Patent GB-473256-A (GB)
Title: Improvements in and relating to the recording and reproduction of sound
IPC: G03B31/00
Filed: 1937-05-08
Abstract: 473,256. Optical sound recording and reproducing. KOLB, O. K. (Schlegel, K. H. F.) May 8, 1937, No. 13212. [Class 40 (ii)] Relates to sound films having one or more sound tracks and a control track adapted to control amplification during reproduction, and consists in arranging the control track 1, Fig. 1, within or substantially within the strip of the film occupied by the sprocket holes 2 at one edge of the film. As a modification, two control tracks may be used, one along each edge. During reproduction, the disturbance caused by the sprocket holes is smoothed out. The control track is conveniently of constant-opacity variable-width symmetrically disposed about the centre line with the opaque or transparent part in the centre, and may be so used that the control current is at zero value when the sound amplitudes are largest or smallest, but preferably the latter. The control track may be derived automatically or from a manually operated volume control potentiometer, and may be impressed on the film at the same time as the sound track or subsequently. The width of the control track may be proportional to the degree of amplification or follow another mathematical law, for example where the control track is to influence the grid bias of an exponential valve, the width of the track should follow a logarithmic law. In the recording circuit, Fig. 5, the voltage produced by the microphone is applied between the. terminals 15, 16 to the grid of an exponential valve 19 followed by an amplifying valve 20, the output of which operates the mirror device 6. Part of the output goes to a potentiometer 23 in the grid circuit of an amplifying valve 21 followed by a rectifier 22, and a filter circuit 24, 25, 26, the resistance 25 of which is in the grid circuit of the exponential valve 19. This results in a reduction of amplification, which may be made to commence at the amplitude desired according to the bias given to the rectifier 22. In the corresponding reproducing circuit, Fig. 6, a photo-cell 28 influenced by the sound track is connected to the grid of an exponential valve 30 followed by a series of amplifying valves 31 feeding a loud-speaker 32 ; a second photo-cell 29 influenced by the control track is connected to amplifying valves 34, 35 followed by a rectifier 36 and a filter 37, 38, 39, the resistance 38 of which is in the grid circuit of the exponential valve 30, whereby the amplification is controlled. Potentiometers 40, 33 are provided for adjustments, and a rectifier 41 may be added to limit the increase of grid bias and thereby limit maximum amplification to an amount within the capacity of the reproducing amplifier. In the film, Fig. 8, two control tracks 1a, 1c are provided for controlling the amplification of the weak and strong sounds respectively. A reproduction circuit for this film comprises three photocells 47, 49, 50, Fig. 9. The photo-cell 47 is affected by the sound track and is connected to the exponential valve 48 of an output amplifier. The photo-cells 49, 50 are affected separately by the control tracks and connected by amplifying devices to oppositely directed rectifying devices 53, 54 leading to resistances 51, 52 in the grid circuit of the exponential valve 48, whereby the amplification of the sound is controlled. A conventional apparatus is described for producing the sound track and a record track by means of two separate mirrors and optical systems. A reproducing apparatus, Fig. 7, is described, in which the sound track and the control track are illuminated from a single source 42 through a slit 43, and separate prisms 45, 46 are used to throw the light from the two tracks on to separate photocells 28, 29. The two tracks might be displaced longitudinally one relatively to the other during recording, or in copying or printing.