US2115001A - Sound reproduction - Google Patents

Description

April 26, 1938. R, BALSLEY 2,115,001

SOUND REPRODUCTION Filed Oct. 29, 1935 2 Sheets-Sheet 1 INVENTOR. N \24 JAMES P. BALsLEY By W ATTORN April 6, 1938. J. R. BALSLEY 2,115,001

SOUND REPRODUCTION Filed Oct. 29, 1935 2 Sheets-Sheet 2 PIE. 3.

JNVENTOR. JAMES R BALSLEY ATTORNEY Patented Apr. 26, 1938 UNITED STATES PATENT OFFICE SOUND REPRODUCTION James R. Balsley, La Canada,

Califi, assignor to Application October 29, 1935, Serial No. 47,288

13 Claims.

This invention relates to sound reproduction and particularly to the reproduction of sound from a record in which during reproduction the volume level is automatically controlled.

In most of the systems of the prior art, a portion of the sound currents are rectified, filtered and are impressed either upon a vacuum tube to change the amplification factor in accordance with the amplitude of the sound modulations, or are impressed directly upon a transmission element for the sound modulated currents so as to vary the change in the volume level range. Filter circuits necessary to filter or smooth out the rectified audio current having an alternating cur rent component thereon introduce time lag and other deleterious effects which may distort the sound reproduced.

One object of the present invention is to eliminate the use of filters for smoothing out the rectified audio frequency current employed to change the volume level range of an automatic volume controlling circuit. This is accomplished by feeding the rectified current obtained through the photoelectric cell due to variations in the average print transmission through the film and/or the rectified sound currents into a transmission element for transmitting the audio fre quency sound currents into a sound reproducer. This rectified current is divided into two paths connecting the detector or rectifying device to a suitable receiving circuit such as a bridge, in the audio current transmission element. A phase changing device is provided in one of the paths to change the phase of the alternating current components in that particular path 180. Also in one of the paths the current variations due to both the average print intensity and rectification appear. Thus when the divided rectified current is fed into the transmission element the alternating current components will be neutralized or balanced out due to the phase opposition of the two current conducting paths leaving only the smooth varying direct current to be fed into the transmission element to change the volume levels of the reproduced sound. The amount of direct current thus obtained may be in accordance with the variation in average current fiow through the cell due to the average print transmission of the sound track portion of the film which in the case of an anti-ground noise type of film varies practically directly as the volume of the sound, or it may vary in accordance with the rectified sound current, or it may vary in accordance with both.

Other objects and advantages of this circuit for expanding sound will be more fully appreciated and the invention more fully understood by reference to the following description read in conjunction with the accompanying drawings in which:

Fig. 1 is a schematic circuit diagram of a sound reproducing apparatus embodying the invention.

Fig. 2 is an enlarged section of a film having a variable area sound track of the anti-ground noise type; and

Fig. 3 is a schematic circuit diagram of a modified form of the invention.

Referring to Fig. 1 the sound record film i may be either of the variable area or variable density type. Also it may have an average print transmission which is proportional to the amplitude of the sound recorded, as in the so-called antiground noise type of record, or it may be of the normal unbiased and unshuttered type of record in which the average print transmission is sub stantially the same throughout the length of the record. In speaking of average transmission I mean the ratio of average transparency to average opacity of a section of sound track which covers more than one sound wave length representation.

Light from an exciter lamp 2 projects a beam of light through a condenser lens 3 and onto the film l in the form of a fine horizontal line of light. The beam of light thus modulated in frequency and intensity by the film l is focused by objective lens 4 upon a photoelectric cell 5. In series with the photoelectric cell 5 are resistance 6, a polarizing battery 1, and a load resistance 8. One side of the load resistance 8 is grounded at 9 and the other side is connected through lead ID with the grid of a detector tube l I. The plate circuit of tube II comprises the primary of a transformer l2, plate battery 13 and potentiometer resistance H in series. The filament of tube II is grounded at l5 to complete the grid circuit while the secondary of transformer i2 is placed in series with a potentiometer resistance i4. Contacts 43 and 44 along the resistance l4 and I1 are connected through leads IB and I 9 respectively to the grids of tubes and 2| respectively. Biasing batteries 22 and 23 are provided in the grid circuits to keep a negative potential upon the grids of tubes 20 and 2|. and obtain a static balance. These tubes 20 and 2| constitute two arms of a bridge circuit 24. Also included in either arm of the bridge circuit 24 is a pair of balanced secondaries 25 and 26 of a transformer 21. A plate battery 28 and output resistance 29 are connected in series across the junction of the two secondary windings 25 and 26 and the junction of the filaments of the two tube-s 20 and 2!.

The primary of transformer 27 constituting part of the audio current transmission circuit comprises, with a plate battery 30 in series therewith, the plate circuit of an amplifier tube 3|. The grid of tube 3| is connected to the photoelectric cell circuit through a lead 32 having a condenser 33 in series therewith. A grid biasing battery 34 and resistance 35 is provided between the grid and filament of tube 3! to maintain a negative potential on the grid of tube 3|. The filament of tube 3| is grounded to complete the grid circuit across the photoelectric cell 5.

A lead 36 and ground connection 31 are connected across the resistance 29 and battery 28 of the bridge circuit 24 thus forming a grid circuit for an amplifying tube 38. A biasing battery 39 and resistance 40 are connected across the grid and filament of tube 38 to maintain a. negative potential upon the grid of tube 38. The output of the plate circuit of tube 38 passes through a transformer 4|. through further amplifying equipment (not shown) if desired, and thence into the loud speaker 42.

In the operation of the apparatus the battery 1 produces a constant potential upon. the photoelectric cell 5. The varying intensity of the light beam reaching the photoelectric cell 5 causes a drop in potential across the resistance 8 which varies in accordance with the print transmission of the film and, without condenser 46 shown in dotted lines, in accordance with the sound currents. This drop in potential is applied through the lead it! upon the grid of the detector tube H. Thus there is applied to the grid of tube Ii an alternating current voltage produced by the sound modulations as well as a direct current voltage due to the average photoelectric cell current produced by the average print transmission. The alternating components of output currents of the plate circuit of tube II when transferred to the secondary of transformer I2 is displaced 180 in phase relation so that the two leads l8 and I9 carry equal and opposite impulses of alternating current. The sliding contacts 43 and 44 are so adjusted along the potentiometer resistances l4 and H that during the passage of a normal unmodulated portion of the film before the photoelectric cell 5 the bridge circuit 24 will be in balance. Upon entering the bridge circuit 24 these alternating current component impulses are neutralized due to their 180 phase relation leaving only the direct current variations applied from contact 44 to change the impedance of the tube 2| and thus unbalance the bridge circuit 24 to produce a voltage drop across the resistance 29 which varies in accordance with the average print transmission of the film and in accordance with the volume variation of the sound by the currents produced by rectification when condenser 46 is not employed. This voltage is then applied through lead 35 and ground connection 31 to the amplifier tube 38.

The frequency of the sound reproduced is determined by the current passing through the lead 32, condenser 33, and tube 3|. The output of tube 3!, having a frequency equal to that of the recorded sound, is applied across the transformer 21 and into the plate circuits of the tubes 20 and 2|.

When condenser 46 is in circuit and by-passes the alternating components from cell 5, or the sound modulations, then unbalance of bridge 24 occurs only by currents produced by the average print transmission of the sound track.

It is thus realized that in using a film of the anti-ground noise type such as shown in Fig. 2, where the average print transmission varies in accordance with the amplitude of the sound modulations, both currents produced by this variation plus those produced by rectification may be used to disturb the balance of bridge 24 to obtain volume expansion in accordance therewith. To utilize both currents, condenser 46 is not used, this condenser being solely for the purpose of eliminating the alternating current modulations from the grid of tube II and volume expansion is to be in accordance with average print transmission only. In the case, however, of a record without noise reduction, then condenser 46 is not employed in order to obtain on the grid of tube I l the sound modulation currents which through rectification in tube II produced the unbalancing desired.

Attention is directed to the method of and circuit employed for eliminating filters in the output of the rectifier which consists of not only reversing the phase of the alternating current components and impressing both phase opposition currents upon the grids of the tubes in the bridge circuit, but also eliminates the direct current components from one of the grid circuits, thus obtaining an unbalance of the bridge.

Referring now to Fig. 3 the same elements 1-5 inclusive are employed as shown in Fig. 1 while the output of this system may comprise the usual amplifier 5D and loud speaker 5|. This circuit also employs a bridge 54 including two tubes 55 and 56 but differs from the bridge circuit 24 of Fig. 1 by employing an output transformer 58 and a tapped balancing resistance 59 for aiding the adjustment of balance of the plate circuits of tubes 55 and 56. The alternating current components are impressed on the bridge through the condensers Gil and amplifier tube 5! having a plate circuit including a primary of the transformer 63 and a plate battery 64 and having a grid circuit including a resistance 65 and biasing battery 66, to maintain a negative potential on the grid of tube 6|. The photoelectric cell 5 is polarized over a load resistance 69 and a resistance 10 by a source of potential "H which also supplies plate potential to a detector tube 13. Tube 13 has a grid circuit employing a resistance 10 and potentiometer resistance 15, which supplies grid bias to the tube 13 and also is a load resistance of the tube 13 for producing a potential for impression upon grids of tubes 55 and 55. A mid-tap 16 is connected to the cathodes of tubes 55 and 56 While the terminals of resistance 15 are connected to the grids of the tubes 55 and 56. Voltage created across the upper half of resistance '15 is therefore impressed on the tube of grid 56 and the voltage produced on the lower half is impressed upon the grid of tube 55. Since the lower terminal resistance 15 will be negative when the upper terminal is positive, alternating components are impressed on the grids of tubes 55 and 56 in phase opposition and will thus be balanced out in the plate circuit of the bridge tubes or, in other words, in the transformer 58, thus producing no output current. The direct current component, however, flowing in resistance 15 changes the biasing potential thereon at a rate to unbalance both sides of the bridge composed of the cathode anode circuits of the respective tubes, thus permitting the alternating current components of cell 5 amplified by tube 6| to be transmitted to amplifier 50 at an amplitude dependent upon the unbalance of bridge 54. This unbalance is directly proportional to the direct current component in the plate circuits of tube 13 flowing through resistance 15.

Similar to the circuit of Fig. 1 a condenser 18 may or may not be employed to shunt resistance 10, that is, if it is desired to obtain unbalance of bridge 54 solely by the variations in the print transmission of an anti-ground noise record shown in Fig. 2, the sound modulations may be by-passed across resistance 16, the direct current components in resistance 15 being due therefore only to the average print transmission. However, with the condenser out of circuit, then the direct currents occurring in resistance 15 are due to both the rectification of the sound modulations and the variations in print transmission in the case of an anti-ground noise record and only to the rectified sound currents in the case of the record not having the noise reduction feature.

What is claimed is:

l. A system of sound reproduction comprising a source of audio frequency currents, means for obtaining from said currents a current varying substantially in accordance with the amplitude variations thereof, means for obtaining said audio frequency currents in phase opposition, a balanced transmission element for transmitting said audio frequency currents, and means for impressing said amplitude varying currents on one portion of said transmission element and said phase opposition currents on two similar portions of said transmission element including said first portion.

2. In a sound reproducing system, the combination of a pair of vacuum tubes, each having a cathode, a plate and a grid and having plate circuits, a portion of which is common, a source of audio frequency current for impression on said individual plate circuits, a detector adapted to have impressed thereon said audio frequency current, the output of said detector being connected to the grid circuits of said pair of vacuum tubes, and means intermediate said pair of vacuum tubes and said detector for impressing on said grids respectively in phase opposition the detected audio currents and on the grid of one tube of said pair a current substantially proportional to the variations in amplitudes of said audio frequency current and an output circuit connected to said common portion of said plate circuits.

3. A sound reproducing circuit comprising a source of audio frequency current, a bridge circuit having a pair of vacuum tubes constituting two arms thereof, the anode cathode circuit of each tube constituting the impedance of each arm, means for impressing said audio frequency currents on the other two arms of said bridge, an output impedance connected intermediate said tubes and said last mentioned arms, a detector circuit, means for impressing said audio frequency currents on said detector circuit and means connecting said detector circuit and the grid circuits of said pair of vacuum tubes for impressing thereon said detected audio frequency currents in phase opposition and on the grid of one of said tubes, at current proportional to the amplitude of said audio frequency currents.

4. A sound reproducing system comprising a photosensitive device, a sound reproducer, means intermediate said photosensitive device and said reproducer for expanding the level of the currents appearing in the output of said photosensitive device, said means including a bridge circuit including a pair of vacuum tubes, a detector and connections between said bridge circult and detector for impressing on the grids of said tubes of said bridge circuit detected currents of said photosensitive device in phase opposition and a current proportional to the amplitude of the output currents of said photosensitive device.

5. The method of obtaining a control efiect proportional to and which varies in accordance with the amplitude of sound frequency currents comprising detecting said currents to produce a current whose amplitude varies in accordance with the average amplitude of said sound variations and deriving therefrom two potentials which vary in volume and frequency, eliminating the volume variations from one of said derived potentials and combining the same with said other derived potentials in phase opposition as respects audio frequency variations, and deriving said control effect from said combined potentials.

6. The method of expanding the volume level of sound currents transmitted through a bridge circuit comprising detecting said sound currents simultaneously with their impression upon said bridge circuit and impressing on said bridge circuit the alternating frequency components of said detected currents in phase opposition and a current proportional to the average volume level of said sound currents for unbalancing said bridge.

'7. The method of reproducing sound from a sound record having sound modulations thereon and an average print transmission varying in accordance with the amplitude of the recorded sound, said reproduction occurring over a bridge circuit comprising translating light varying in accordance with said sound record into electrical currents, impressing the alternating current components of said currents upon said bridge circuit and impressing the alternating current components of the detected currents upon said bridge circuit in phase opposition and also a detected impulse which varies with the average print transmission of said record.

8. In an apparatus for reproducing sound from a sound record, means for translating the sound record into varying electrical currents, means for detecting said current, a bridge circuit, means for passing the alternating components of said detected current into opposite sides of said bridge circuit, means for passing the direct current component of said detected current into one side of said bridge circuit, and an output circuit across said bridge circuit,

9. In a sound apparatus a source of audio frequency current, a sound reproducer, means intermediate said source and said reproducer for expanding the volume level of said audio current, said means comprising a detector adapted to have impressed thereon said audio current, a pair of vacuum tubes, the output of said detector being connected to the grids of said vacuum tubes, a transformer between said detector and the grid of one of said tubes, a common plate circuit for said tubes, means for impressing said audio current on said plate circuits, and means connecting said plate circuits to said reproducer.

10. A sound apparatus comprising a source of audio frequency current, a detector adapted to have impressed thereon said audio current, a resistance in the plate circuit of said detector, a pair of vacuum tubes, the grids of said vacuum tubes being connected across said resistance, a bridge circuit, the plate circuits of said vacuum tubes comprising two arms of said bridge, means for impressing said audio current on either arm of said bridge, and an output circuit across said bridge.

11. In an apparatus for reproducing sound from a sound record, a photo-electric device for translating the sound record into an audio frequency current, a transmission circuit for said audio current, a pair of vacuum tubes having their cathode-anode circuits connected to said transmission circuit, a detector tube, means for impressing on the grid of said detector tube said audio current, a transformer having its primary winding in the cathode-anode circuit of said detector tube, means for impressing on the grid of one of said vacuum tubes the current flowing in the cathode-anode circuit of said detector tube and means for impressing on the grid of the other of said vacuum tubes the current flowing through the secondary winding of said transformer.

12. In an apparatus for. reproducing sound from a sound record, a photo-electric device for translating the sound record into an audio frequency current, a transmission circuit for said audio current, a detector tube, means for impressing said audio current on the grid of said tube, a transformer and resistance in the cathode-anode circuit of said tube, a resistance in the secondary circuit of said transformer, and means for impressing on said transmission circuit a current controlled by the drop in potential across both of said resistances.

13. An apparatus for reproducing sound from a sound record comprising a photo-electric device for translating the sound record into an audio frequency current, a transmission circuit for said audio current, a pair of vacuum tubes having their cathode-anode circuits connected to said transmission circuit, a detector tube, means for impressing said audio current on the grid of said detector tube, a resistance in the cathode-anode circuit of said detector tube, and means connecting the grids of said vacuum tubes across said resistance whereby the alternating current components impressed upon the grid of one of said vacuum tubes will be in phase opposition with the alternating current components impressed on the grid of said other vacuum tube.

JAMES R. BALSLEY.

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