Patent GB-1526195-A (GB)
Title: Transmission or recording of quadraphonic signals
IPC: H04S3/02
Filed: 1975-11-07
Abstract: 1526195 Quadraphonic systems BRITISH BROADCASTING CORP 7 Nov 1975 [7 Aug 1974] 34839/74 Heading H4R In a matrix quadraphonic encoder which is particularly compatible with stereophonic and monophonic reproducers the input audio signals are coded into two channels the signals in which are formed of combinations of the input signals weighted in amplitude and phase shifted such that a sound source moving in a circle round a datum position results in two output signals related in amplitude and of relative phase such that if the amplitudes are A and B the phase difference between the signals is #, and α/ 2  = tan<SP>-</SP><SP>1</SP>A/B a plot of α and # on the surface of a sphere does not enter the forbidden zone shown on Fig. 10 where α is plotted as radial distance with the centre as α = 0 and periphery of the figure as α = 90 and # is the angle of rotation around the figure. The figure also shows experimentally determined permissible areas of the locus of α and # for centre front (CF), right front (RF), centre right (CR), right back (RB) and centre back (CB) sound sources, to which the described embodiments conform. Preferred matrix for both coding and decoding is shown in Fig. 17 which has the characteristics This can be implemented direct for pan pot and cardioid sources or combined with a different matrix for mixing in signals from a large number of microphones. A better characteristic (chain dotted in Fig. 9, not shown) is obtained with four coincident hypercardioid microphones arranged to face out along the diagonals of a square corresponding to L F , R F , L B , R B , directions with the hypercardioid response having a front to back ratio of 5.831. The response can be approximated using cardioid microphones with the outputs from the diagonally opposite pairs passed through sum and difference networks attenuating the sum signal 3dB and recombining in sum and difference networks, Fig. 11 (not shown). Such a network can be incorporated in the encoding matrix by using different matrix coefficients. In an alternative arrangement, Fig. 18 (not shown), the phase shifted signals are generated by mixing orthogonally phased components according to the matrix equation The decoding matrices for the two embodiments referred to above can be obtained by taking the complex conjugates of the coefficients in the coding matrix equations, e.g. Modified forms of this matrix which are said to provide improved characteristics in reproduction and Alternatively a non-linear decoder, i.e. with control logic, of the type described in Specification 1,402,320 may also be used provided that phase shift circuits give 60 degrees phase difference are inserted in the decoder input paths, Fig. 19 (not shown). For transmission the L' and R<SP>1</SP> signals can be encoded as sum M and difference S in the normal way. Alternatively the signals M = L F  + R F  + L B  + R B  S = L F  - R F  + L B  - R B  T = L F  + R F  - L B  - R B  Q = L F  - R F  - L B  + R B  can be formed and encoded for transmission as M<SP>1</SP> = 0À889M - j0À117S + 0À182T - j0À030Q and S<SP>1</SP> = - j0À117M + 0À889S + j0À590T - 0À108Q. In this arrangement the Q signal may be ignored since its effect is small, and zero for truly coincident cardioids and hypercardioids.