Patent GB-768956-A (GB)
Abstract: 768,956. Automatic volume control. PERSPECTA SOUND, Inc. Sept. 27, 1954 [Oct. 8, 1953; July 2, 1954], No. 27899/54. Class 40 (4). In a system wherein the respective outputs of a number of spaced loud-speakers are controlled in accordance with a number of control signals, the control unit includes means whereby the output of at least one loud-speaker can be made independent of the control signals when a predetermined condition of the signals exists. The control signals which are separated from the combined intelligence and control signal input to terminals 111, Fig. 6, by filter 92 are pre-amplified at 95 and separated by filters 93 to produce direct control voltages in amplifierrectifiers 117 for control of the respective intelligence reproducing circuits 32, 33, 34. The control signals are also integrated in a resistor 121, which serves also for adjusting sensitivity and rectified in 123 to energize relay 124. The relay is released when the total signal level falls below a predetermined minimum and connects the power amplifier 33 direct to the output of filter 91. More loud-speaker circuits may be similarly directly connected. The circuit of the control amplifiers 122, 125 and rectifier 123 is described with reference to Fig. 7 (not shown). The system described is intended to create an impression of stereophonic sound in a cinema, the control signals which are small in amplitude compared with the programme signal (at least - 18 db.) and may be above or below the audio band being derived from a single recording track. The relay arrangement besides providing a safeguard against the interruption of the programme in the event of a fault in the control circuit, enables the reproduction of standard films (i.e. films without stereophonic control signals) by the same apparatus. The composite signal recording arrangements wherein the control frequencies have their levels adjusted manually as desired prior to mixing, are described with reference to Fig. 1 (not shown) and an arrangement in which the control signals are regulated automatically by spaced microphones is described with reference to Fig. 5 (not shown). A mechanical delay arrangement comprising a recorder and a reproducer co-operating with a moving continuous recording medium, described with reference to Fig. 4 (not shown), may be incorporated in one of the loud-speaker inputs to create a reverberation effect. A time delay circuit is incorporated in the relay circuit to prevent its release during periods of silence, and a much longer time delay is incorporated in the operate circuit to prevent false actuation by noise when a track not having control signals is being used. The film has a length of control track preceding the sound track to compensate for the latter delay. An additional delay circuit is provided to prevent control signal waver due to intermodulation and the control signals are advanced relative to the sound track. Sound signals at terminals 201, Fig. 8, selected by high-pass filter 204 are amplified in a valve which includes a phase inverter stage, and applied to parallel connected push-pull amplifiers V10, V11-V14, V15 to feed the respective loud-speakers over output transformers 210, 212, 213. The control signals which are selected by filter 208 and separated by filters 214-216 are rectified in diodes V7-9 to vary the bias to the grids of amplifiers V10-15 from source 224 during stereophonic working and are applied to the cathode follower stage of valve V3 to control relay 234 through the right-hand portion of valve V2 and the lefthand portion of valve V3. When relay 234 is released due to the absence of control signals, valves V12, V13 are biased for normal working by source 226, and the other two amplifiers are cut-off by source 225. The filters 214-216 and 218-220 must individually or jointly have a good flat topped characteristic to avoid intermodulation. The system may also be used to give a stereophonic reproduction of orchestral work. Booming is prevented on changeover from one to three loud-speakers by poor power supply regulation.