GB477726A - Improvements relating to photographic sound records and methods of producing same - Google Patents

Abstract

477,726. Recording photographic sound-films. BRITISH ACOUSTIC FILMS, Ltd., POULSEN, A., KOLB, O. K., PRATT, A. S., and BOSTON, D. W. Nov. 17, 1936, No. 31501. [Class 40 (ii)] A photographic sound -film of the " noiseless " type, i.e. having low mean transparent width or low mean transparency during periods of low sound intensity, but increasing in mean transparent width or mean transparency as the intensity of the sound increases, is recorded by apparatus in which the biassing device governing the mean transparent width or mean transparency is controlled by two or more circuits having different time constants so that whereas the biassing device responds rapidly to an increase in the intensity of the sound to be recorded, it responds more slowly to a decrease in sound intensity in a manner dependent on the duration and intensity of the sound immediately preceding such decrease. The Specification states that the varying sensitivity of the ear is thus compensated for, enabling an aural impression of constant low background noise to be created. A microphone 1 is connected to an amplifier 2, the output of which is connected through the lines 3 to the primary of a transformer 4 whose secondary supplies current of audio-frequency to the recorder 5. The amplifier output is applied also over lines 6 between the grid and cathode of an amplifying valve 7 biassed by a resistance 8 and condenser 9. The anode of this valve is fed from a battery 10 through the primary of a transformer 11 whose secondary has connected across it the two diodes 12, 13 forming a full-wave rectifier. The output of this rectifier is smoothed by a filter comprising resistances 14, 15 and condensers 16, 17. Two rectifiers 18, 19 connected across the secondary of the transformer 11 have their output smoothed by a filter comprising resistances 20, 21 and condensers 23, 24 and then applied through a further rectifier 31 to the anodes of the diodes 12, 13. The output from the smoothing filter comprising resistances 14, 15 and condensers 16, 17 is applied to the grid of a valve 25, bias being provided by the battery 26 shunted by the byepass condenser 27. The high-tension voltage for the valve 25 is supplied by a battery 30 and the anode current of this valve is passed through the recording device 5 to provide the necessary bias in response to the intensity of the sound being recorded. A blocking condenser 28 prevents anode current from flowing in the winding of the transformer 4. The time constant of the circuit including the diodes 12, 13 and following smoothing filter is relatively short, but the time constant of the rectifiers 18, 19 and following smoothing filter is considerably longer. According to a modification, a single fullwave rectifier 12, 13, Fig. 2, may be employed, followed by one circuit comprising a resistance 34 and condenser 35 in parallel, with a short time constant, and by a second circuit consisting of a resistance 32 and condenser 33 in series with a relatively long time constant.