Patent GB-965705-A (GB)
Title: Improvements in compatible tv and fm stereophonic system
IPC: H04N7/06
Filed: 1961-10-27
Abstract: 965,705. Stereophonic broadcasting systems. GENERAL ELECTRIC CO. Oct. 27, 1961 [Oct. 31, 1960], No. 38529/61. Heading H4L. Relates to a television system in which stereophonic sound signals are transmitted and are of the type wherein the L - R signal amplitude-modulates a subcarrier and is added to the L+R signal, the combined signal frequency-modulating the main carrier, the L - R and L+R signals being provided with preemphasis of the higher audio-frequencies in accordance with conventional F.M. practice, and the sub-carrier being derived from the synchronizing pulse generator of the vision transmitter, the sub-carrier normally being suppressed. The receiver is provided with a circuit which isolates the amplitude modulation components and introduces de-emphasis for the L - R and L+R signals. In a receiver for TV sound, Fig. 4, sync. pulses 48 from the receiver 46, are supplied to a frequency doubler and sine wave generator 50. The output of the F.M. detector 28 is supplied to an L+R deemphasis network 38 and also to a band pass filter 30 selecting the L - R amplitude modulation sidebands. The generator 50 reconstitutes the suppressed sub-carrier which is combined with the L - R sidebands at 30, and after amplitude detection at 32 and L - R deemphasis at 34 the L - R and L+R signals are combined in a matrix 36 to supply the L signal to an A.F. amplifier 40 and the R signal to an A.F. amplifier 42. Fig. 5 shows the circuit contained in the dotted rectangle 44. The L +R signal is selected and de-emphasized by a resistor 52 and capacitor 54 connected to the output of the F.M. detector 28, the time constant of the circuit 52, 54 being the same as that of the pre-emphasis network at the transmitter. To extract the L - R signal and reject the L+R signal the output of the detector 28 is supplied via a coupling capacitor 56 to a circuit 58, 60 tuned to provide maximum output at the sub-carrier frequency and having a Q value such that the sidebands are attenuated to provide the correct de-emphasis. The deemphasized sidebands are applied to oppositely poled rectifiers 63, 64, and their associated load circuits provide signals R - L and L - R respectively. The signals L+R and L - R are combined via resistors 74, 76 to produce a resultant signal L and the R signal is obtained by similarly combining signals L +R and R - L via resistors 80, 82. If no sub-carrier is transmitted it is reconstituted from line sync. pulses from 51. The diodes 63, 64 may be replaced by a single diode having two load circuits providing signals L - R and R - L respectively. In an alternative circuit, Fig. 8, the F.M. detector 106 via de-emphasis networks 108, 110 and 114, 116 selects signals L+R and - (L+R) respectively. The L - R signal is derived by a tuned circuit 120, 122 operating as in Fig. 5, and push-pull detector 128, 130, and combined in resistor 138 with signal L+R to produce the output signal KL, and in resistor 140 with signal - (L+R) to produce the other output signal - KR which is phase inverted at 148. The movable contacts 142, 144 are adjusted for correct balance. The carrier is re-constituted as before from line sync. pulses from 152. In a modification of Fig. 4, Fig. 9 (not shown), a series resonant circuit is used instead of the parallel circuit 58, 60.