELAR.