APPARATUS FOR REPRODUCING QUADRAPHONIC SOUND

1387874 Quadraphonic systems CBS Inc 23 Feb 1972 [15 March 1971] 8412/72 Heading H4R In a quadraphonic system in which the four signals L f , R f , L b , R b are matrixed on to two transmission channels each channel carries a respective predominant signal R f or L f together with sub-dominent signals R b and L b . The signals are combined after phase shifting such that, in each channel, the two sub-dominant signals receive a phase shift differing one from the other by 90 degrees, with the signal R b receiving the greater phase shift in one channel and the signal L b the greater phase shift in the other. The signals R f and L f in each channel receive the same phase shift, and the same phase shift as, respectively, the subdominant component L b or R b in its own channel. Alternatively the phase shift may be such that the subdominent components in one channel are in antiphase with the positions referred to above. Fig. 7 shows a preferred embodiment of one encoding matrix and Fig. 5 (not shown) a second similar matrix. The alternative arrangement, Fig. 8, having the subdominant components in one channel in antiphase to those shown in Fig. 7, is said to give better discrimination between centre front and centre back signals than the arrangement of Fig. 7. The decoder used can be the same as that of Specification 1,387,873 but an alternative form, Fig. 12, is more suitable for the coding system of Fig. 8. The major portion of the arrangement of Fig. 12 is equivalent to that of Figs. 4a and 4b of Specification 1,387,873, with appropriate phase change in combiner 326, but an additional logic circuit is provided, 354 to 378, which compares the sum of the two channel signals with the difference of the two channel signals, which is shown to discriminate a centre front signal from a centre rear signal, and the result of this comparison is combined with the output of the conventional logic circuit 340 to operate the controlled gain amplifiers 312, 314, 322, 328. The outputs of the two circuits can be added, as shown in Fig. 12, or the maximum signals picked off as described with respect to Fig. 12A (not shown).