Patent GB-970051-A (GB)
Title: Compatible stereophonic transmission and reception systems,and methods and components characterizing same
IPC: H04H1/00
Filed: 1961-02-10
Abstract: 970,051. Stereophonic radio systems. L. R. KAHN. Feb. 10, 1961 [Feb. 11, 1960], No. 5139/61. Heading H4L. A system for the transmission of two stereophonically related signals comprises means for amplitude modulating a carrier wave with the sum of the stereophonic signals and means for phase modulating the carrier wave with the difference between the stereophonic signals in quadrature relation to the sum signals, whereby stereophonically distinguishable components of one signal appear as a first order, upper sideband and stereophonically distinguishable components of the other signal appear as a first order lower sideband. As shown in Fig. 2 the stereophonic signals A and B are added at 60, and subtracted at 62 after phase reversing at 64 if required, and the A + B and A - B signals are given a quadrature relationship by phase shifters 70 and 72 which is maintained over most of the stereophonically distinguishable spectrum, i.e. from about 150 c/s. to 5000 c/s. The A-B signal is supplied to a balanced modulator 76 together with a carrier signal generated at 80, a portion of the carrier after a 90 degrees phase shift at 82 being'combined at 86 with the output of modulator 76 to give a resultant phase-modulated signal. The resulting signal is frequency multiplied to the required transmission frequency at 42 and this phase-modulated wave is then amplitude modulated by the A + B signal from 70 to provide the resultant signal consisting of a carrier with the lower and upper sidebands carrying stereophonic signals A and B respectively as amplitude modulation. Vector diagrams are given showing how this is achieved by the cancellation of one set of phasemodulated and amplitude-modulated sidebands. Fig. 7 (not shown) shows an arrangement in which a low-pass filter passing frequencies below say 5000 c/s. is included in the phasemodulation chain thereby restricting the stereophonic effect to frequencies below this value and a compensating phase equalisation network is included in the amplitude modulation chain. A plurality of mixers are included in the phase-modulation chain to provide cancellation of carrier drift and unwanted frequencies. The signals are received as monophonic signals on a conventional receiver employing envelope detection, or as stereophonic signals by the use of two such conventional receivers one being tuned slightly above the carrier frequency and the other slightly below to select the respective sidebands. In a special receiver, Fig. 11, a conventional R.F. amplifier 202, mixer 206 and oscillator 204 feed an I.F. amplifier 208 whose output is split, one portion being demodulated by an envelope detector 210 to isolate the A + B component of the signal which is given a 45 degrees phase lead in a circuit 212. The other portion feeds a known discriminator and deemphasis circuit 216 via a limiter 214 and is given a 45 degrees phase lag at 218 to provide the A-B signal. The A + B and A - B signals are fed to sum and difference circuits 220 and 222 providing as outputs the A and B signals respectively. In an alternative receiver, Fig. 12 (not shown), conventional R.F. and mixer stages feed a pair of I.F. amplifiers in parallel, one being tuned to accept the upper sideband and the other the lower sideband of the intermediate frequency wave and feeding individual envelope detectors for the A and B signals. A small part of the B signal is fed into the A channel to eliminate cross-talk in the A channel possibly introduced by the detector circuit described. In a modification, Fig. 13 (not shown), three I.F. amplifiers are fed in parallel, two arranged as in Fig. 12 but emphasizing the higher frequencies in the respective sidebands, and the third being tuned to receive both sidebands and to emphasize the lower frequencies. The three signals are supplied to respective detectors and amplifiers, that including both sidebands being supplied to a central " woofer " loud-speaker and also utilized to provide A.G.C. and the A and B signals being supplied to respective " tweeter " loudspeakers appropriately spaced from the " woofer." This loud-speaker arrangement may be applied to the receiver shown in Fig. 11, the A + B signal being supplied to the central " woofer " loud-speaker.