Patent GB-1053877-A (GB)
IPC: H04N9/72
Abstract: 1,053,877. Colour television. COMPAGNIE FRANCAISE DE TELIVISION. Aug. 4, 1964 [July 30, 1963; June 15, 1964], No. 31215/64. Heading H4F. The invention is concerned with signal level clamping in a receiver for a colour television signal of the type comprising a luminance signal together with a sub-carrier which is frequency modulated alternately at line rate by two colour signals, the colour signals being subjected to pre-emphasis of the higher frequencies at the transmitter and the subcarrier also being suppressed during that portion E, Fig. 1, of the horizontal blanking interval J which includes the synchronizing pulse S. During the remaining parts p and P of the blanking interval the colour signals are at zero reference level which results in the subcarrier being at its reference or resting frequency. The output of a frequency discriminator at the receiver during the blanking interval may take the form shown in Fig. 2 where level M is subject to variation due to drift in the discriminator. The signal after frequency deemphasis to compensate for the pre-emphasis at the transmitter then takes the form shown in Fig. 3 where the reference level during the early part of interval I is disturbed due to the step between levels M-N. Furthermore, the frequency discriminator is susceptible to noise during the interval when the subcarrier is suppressed which has a similar effect due to the frequency de-emphasis in disturbing the reference level. The reference level P is used at the receiver for signal clamping, but in accordance with the invention this is effected: (a) before de-emphasis so as to avoid completely the disturbance of level illustrated in Fig. 3; (b) after de-emphasis but within only the interval I<SP>1</SP>, Fig. 3, when the disturbance is substantially ceased; or (c) by blocking the sub-carrier channel for a period comprising at least the end period of period E, Fig. 1, when the subcarrier is suppressed and noise may disturb the reference level. In each case the clamping may be effected on: (1) the original two colour signals derived from the subcarrier and rendered simultaneous by known means; (2) the three colour-difference signals obtained by matrixing the two colour signals; or (3) the three colours component signals which are finally applied to a three-gun cathode-ray tube. The Specification describes two receivers with reference to Figs. 5 and 8 (not shown) where the clamping is applied before de-emphasis to the final colour component signals. The luminance signal employed for matrixing is either subjected to a pre-emphasis similar to that applied to the colour signals, or a luminance correction signal subjected to pre-emphasis is added to the final signals, e.g. at the cathodes of the cathoderay tube. Where the clamping is applied after the de-emphasis, Fig. 9 (not shown), the clamping is controlled by a pulse generator during only the reduced period I<SP>1</SP>, Fig. 3, and the pulse generator is triggered from the rear edge of the synchronizing pulse S by way of a monostable circuit which introduces the appropriate delay. The third method of operation, i.e. blocking the subcarrier channel, is applicable only where the frequency discriminator delivers a sufficiently stable level N, Fig. 2. A circuit is described, Fig. 10 (not shown), in which a first pulse generator provides a blocking voltage which starts with the leading edge of the synchronizing pulse S and lasts at least throughout interval E, Fig. 1, and preferably throughout the clamping interval as well, and a second pulse generator provides a clamping voltage which commences a suitable interval after the start of the blocking voltage, so as to allow for the decay of any noise which occurs before the blocking commences, and which lasts throughout the remainder of interval E plus the whole of interval I. The blocking may be applied directly to the frequency discriminator or to a preceding circuit, e.g. a limiter. A detailed circuit of a frequency discriminator with blocking is described with reference to Fig. 11 (not shown). Where the noise level is lower than that of the sub-carrier, blocking may be achieved throughout interval E, i.e. the interval where the subcarrier is suppressed, by applying an appropriate level of fixed bias to the diodes in the discriminator circuit or to a class C amplifier in the subcarrier channel. Transistor clamping circuits which are described are conventional.