GB947371A - Improvements relating to sound transmission systems - Google Patents

Abstract

947,371. Automatic volume control; stereophonic systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. March 29, 1960 [Dec. 30, 1958], No. 41976/58. Heading H4R. In a sound transmission system control means are provided for controlling the loudness of a sound to be reproduced, as a function of the direction or position of a source of the sound. Spaced microphones 1, 2 receive signals, which differ in dependence upon the position of the sound source, and feed these signals to pre-amplifiers, volume and tone control circuits 3, 4. The left- and right-hand signals V L and V R are combined linearly at 6 to form the composite audio signal V A and they are also passed into a processing circuit 5 which derives a control signal V C . The relationship between the value of V C and the direction of the source is shown at 7 and V C has the form V L <SP>11</SP>/(V L <SP>11</SP>+ V R <SP>11</SP>), the derivation of which by the control circuit is fully described in Specification 871,992. V C is then applied to a non-linear network 8 which is arranged to modify the control signal in a non-linear way; for example using a network similar in construction to that described in Specification 932,557, the relationship between its value and the source direction is arranged to be that represented by the diagram 9, i.e. V C (modified) has a maximum value for central signals and is less for signals originating from any other direction. The modified control signal and the audio signal V A are then multiplied together at 10, using for example Hall effect elements such as those described in Specification 932,557, the modified audio signal and the control signal then being transmitted in any known way. Non-linear network 8 may have a manual control for emphasizing signals from directions other than the central one. Fig. 2 (not shown) illustrates the invention as applied to a stereophonic reproduction system. In this case V L and V R are not combined, but are each multiplied by a modified control signal obtained from separate non-linear networks although it is stated that a single non-linear network only need be used. The invention may be applied to a system for increasing the number of audio channels available for producing stereophonic reproduction as described in Specification 932,557. Specification 890,833 also is referred to.