Patent GB-473772-A (GB)
Title: Improvements in or relating to sound recording and reproducing systems
IPC: G11B7/00
Filed: 1936-05-12
Abstract: 473,772. Optically recording or reproducing sound. ELECTRICAL RESEARCH PRODUCTS, Inc., and LATIMER, K. E. May 12, 1936, No. 13466. [Class 40 (ii)] A sound-recording apparatus has means for amplifying to a substantially constant amplitude, signal currents modulated by the sounds to be recorded, and records these currents thus modulated, together with currents which are a measure of the volume variation introduced in order to obtain the substantially constant-amplitude signal currents, in a single trace. In the sound-reproducing apparatus for records so produced, the currents derived from the record are divided into two components, one corresponding to the frequencies of the substantially constant-amplitude soundmodulated currents, and the other corresponding to the volume variation introduced while recording, and this latter component is used to control the gain of a reproducing amplifier so that the output thereof will correspond to the sounds originally recorded. The volume variation is preferably measured by a low-frequency current below the range of the sound frequencies to be recorded. The recording apparatus comprises a microphone 9, Fig. 1, translating sounds into a modulated electric current which is supplied to an amplifier 10 whose output is fed to a compressor unit 13 preferably comprising a pair of variable mu thermionic valves operating in push-pull. An amplifier detector 11 is bridged across the output of the amplifier 10 and develops a direct current voltage which varies according to the envelope of the sound current, and the output of the amplifier detector is applied to two circuits, one a low-pass filter 12 and the other the compressor unit 13. The output of the amplifier detector 11 thus controls the gain of the compressor unit 13 so that all waves are amplified to an approximately constant level. The output from the compressor unit 13 is applied through an amplifier unit 14 to a high-pass filter 15 which is arranged with the low-pass filter 12 so that they terminate each other for frequencies near a cross-over point at about 50 cycles per second. The outputs of the filters 12, 15 are combined and applied to a sound-recording device, e.g. a light valve 16, the connections being such that, in known manner, the spacing of the light valve ribbons is increased by the amplifier detector 11 when loud sounds are being recorded, but diminished when soft passages are being recorded, thereby producing a record of " noiseless type. The low-pass filter 12 limits the frequencies used for the control current which is a measure of the variation of volume, to a range below that used for recording the sounds, whilst the high-pass filter eliminates from the sound current, the frequencies of the order used for the control current. In the reproducing apparatus, a light beam is passed through the film carrying the sound track, to fall upon a photo-electric cell 20, Fig. 2, whose output is fed to a preliminary direct-current amplifier 21. The output of this amplifier is divided by a low-pass filter 22 and high-pass filter 23 into control and sound-reproducing currents respectively. The control current is passed through an amplifier 24 to an expander unit 25 (similar to the compressor unit 13, Fig. 1), which receives also the output of an amplifier 26 for the sound current. The output of the expander unit is applied through an amplifier system 27 to a sound-reproducing device 28, or to a re-recording device. A switch 29 is provided in the recording apparatus whereby the operator may anticipate the arrival of sounds of sudden large amplitude, to compensate for the time lags in the responses of the amplifier detectors &c, In a modification, the input of the amplifier detector in the recording apparatus may be derived from a separate microphone placed nearer the source of sound than the microphone 9 so that the compensation for time lag is approximately automatic. When recording by the variable-density method, the sounds may be recorded in two tracks which are similar but reversed, i.e. one is light where the other is dark, and the volume compression is then superimposed on either or both of the tracks. With this system, the control of the expander unit in the reproducing apparatus is derived from the output of the two photo-electric cells in parallel, whereas the sound currents are obtained from the same two cells in push-pull. The amplifier detector may have a characteristic to compensate for any distortion resulting from the non-linearity of the H and D curve of the photographic record. In a further modification, the compressed speech current may operate one light valve through a high-pass filter and the amplifier detector may operate another light-valve through a lowpass filter, one light-valve producing a variable density record and the other light-valve varying the area of this track. Specification 456,734 is referred to.