Patent GB-1504392-A (GB)
Title: Compatible four channel radio broadcast and receiving system
IPC: H04H20/89
Filed: 1975-04-18
Abstract: 1504392 Quadraphonic systems CBS Inc 18 April 1975 [18 April 1974] 16130/75 Heading H4R In a quadraphonic broadcasting system primary and secondary channels of the system carry respectively sum and difference of the signals L T  and R T , where L T  contains an audio signal L f  in a dominant proportion together with sub-dominant proportions of the audio signals L b  and R b , one of which has been phase shifted relative the other, and R T  contains an audio signal R f  in a dominant proportion together with sub-dominant proportions of the audio signals L b  and R b  one of which has been phase shifted relative the other, the primary and secondary channel signals to provide signals compatible with existing monophonic and stereophonic broadcast standards, while one or more auxiliary channels carry further signals which can be combined with the quadraphonic signals, recovered from the primary and secondary channels by a first matrixing operation, to provide quadraphonic signals in which the separation between the channels is improved, or made complete. As described the quadraphonic source signals are matrixed as shown in Fig. 3 to produce L T  and R T  signals 74 and 75 which are transmitted as a sum signal L T  + R T , as a main channel signal in base band, and as a difference signal L T -R T , modulated on a 38 kHz sub-carrier, so that the signals are compatible with conventional mono, and stereo, broadcast signals. At the receiver these signals, after feeding through a matrix Fig. 6 (not shown), provide conventional matrix output signals L<SP>1</SP> f  L<SP>1</SP> b  R<SP>1</SP> b  and R<SP>1</SP> f  each of which contains a dominant component, e.g. L f  in L<SP>1</SP> f , with subdominant components, e.g. L b  and R b  in L<SP>1</SP> f <SP>1</SP>. In one embodiment, Fig. 7A (not shown), a single auxiliary signal S, is transmitted on a 38 kHz subcarrier in quadrature with the sub-carrier carrying the difference signal (L T -R T ), the auxiliary signal S, being At the receiver, Fig. 7B (not shown) the signal S 1  is combined with the signals L<SP>1</SP> f , L<SP>1</SP> b , R<SP>1</SP> b , R<SP>1</SP> f , to provide an increase in separation between these signals. In a second embodiment, Fig. 4a (not shown), two auxiliary signals S, and S2, are transmitted S, on a 38 kHz quadrature sub-carrier and S2 on a 76 kHz sub-carrier. The signals S, and S 2  are signals which are quasi complementary to the signals L T  and R T , i.e. the main components L f  and R f  are in phase with the same signals in L T  and R T  but the back signals L b  and R b  are in antiphase with the same signals in L T  and R T . At the receiver Fig. 4b (not shown) the signals S, and S2 are fed through a second matrix of the type shown in Fig. 6 and the outputs of the second matrix combined with the outputs of a first matrix fed with the signals L T  and R T  to provide discrete outputs L f L b  R b  and R f . In a further alternative Fig. 8A (not shown) the signal and At the receiver Fig. 8B (not shown) combination of the signal S, with the outputs from the matrix of Fig. 6 provides a first stage of increase in the separation of the signals, obtainable by relatively simple equipment (130A) while further combination with the signal S2 provides complete separation of the signals. A further alternative embodiment Fig. 9A transmits three auxiliary signals S 1 , on a 38 kHz quadrature carrier, and S 2  and S 3  on respective quadrature related 76 kHz carriers. The signal S, is the same as the signal S, in the embodiment described above, and At the receiver Fig. 9B the signals from the matrix of Fig. 6 are combined with the signal S, to provide a first stage of increase in separation in the signals L<SP>11</SP> L<SP>11</SP> b , R<SP>11</SP> b , R<SP>11</SP> f , by combination of these signals with the signal S 2  at 130C a further improvement of separation is provided in the signals L<SP>111</SP> f , L<SP>111</SP> b , R<SP>111</SP> b , R<SP>111</SP> f , at a relatively small increase in complexity of receiving equipment, while by combining these signals with the signal S 3  at 130D completely discrete signals L f , L b , R b , R f , are obtained. A receiver can be equipped with as many stages of separation improvement as is thought desirable. Alternatively the transmitter may be modified to transmit only S 1 , or S 1  and S 2  rather than S 1 , S 2 , and S 3 .