Patent GB-417874-A (GB)
Title: Improvements in or relating to photographic sound recording apparatus
IPC: G11B7/00
Filed: 1933-04-13
Abstract: 417,874. Current and pressure meters. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.- (Assignees of Silent, H. C. ; 2409, Detour Drive, and Scoville, R. R. ; 108, South Harper Street, both in Los Angeles, U.S.A.) April 13, 1933, No. 11094. Convention date, April 14, 1932. [Class 37.] Relates to photographic sound recording processes of the kind in which alternating electric currents representing the sound waves (which currents may be predistorted as by amplitude frequency equalization or not) govern the exposure or the exposed width of track, and the average of this for several cycles is controlled by a " dynamic " bias voltage or current set up in the control circuit and acting in opposition to a "static" bias, and consists in methods of adjusting said dynamic bias and static bias and the relation between the dynamic bias and the magnitude of the alternating currents by means of a test oscillator, amplifiers and measuring instruments. The apparatus used comprises a test oscillator and control network, Fig. 1, an amplifier, Fig. 2, a second amplifier with input control potentiometer, Fig. 3, a control circuit and recording device, Fig. 4, an input circuit, Fig. 5, and a circuit, Fig. 6, for measuring the radiant output of the recording device. The test oscillator and control network, Fig. 1, comprises an oscillator 10 connected to a series of resistances 11-16 and also through resistances 28, 29 to a balanced constant impedance potentiometer P of known type having a pair of simultaneously movable sliders 30, 31. These sliders are connected to terminals 40, 41 through two pairs of switches C, C<1> and D, D1 so that according to the position of these switches the whole voltage between the sliders may be transmitted to the terminals 40, 41 or only a definite percentage of the whole determined by the system of tapped resistances between the switches C and C<1> and D and D<1> respectively. The mid points of all the resistance systems are connected to an earth terminal 60. The output of the oscillator 10 can be measured by a milliametre 27, which can be connected by a switch A to various tappings between the resistances 11-16. The same instrument can be used to measure the amplification of the amplifiers, Fig. 2 and Fig. 3. For this purpose, the amplifier to be tested is fed from the terminals 40, 41 and the output from the amplifier is led to terminals 79, 80 shunted by tapped resistances and connected to a switch B which can be set to connect the milliametre 27 to the oscillator 10 or to the output from the amplifier. The control and recording circuit, Fig. 4, comprises a light valve 128 having a shutter 131, 132 connected by wires 144, 145 to a bias battery 143 and also to a rectifier 155-158, which is supplied from the amplifier, Fig. 3, through terminals 124, 125 and 152, 153, the amplifier, Fig. 3, being itself fed from the amplifier, Fig. 2, through terminals 75, 76 and 101, 102. The fixed bias from the battery 143 is adjusted by a variable resistance 148 and the rectifier superimposes a control depending on the amplitude of the current from the amplifier, Fig. 3. This current to be recorded is supplied to the light valve 128 through a condenser 175 and terminals 166, 167 adapted to be connected to terminals 164, 165 of the input circuit, Fig. 5, comprising a transformer 172 and a potentiometer 170, which is connected through terminals 162, 163 to the terminals 77, 78 of the amplifier, Fig. 2. In use, the terminals 73, 74 of the amplifier, Fig. 2, are connected to a microphone, and the light valve is used to impress a record ; but for making the adjustments in accordance with the invention, the terminals 73, 74 of the amplifier, Fig. 2, are connected to the terminals 40, 41 of the oscillator network, Fig. 1, and the light valve is made to affect a photo-cell 130, Fig. 6, connected to a thermionic valve 138 containing in its anode circuit a meter 140 shunted by a battery 141 and an adjustable resistance 142 set to balance the normal current from the valve. So long as the light-valve works within the limits of its capacity, the meter 140 is unaffected ; but when the light-valve is overloaded for its capacity in one direction, the distortion is indicated by the meter 140. The method of the invention is as follows : Current is supplied from the oscillator through the amplifiers and the input circuit, Fig. 5, to the light-valve, and the potentiometer 170 is adjusted until the light-valve is fully loaded. Switches C, C<1> are then opened to reduce the input, the resistances being designed to make a reduction of 10 to 15 decibels. Switch H, Fig. 4, is then closed and resistance 148 adjusted to reduce the capacity of the light-valve until it is again fully loaded by the reduced input. Switches C, C<1> are then again closed, switches D, D<1> are opened, and switch G, Fig. 4, closed to introduce the automatic control of the bias of the light-valve. The attenuation caused by the opening of the switches D, D<1> corresponds to the ratio between the input power which fully loads the valve and the maximum input, this margin being required to prevent overloading while the automatic bias control is building up. The potentiometer 109 of the rectifier, Fig. 3, is then adjusted until the automatic control or " dynamic " bias just balances the " static " bias previously determined by the adjustment of the variable resistance 148, this balance being indicated by the reading of the meter 146, Fig. 4, being reduced practically to zero. The adjustments are now complete, and the test oscillator may be replaced by a microphone, and the measuring device, Fig. 6, by a record impressing device.