GB931422A - Improvements relating to stereophonic sound transmission systems - Google Patents

Abstract

931,422. Stereophonic systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. Oct. 12, 1959 [Oct. 11, 1958; Nov. 8, 1958], Nos. 32522/58 and 35920/58. Class 40 (4). [Also in Group XL (c)] In a stereophonic system two directional signals VL and VR derived from sound are linearly combined to form two signals A and B each of which is compatible (viz. such that A and B are never zero for any non-negative values of VL and VR, excluding the case where VL and VR are both zero), from which signals the directional signals VL and VR may be regenerated. As described, the signals A and B are broadcast and the directional signals VL and VR are regenerated at a receiver to effect stereophonic reproduction. Apparatus for deriving two compatible signals.-The left and right signals VL and VR, which may be derived from crossed directional microphones, appear in channels 1 and 2, respectively (Fig. 1). ). VL is applied to two summing circuits 3 and 4, directly in the case of 4 but through an amplifier of gain two in the case of 3, while VR is applied directly to 3 and 4. The output of 3 is therefore 2VL + VR and that of 4 is VL + VR. These two signals, which constitute the signals A and B, amplitude modulate a carrier wave from oscillator 8 in modulators 6 and 7, the filters 9 and 10 suppressing the lower and upper sidebands respectively, and the remaining sidebands being mixed at 11. Stereophonic reproduction from signals A and B.-The received signals are applied to filters 14 and 15 (Fig. 2), corresponding to 9 and 10 (Fig. 1), which select the upper and lower sidebands respectively and apply them to demodulators 16 and 17 to derive signals A and B. Signals A and B are applied to the differencing circuit 18 which produces A-B, namely VL Signal B is also applied via an amplifier 19 of gain two to the differencing circuit 20 where A is subtracted to produce 2B-A, namely VR. The signals VL and VR are amplified at 21, 22 and fed to spaced loud-speakers 24, 25. Modified arrangements.-In the arrangement of Fig. 3 (not shown) the signals VL and VR are applied to a circuit which produces a relative phase shift of 90 degrees between them to produce VL and jVR. These signals are added to produce A and subtracted to produce B. At the receiver (Fig. 4, not shown) the signals A and B derived by filtering and demodulating the sidebands are summed to produce 2VL and differenced to produce 2jVR, these signals then being passed through a circuit which produces a relative phase shift of 90 degrees between them to derive signals representative of VL and VR which are fed to the loud-speakers.