Patent GB-1430195-A (GB)
Title: Four-channel stereophonic sound reproducing system
IPC: H04H20/89
Filed: 1973-03-07
Abstract: 1430195 Quadrophonic decoder logic circuits TOKYO SHIBAURA ELECTRIC CO Ltd 7 March 1973 [7 March 1972] 11050/73 Heading H4R In a quadrophonic system in which composite signals and are decoded into signals containing crosstalk, one or more judgment signals C are evaluated by level detecting and mixing the decoded signals in order to control the gains of the respective channels to reduce the level of those channels containing most crosstalk. It is shown that where, e.g., only Lf and Rf signals are supposed to be reproduced, crosstalk in the front channels can be eliminated by various mixing techniques, e.g. adding Lf1 and Rf1 in antiphase and with a relative amplitude 1/#2, or by adding JRb1 and -jLb1 to Lf1 and Rf1 respectively. The rear channels can be simply attenuated, or cancelled by similar mixing techniques. Recognition of conditions such as these, i.e. Lf and Rf high level and Lb, Rb low level, is not always possible by comparison of the envelopes of the sum and difference, respectively, of Lf and Rf. Therefore, the judgment sensor of Fig. 2 is employed. The amplitudes of the decoded signals are detected, 6-9, mixed in non-diagonal pairs 10-13 and compared, 15-18, with relative amplitudes of 1/1 + #2. Optionally, diagonals may be directly compared, 14, 19, to generate additional judgment signals C5, C6. The signals C1-C6 are proportional to the sum of the amplitudes of different pairs of decoded signals and are each fed to two of four generators 30-33 which pass the smallest of their input signals to control, 34-37, the amplitudes of the decoded signals. For example, C2, which is proportional to the sum of the amplitudes of Lb and Rb, is utilized to control the gains of the Lf and Rf channels. The attenuation reduces with increasing judgment signal level. In Fig. 7 (not shown) the composite signals L, R, are split into quadrature components and decoded into quadrature components of Lf, Rf, Lb and Rb. Diagonally opposite quadrature components are mixed (54-57) and attenuated (58-61) in response to C5 and C6. The attenuator outputs are mixed with the other components, attenuated by C1-C4, in the manner mentioned above, i.e. adding Lf1 and Rf1 in antiphase at a relative amplitude of 1/#2 and similarly for Lb1 and Rb1, in order to reduce crosstalk. These mixer outputs are further attenuated, 65-68, in response to C1-C4 with the same attenuation characteristic as before but the other attenuators have increasing attenuation with increasing judgment signal level. In a simplification (Fig. 9, not shown) the decoder produces three phase-shifted components for the rear but only one in phase component for each front channel. Lf1 and Rf1 are mixed in antiphase and at an amplitude ratio of 1/1 + #2 with 90 degree-shifted Lb1 and Rb1 components to generate a single control signal (C) which directly controls the in-phase rear components and the phase lagging rear components which are subsequently mixed (73, 74) with the diagonally opposite front signals to produce the final Lf2 and Rf2 signals. The factor 1/1 + #2 takes account of the relative amplitudes of the front and rear channel sums. A similar circuit (Fig. 10, not shown) depending on phase-shifting in the encoder generates a single control signal (C) as above which controls the levels fed to mixers (73, 74) also as before but additionally controls attenuators (75-78) in each output path. A normalization circuit (Fig. 11, not shown) may be included whereby relative ratios of signals rather than absolute levels may be compared. Embodiments preserving the acoustic output of the four channels operate by adding a portion k of the back centre signal Bc to Rf and subtracting the portion from Lf and similarly adding u.fc to Lb and subtracting from Rb where k and u are selected to give complete or partial cancellation. In Fig. 12 (not shown), the decoder produces the normal 4-channel signals and additional front, back, left and right centre signals which are multiplied by the appropriate factors (98-101), fed to attenuators (102-105) controlled by signals C1-C4 as before, and mixed with similarly attenuated but not factored centre signals in summing mixers (90-93) to which the four channel signals are also connected. The mixer outputs are phase-shifted (106-109). In a further modification (Fig. 15, not shown), the centre signals are controlled by signals C1-C4 and are each fed to two mixers (115- 118) where they are combined with the remotest corner signal, the outputs being fed to phase-shifters at the output and adjacent channels being selectably blended together by resistors controlled by the signals C1-C4. It is also stated that a corrective signal of the form A=xL+yR may be generated to cancel crosstalk.