GB397514A - Improvements in and relating to photographic sound records and systems for recording sounds on a light sensitive film - Google Patents

Abstract

397,514. Valve amplifying circuits. BRITISH ACOUSTIC FILMS, Ltd., Film House, Wardour Street, London. Feb. 22, 1932, No. 5299. Convention date, Feb. 26, 1931. [Class 40 (v).] Photographic soundrecords of the variable width or of the variable density - type and in which the position of the zero line of the sound trace in relation to the edge of the record track, or the average transparency, is displaced as the amplitude of the sounds recorded varies are so produced that the displacement of the zero line or the average transparency is varied more rapidly than the amplitudes as these increase. It is further provided that the "displacement" is effected more slowly than the amplitudes as these decrease.' A record so produced not only masks ground noises but avoids sudden changes in passing from reproduction to silence, and vice versa, owing to the zero line in the case of a varying width record being produced with the form o, Fig. 2. This result is attained by diverting and rectifying a portion of the microphone current being recorded and employing this to adjust the coefficients of conductivity in the electrical recording circuit whereby variations in the rectified current are effected more quickly when the current increases than when it decreases. In one arrangement the microphonic recording current influencing through a transformer T, Fig. 3, an oscillograph e or a recording light source has a portion diverted through a transformer T<1> to a rectifying valve S whose plate circuit includes a smoothing condenser u and a resistance t in parallel, a variable connection from the resistance passing to the grid of an amplifier V whose output is superposed on the recording current. The variation of the internal resistance of the valve S by the incoming current variations changes the time constants of the plate circuit during charging and discharging of the condenser, and by suitable selection of the values of the condenser u and resistance t in relation to the internal resistance of the valve S the shape of the curve representing the rectified current is modified so that variation of current due to a given increase in amplitude takes place more rapidly than variation due to a corresponding decrease in amplitude. This effect is superposed on the recording current to effect the corresponding displacement of the zero line or variation of average transparency. To avoid excessive time of discharge of the condenser, an inductance mav be inserted in series with the resistance t, or in series with the grid leak in the input circuit of the valve S. Specifications 377,675 and 384,333, [Group XXXVIII], are referred to.