GB1012160A - Improvements relating to stereophonic transmission systems and receivers therefor - Google Patents

Abstract

1,012,160. Stereophonic transmission systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. Jan. 26, 1962 [Jan. 26, 1961], No. 3081/61. Heading H4R. In a stereophonic transmission system in which two audio signals representative respectively of the sum and difference of two further audio signals which are derived from a sound source and which differ one from another in dependence upon the direction or position of said source, the difference signal is subjected to amplitude compression before it is actually transmitted. Rising transients occurring in the envelope of the sum signal may also be compressed. It is stated that by these means the bandwidth (in a frequency modulation transmission system) allotted to the sum signal may be increased without degrading the difference signal which carries the stereo information. Signals derived from left and right microphones 1, 1<SP>1</SP> are combined in circuits 2, 3, part of the sum signal then being passed via a delay network 21 to an amplifier 22 and part being passed via circuits 18, 19, 20 so as to produce a signal which controls amplifier 22 whereby amplitude compression of transients in the sum signal is obtained. A filter responsive to transients is depicted in Fig. 3 (not shown). The difference signal is treated in a similar manner but in this case the circuit 7 (Fig. 2, not shown) takes the form of a low-pass filter which controls the rise and fall times of the envelope of the difference signal as derived by rectifier 6. A receiver for the above system is described with reference to Fig. 4 (not shown) in which receiver the difference signal only is expanded before being combined with the sum signal so as to produce a stereophonic output.