Patent ES-336680-A1 (ES)
Title: Improvements in or relating to Colour Television Receivers
IPC: H04N9/47
Filed: 1967-02-10
Abstract: The invention relates to a colour television system in which there is transmitted a chrominance signal the composition of which is of one kind during alternate line scans and of a second kind during intervening line scans, as indicated at F1 and F2 in Fig. 1, and in which colour synchronizing signals are transmitted in the intervals between lines and changed in phase by 90 degrees in successive lines as indicated at B1 and B2 in Figs. 1 and 3. According to the invention, a control signal for operating a switching means to adapt a receiver to the change in composition of the chrominance signal is developed by combining each synchronizing signal with the signal transmitted in the previous interline interval. The invention is described in detail only as applied to the P.A.L. system, but is stated to be applicable to the SECAM (Registered Trade Mark) system and the N.I.R. System described in the " Wireless World," February 1966, pp. 73, 74. In the latter system the chrominance signal comprises in alternate lines an N.T.S.C. type of subcarrier signal and a sub-carrier signal of reference phase. In a first application of the invention to a receiver for the P.A.L. system, Fig. 3, a gate 5 separates each synchronizing signal from the composite received signal, and after a phase shift of 90 degrees in a circuit 9 it is combined in a multiplier M (a synchronous demodulator or ring modulator) with the signal from the previous interline interval which is available at the output of the line duration delay element 1 in the conventional delay line demodulator 1, 2, 3. The output from the multiplier is passed through a low-pass filter 8 and results in a pulse train 7 in which alternate pulses are positive and negative. The pulse train is applied to control the switching means (not illustrated). According to modifications the phase shifter 9 may be included in the path of the delayed signal. The gate 5 may likewise be included in the delayed signal path or may appear after the multiplier. The phase shift introduced by element 9 may be other than 90 degrees. The pulse train 7 may be filtered to produce a sinusoidal signal, Fig. 4 (not shown), or may be converted to a sawtooth wave. In a second application of the invention to a receiver for the P.A.L. system, Fig. 5, the synchronizing signals are used as they appear at the outputs of the adding and subtracting stages 2 and 3 in the delay line demodulator circuit. Each signal is formed of the combination of two successive signals, the resultant 14 in the case of the adding stage comprising signals of invariable phase and the resultant 15 in the case of the subtracting stage comprising signals alternately of opposite phase. The signals 14 are shifted by 90 degrees in an element 9, added to signals 15 in adder 11 and the combination passed through a gate 5, which operates during the synchronizing signal intervals, to an amplitude detector 13. The final signal 7 has alternate pulses cancelled and thus comprises a pulse train at half line frequency. In a modified. tion adder 11 is replaced by a multiplier the output from which comprises alternate positive and negative signal. Detector 13 is dispensed with and gate 5 may be located at a point in the circuit preceding the multiplier.