Patent GB-1499990-A (GB)
Title: Demodulation circuit for colour television signals
IPC: H04N9/455
Filed: 1974-12-11
Abstract: 1499990 Colour television INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 11 Dec 1974 53500/74 Heading H4F A PAL chrominance signal is demodulated along axes each of which is spaced at 45 degrees from the modulation axes. The PAL chrominance signal may include "swinging" subcarrier reference bursts or, in certain embodiments, a subcarrier reference burst only in certain line blanking periods, for example every second or third such period, the remaining line blanking periods being used for the transmission of supplementary information, such as facsimile information, the time, or supplementary sound information. In a first embodiment, Fig. 1, the received signal is supplied to two synchronous demodulators 8, 9 each receiving the output of a respective subcarrier regenerator 6, 7. Each subcarrier regenerator receives alternate subcarrier reference bursts from a change-over switch 4 controlled by a half line frequency signal from a generator 5, the changeover switch 4 receiving the output of a burst gate 2. For one phase of operation of generator 5 regenerator 6 generates a subcarrier of phase R I F, Fig. 3 and regenerator 7 generates a subcarrier of phase R1F. For a received signal which is S +  in alternate lines and S -  in the intervening lines demodulator 8 generates the signals I, II, I, II whereas demodulator 9 generates the signals II, I, II, I. If the phase of operation of generator 5 is reversed the phases of regenerators 6 and 7 are transposed as are the operations of demodulators 8, 9. The outputs of demodulators 8, 9 may either be supplied to a permutating means (21, Fig. 4, not shown) controlled by generator 5 and feeding a matrix to derive red, green and blue primary colour signals, or the outputs of demodulators 8, 9 may be supplied to an adder (37, Fig. 5, not shown) to derive the blue colour difference signal and to a subtractor (35, 36) the output of which is fed via a change-over switch (38) operated at half-line frequency and feeding a red colour difference signal output directly and via an inverter (40). In an alternative embodiment only one subcarrier regenerator (14, Fig. 2, not shown) is provided; this regenerator receives alternate subcarrier reference bursts via a gate (13) controlled by generator 5 and supplies both demodulators 8, 9. The received signal is supplied to demodulator 8 via a delay circuit (12) supplying a delay equal to an odd number of line scanning periods. In a modification of this embodiment an amplitude detector (46, Fig. 6, not shown) receives the output of the subcarrier regenerator (14) and controls a colour killer (47) and a gate (42) which blocks the application of line frequency pulses to the generator 5 when the colour killer (47) is operated. In a further embodiment (Fig. 7, not shown) the input signal is connected directly to one demodulator (104) and via a one line delay line (102) to a second demodulator (103). The reference subcarrier inputs of the two demodulators (103, 104) receive signals with a phase difference of 180 degrees therebetween, the reference subcarrier input to the first demodulator (104) being derived from a changeover switch (113) receiving inputs from two subcarrier regenerators (111, 112) each of which is supplied with alternate subcarrier reference bursts from a change-over switch (110), the two changeover switches (110, 113) being operated in synchronism. The first demodulator (104) always receives a reference subcarrier in phase with the subcarrier reference burst of the received line and provides the signal I, Fig. 3, whereas the second demodulator (103) always receives the inverted reference subcarrier and the delayed received signal and provides the signal II. In a modification of this embodiment (Fig. 14, not shown) only one subcarrier regenerator is used, the phase of this regenerator being changed from line to line so as always to be in phase with the subcarrier reference burst of the received line. In a further modification the delay line connected to the second demodulator (103) is omitted and the reference subcarrier fed to the first demodulator (104) is fed to the second demodulator (103) via a one line delay (520). In another embodiment the input signal is supplied via a colour killer circuit 120, Fig. 9, to two demodulators 103, 104. Two subcarrier regenerators 111, 112 each receive alternate subcarrier reference bursts from a change-over switch 110 and supply signals to two change-over switches 121, 122 operated in synchronism with change-over switch 110 and connected to demodulators 104, 103. Demodulator 104 always receives from switch 121 a reference subcarrier in phase with the subcarrier reference burst of the received line and provides the signal I, Fig. 3, whereas demodulator 103 always receives from switch 122 a reference subcarrier in quadrature with the subcarrier reference burst of the received line and provides the signal II. The colour killer circuit 120 connects its output to earth when the output of a detector 123 indicates reception of a monochrome signal. In a further embodiment a change-over switch (401, Fig. 13, not shown) supplies the input signal during alternate line scanning periods to a first burst gate (403) feeding a first subcarrier regenerator (409) and also to first and second demodulators (404, 405) and during the intervening line periods to a second burst gate (406) feeding a second subcarrier regenerator (410) and also to third and fourth demodulators (407, 408). The output of the first subcarrier regenerator (409) is supplied to the first and third demodulators (404, 407) and the output of the second subcarrier regenerator (410) is supplied to the second and fourth demodulators (405, 408). The outputs of the first and fourth demodulators (404, 408) are connected together and supply the signal I, Fig. 3 and the outputs of the second and third demodulators (405, 407) are connected together and supply the signal II.