Patent GB-421615-A (GB)
Title: Improvements in or relating to photographic sound-recording apparatus
IPC: G11B7/00
Filed: 1933-06-27
Abstract: 421,615. Optical sound - recording apparatus. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Silent, H. C.; 2409, Detour Drive, Los Angeles, U.S.A.) June 27, 1933, No. 18307. Convention date, June 30, 1932. [Class 40 (ii)] Relates to photographic sound-recording apparatus in which a light valve or the like controlled by alternating currents corresponding to the sound waves to be recorded is also controlled by a slowly varying bias voltage (" dynamic bias "), derived from a circuit which includes a condenser or other storage device fed by a rectified component of the audio-currents, the storage device having separate charging and discharging circuits. In such a system, which is designed for minimizing background noise in sound reproduction, the recording device is so set and biased that in the absence of alternating currents the photographic exposure is a minimum, but upon occurrence of and increase in alternating sound currents a rapid increase in the average exposure is effected by the dynamic bias set up by the charging of the condenser. Upon decrease of the alternating currents, in response to decrease of sound intensity, a restoring effect is brought about by the discharge of the condenser in the control circuit, but this action need not be so rapid, as in this case there is no risk of over-modulation. According to the invention, both the operating time and the restoring time of the control circuit are made independently adjustable by providing in the charging circuit of the storage device an adjustable impedance connected in series, and an adjustable impedance connected in shunt, and in the discharging circuit an independent adjustable impedance connected in series, and an independent adjustable impedance connected in shunt. The impedances are preferably resistances. In the system shown, an amplifier 15, connected to a microphone 14, supplies currents to a transformer 16, the output of which is transmitted through a condenser 17 to the ribbons 11 of a light-valve by which a record is made on a film 6. A constant current flows from a battery 18 through the ribbons, the current passing through an inductance 23, variable resistance 22 and an asymmetrically conductive device 21. The strength of the " static current" is controlled by the resistance 23, to adjust the normal spacing of the ribbons 11. The amplifier 15 also supplies a second amplifier 36, the output of which is fed through a transformer 37 to a full-wave rectifier 31. A condenser 25 is connected between the rectifier and the battery 18, impedances 29, 35, 30, 28, 26, 27 being arranged as shown. The battery 18 charges the condenser 25, the charging time being mainly determined by adjusting the resistances 26, 28. When current fiows into the condenser from the rectifier 31, it reverses the charge previously received from the battery, the time taken for this reversal largely depending on the values of the resistances 35, 30. Current then flows through the network in which the battery is included, and the voltage supplied by the battery to the ribbons 11 is reduced, thereby increasing the mean spacing of the ribbons. For rapid operation, the resistances 35, 30 should be small, and the resistance 29 infinite. When the rectified potential applied to the condenser is sufficient to neutralize fully the voltage of the battery 18, the ribbons have a maximum spacing, and the light valve can record currents of maximum amplitude. The device 21 prevents any further increase in the rectified potential from increasing the spacing of the ribbons. When the rectified potential falls again tho zero, the condenser discharges through the resistances 28, 26, 22, and the battery is eventually effective to reduce the spacing of the ribbons of the light valve.