Patent GB-505011-A (GB)
Title: An improved process and apparatus for recording sound on film
IPC: G11B7/00
Filed: 1937-09-02
Abstract: 505,011. Optical sound - recording. KOLB, O. K. (Schlegel, K. H. F.) Sept. 2, 1937, No. 23995. [Class 40 (ii)] Relates to the production of a photographic sound record by directing a beam of light on to a moving film and varying the exposure of the film, both in a manner dependent on' the wave form of the sound oscillations and in a manner dependent on the volume of the said oscillations and consists in a method of controlling the variation of the average amount of light impinging on the film, within an interval of time in which a distinct wave train persists,- a distinct wave train being considered as one occurring between periode of silence or substantial quiescence-by increasing this average amount, of light by the biassing of the sound recording light valve or like means in accordance with the volume of a single half-wave of the wave train in question, which half-wave is directed in such sense that it must be recorded by effecting an instantaneous decrease of the light falling on the film. The method, according to the invention may be employed to record a sound comprising either substantially constant or continuous oscillations or one comprising damped or discontinuous oscillations so as to produce either a noiseless variable area or variable density record. Fig. 1 shows, graphically, a discontinuous trace of oscillations. In recording such oscillations according to the invention, the direction of the connections between the light valve and the recording amplifier is arranged so that the half-waves having the amplitude a are recorded by effecting an instantaneous increase of the light falling on the film whilst the half-waves having the amplitude b are recorded by effecting an instantaneous decrease in such light. A single half-wave having the amplitude b is therefore utilized for effecting the necessary displacement of the zero line. Fig. 3 shows a circuit arrangement according to the invention in which the sound currents from a microphone M are amplified at V and fed through a transformer T<1> to a light valve L adapted to oscillate a light beam by means of which the sound oscillations are recorded on the film. A portion of the sound currents is fed to the primary winding of a transformer T2, the secondary winding of which is connected up in two different circuits, one of which-the " closing circuit-includes a rectifier D' and a condenser C1 in series, whilst the other or "opening" " circuit includes a rectifier D2 and a condenser C2 in series. The rectifiers are so arranged that the half-waves of the portion of the sound current fed thereto by the transformer T2 correspond to the halfwaves b, Fig. 1, and the time constants of the two circuits are so chosen that the said halfwave, when appearing, charges both of the condensers practically simultaneously so that the condenser C<1> is charged to a voltage V1 which is somewhat higher than the voltage V2 to which condenser C2 is charged, and so that both of these voltages are sufficiently high to prevent the consecutive half-waves having a smaller amplitude than the half-wave b passing the rectifiers D1, D2. The mean position of the light valve L is determined by the anode current in a valve N when a grid bias of the value-E1 is applied thereto, the maximum displacement of the zero line occurring when the grid bias is increased to -E1+8. The mean position of the light valve is controlled by the charge of a condenser K2 included in the grid circuit of the valve N, this condenser, in series with a resistor r2, being connected across condenser C2 and forming part of the " opening " circuit. The time constant of this " opening circuit is so chosen that the charging of condenser K2 is effected at the rate at which it is desired to vary the average amount of light impinging on the film when a sound oscillation having a second half-wave of the magnitude b appears. A rectifier D4, connected across the condenser K2 in series with a portion, having the voltage e, of the high voltage source, prevents the voltage of the condenser K2 from exceeding the value e. The return motion of the zero line depends upon the discharge of the condenser K2. This condenser is discharged through a rectifier D3 and a pentode valve U in series whilst the condenser C1 in the " closing" circuit is discharged directly through the valve U. The rectifier D3 connected up between the condensers C1, C2 prevents the latter from being discharged until the voltage across condenser C1 has decreased to the same value as the voltage V2. Condenser C1 thus delays the discharge of condenser C2 so that the average amount of light falling on the film, after having been increased to a certain value by voltage V2, is maintained at that value through a period of time depending on the rate of discharge of condenser C1. In order to prevent the voltage V1 from increasing to too high a value, a detector D7 is inserted between the anode of the valve U and a point of the high voltage source having the potential -E+e1 so that voltage V1 cannot increase to a value higher than -E+e1. Specifications 377,675 and 393,610 are referred to.