GB414680A - Improvements in or relating to photographic sound recording apparatus - Google Patents

Abstract

414,680. Valve amplifying circuits. SIEMENS & HALSKE AKT.-GES., Siemensstadt, Berlin. Feb. 8, 1933, No. 3854. Convention date, Feb. 16, 1932. [Classes 40 (iii) and 40 (v).] Relates to sound-recording apparatus of the type in which alternating currents corresponding to sound waves (which currents may be predistorted, e.g. by volume-level equalization) are used to produce a photographic record in any usual manner, and in which the average exposure and/or average width of the exposed track on the photographic surface, for a length representing several cycles of the alternating currents, is also controlled by means of a relatively slowly varying bias voltage set up in a control circuit with which is associated a device for limiting the magnitude of the bias voltage. According to the invention, the limiting device consists of a device of which the impedance changes rapidly at or about some definite value of applied voltage, and is connected in shunt with the control circuit. The limiting device is preferably a rectifier of the copper oxide type. In the arrangement shown, a portion of the microphone currents applied to the input terminals 1 passes to a valve 2, the output of which, amplified at 18, controls a recording-device comprising a Kerr cell 15, which is connected to a battery 16. Another portion of the microphone currents passes through a transformer 3, the secondary of which is connected to a control circuit which includes a rectifier 4 and a filter 5, 6, 7 for suppressing high frequencies. The rectifier produces a slowly varying voltage in the circuit. Passing over fixed and variable resistances 8, 9, the rectified currents are delivered to the biaslimiting device. This comprises a rectifier 11, connected together with a resistance 10 and a battery 12 in shunt with the control circuit. A resistance 13 is connected to the rectifier 11 and to another battery 14 as shown, and a bias voltage for the grid of the amplifier 2 is tapped off at 21 from the resistance 13, whilst another tapping at 22 provides a bias potential for the Kerr cell 15. At the ends of the resistance 8 there is a voltage which varies continually in accordance with the volume of the sounds being recorded. The voltage of the battery 12 is such that so long as the voltage across the resistance 8 does not exceed a predetermined limit, no current can flow through the branch 11, 12, the voltage drop along the resistance 13 varies continually, and the bias voltage applied to the Kerr cell varies accordingly. When however the voltage supplied caross the resistance 8 exceeds the determined limit, current flows through the rectifier 11 and the resistance 10 in the direction opposite to that shown by the arrow A, and the Kerr cell bias voltage remains practically constant. Instead of applying the limited voltage tapped off from the resistance 13 to the Kerr cell, in order to vary the position of the zero line on the record, it may serve the same purpose by controlling a shutter or influencing the current in the recording lamp.