Patent GB-1398786-A (GB)
Title: Multisignal transmission apparatus
IPC: H04S3/02
Filed: 1972-08-01
Abstract: 1398786 Quadraphonic systems SONY CORP 1 Aug 1972 [6 Aug 1971 16 Aug 1971] 35943/72 Heading H4R In a quadraphonic decoder two automatic gain controlled amplifiers are provided in association with logic circuits for detecting and thereby minimizing crosstalk. Input signals for waveform matching logic circuitry and for front to back logic circuitry are provided and are derived from the output signals of the two automatic gain controlled amplifiers. L T  and RT phasor groups 38, 40 (Fig. 2a) from an encoder (Fig. 1 not shown) are applied to input terminals 40, 42 of a decoder and phase shift networks 46, 48, 50, 52 ( 4 is a function of frequency) provide signals in four channels 54, 56, 58, 60. Summing junctions 70, 72 are provided and output signals are applied through gain controlled amplifiers 74, 76, 78, 80 to loud-speakers 82, 84, 86, 88 which would, in the absence of logic circuits, provide output signals having phasor groups 90, 92, 94 and 96. The signals in channels 60 and 50 are, however, also applied to logic circuitry (Fig. 2b) through high pass filters 110, 112 and amplifiers 116, 118. The high pass filters have a L.F. cutoff of 50 Hz and a loudness characteristic. The outputs of amplifiers 116, 118, by the use of summing junctions 112, 126 provide signals in four channels and these are rectified in full wave rectifiers 134, 136, 138, 140 having time constant circuits 142, 144, 146, 148 providing rapid attack and a relatively slow decay. The four rectified signals are added in a junction 150 and applied as feedback to amplifiers 116, 118 to maintain their outputs substantially constant. Waveform matching logic.-The outputs of the four channels are fed through full wave rectifiers 152, 154, 156, 158 to subtracting junctions 160, 162. The output signals from junctions 160, 162 are further rectified by full wave rectifiers 164, 166 and their output signals are subtracted in junctions 172 and 178. The output terminals 174, 180 provide waveform matching logic signals which control the gains of amplifiers 74, 76, 78, 80. If either or both the L f  and R f  signals are present a positive signal is produced at terminal 174 to control amplifiers 74, 80 and enhance the L f  and R f  signals reproduced by loud-speakers 82 and 88. Similarly if either or both the L b  and R b  signals are present a positive signal is produced at terminal 180 to control amplifiers 76, 78 and enhance the signals L b  and Rb reproduced by loud-speakers 84 and 86. In order to avoid reproduced sounds skipping from one loud-speaker to another the logic circuits may include time constant circuits. Thus the circuit from rectifiers 164, 166 to junctions 176, 182 may be replaced by a circuit 240 (Fig. 4) having two differential amplifiers 254, 256 and two time constant circuits 250, 252. Front-to-back logic.-If a centre-front or centre-back signal exists in the original sound field, crosstalk signals are reduced by applying output signals from the automatic gain control amplifiers 116, 118 (Fig. 2b) to # networks 204, 206 through conductors 200, 202. The output signals of networks 204, 206 (shown as phasor groups 208, 210) are applied to a summing junction 212 and a subtracting junction 214. The sum signal at junction 212 and the difference signal at junction 214 are rectified by rectifiers 220, 222 respectively. A subtracting junction 224, at its output 226, provides a difference signal which is applied to summing junction 176 for controlling the gains of amplifiers 74, 80. A subtracting junction 228, at its output terminal 230, provides a difference signal which is applied to summation junction 182 for controlling the gains of amplifiers 76, 78. Centre-front signals in the composite signals L T , R T  thus act to increase the gains of amplifiers 74, 80 and reduce crosstalk in the back channels. Centre back signals in the composite signal L T , R T  act to increase the gains of amplifiers 76, 78 and reduce crosstalk in the front channels.