Patent GB-1078498-A (GB)
Title: Improvements in and relating to receivers for receiving a colour television signal including a frequency-modulated subcarrier
IPC: H04N11/18
Filed: 1965-07-09
Abstract: 1,078,498. Colour television. COMPAGNIE FRANCAISE DE TELEVISION. July 9, 1965 [July 15, 1964], No. 29299/65. Heading H4F. The invention relates to a receiver capable of receiving a colour television composite video signal including a luminance signal and a subcarrier which is frequency-modulated by a colour information and superimposed on the luminance signal in a portion of the bandwidth thereof, e.g. a "SECAM" (Trade Mark) system, the frequency modulated subcarrier having been, at the transmitting end, before its addition to the luminance signal, passed through a coding filter whose amplitude-frequency characteristic in the frequency bandwidth of the subcarrier channel increases on both sides of a predetermined frequency Fo, i.e. the subcarrier frequency. Accordingly the receiver comprises in the luminance channel a trap whose amplitude frequency characteristic shows a relative maximum for frequency Fo, and two minima for two frequencies Fo-f and Fo+f located on both sides of frequency Fo, Fig. 2 (not shown), the trap attenuating each of the two frequencies Fo-f and Fo+f relatively to frequency Fo substantially in the same ratio as it was enhanced by the coding filter relatively to frequency Fo, Fig. 1, not shown. This trap thus sufficiently eliminates the parasitic signal which the subcarrier builds up for the luminance signal without undesirable deterioration of the luminance signal. Further in the SECAM system in order to improve the transmission of the signals, the colour information video frequency signals are passed through a preemphasis filter which enhances the higher frequency components relatively to the lower frequency components before they frequency modulate the subcarrier. The frequencies Fo-f and Fo+f referred to above are the instantaneous frequencies of the subcarrier corresponding to the transmission of the maximum negative and positive levels which appear at the output of the pre-emphasis filter. As shown, Fig. 3, the trap comprises a triode 3 mounted in a cathode-follower circuit with its input 1 on the grid and its output coupled to the cathode. The anode load of the cathode follower comprises a primary parallel resonant circuit 4, 5, tuned to the frequency Fo, inductively coupled to a damped secondary resonant circuit 6, 7 also tuned to the frequency Fo. For a certain degree of coupling the impedance of circuit 4, 5 loaded by circuit 6, 7 divides into two maxima located on either side of Fo and separated by a relative minimum for Fo, the frequency interval between these two maxima growing with the degree of coupling. The efficiency of the trap may be improved by connecting the anode of tube 3 to the cathode through a capacitor 8 and a resistor 9 connected as shown. Although the cathode resistor could be a single resistor it is shown as a potentiometer resistor 10 with the amplifier output connected to a slider 11, in which case resistance 10 may also act as the resistance of the gain control potentiometer allowing for adjustment of the gain of the luminance channel. Black and white receivers adapted to receive colour video signals and to provide a black-and-white picture by means of the luminance signal may also be provided with a trap.