Patent GB-1041726-A (GB)
Title: Improvements in colour television systems using at least one frequency-modulated subcarrier
IPC: G09B29/10
Filed: 1963-06-17
Abstract: 1,041,726. Colour television; frequency modulation systems. COMPAGNIE FRANCAISE DE TELEVISION. June 17, 1963 [June 18, 1962; June 10, 1963], No. 24112/63. Headings H3R and H4F. Relates to a colour television system of the type using a first wide band picture signal within the frequency band of which is located a subcarrier signal which is frequently modulated by a second picture signal and is concerned with improving the noise protection of the subcarrier signal in such a manner as not to disturb reception of the wide-band signal. In broad terms the noise protection is achieved by including in a receiver for the system a filter which is located in the subcarrier channel and has a characteristic as shown in Fig. 3(b) which emphasizes frequencies around the subcarrier frequency F 0  in relation to the side-band frequencies F 3 , F 4 . (F 1 -F 2  indicates the frequency deviation.) In addition, at the transmitter, in the channel for the second signal to the frequency modulator, there is provided a filter as shown in Fig. 2(a) which lowers the amplitude of the lower frequencies so as to increase the time that the subcarrier frequency is located within the range F<SP>1</SP> 1 -F<SP>1</SP> 2  of the receiver filter where substantial emphasis, and hence noise protection, is provided. For example, where the band-width fm of the second signal is 1 Mc/s., f in Fig. 2(a) may be made equal to 700 kc/s. and the ratio G 1 /G 0  approximately equal to 3. A filter, Fig. 2(b), having a characteristic complementary to Fig. 2(a) is provided following the frequency discriminator at the receiver, and a filter, Fig. 3(a), having a characteristic complementary to Fig. 3(b) is provided in the subcarrier channel at the transmitter. This latter filter is provided to compensate the phase distortion due to the corresponding receiver filter. Amplitude modulation imparted to the subcarrier at the transmitter by the use of the filter. Fig. 3(a), is removed by either a limiter following the filter, or an amplitude modulator preceding the filter which is connected in a feed-back circuit including an envelope detector connected to the output of the filter. The feed-back circuit may include a low-pass filter so as to be operative only for low frequency signals. The steps of amplitude suppression is an essential feature in preventing the disturbance of reception of the first picture signal by the subcarrier signal. The invention is described more particularly as applied to the " SECAM " (Registered Trade Mark) system where the first picture comprises a luminance signal and the second signal comprises red and blue colour difference signals alternating at line rate. Reference is made to 1,011,706 in connection with additional amplitude modulation of the subcarrier signal as a function of the level of the components of the luminance signal within the band common with the subcarrier signal. This may be effected by an amplitude modulator in the subcarrier channel, or, where amplitude suppression by feed-back is used as described above, the modulation signal may operate in a subtractor circuit in the feed-back path.