Patent GB-1026585-A (GB)
Title: Improvements in or relating to circuit arrangements in colour television receivers
IPC: H04N9/68
Filed: 1964-08-27
Abstract: 1,026,585. Colour television receivers; automatic gain control. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Aug. 27, 1964 [Aug. 27, 1963], No. 35120/64. Headings H3Q and H4F. In a colour television receiver for use with the N.T.S.C. type of signal (i.e. a signal including two colour signals modulated in quadrature on a sub-carrier with phase reference burst signals between each line of the picture), in which the amplitudes of both the luminance signal and the colour signals are varied together by means of a contrast control, e.g. by altering the A.G.C. characteristic for the common intermediate-frequency amplifier, a gain control signal is applied to the colour amplifier channel (which amplifies both the burst signals and the modulated colour signals) to automatically reduce any variation in the amplitude of the burst signals without affecting the modulated colour signals. Thus, colour faults are prevented since any variation in the amplitude of the burst signals would cause a variation in the phase of the regenerated sub-carrier signal used in the colour synchronous demodulators. As shown, a colour signal separated from the main carrier is applied through a lead 1 to the first control grid of amplifying tube 2 which amplifies the colour and burst signals, and a gain control signal is applied to the second control electrode 5 (see later). Circuit 3 is tuned to the sub-carrier frequency so that the signal fed via amplifier 4 to the synchronous demodulators 6 and 14 contains only the colour and burst signals. Demodulator 6 demodulates the colour signal in a phase which differs by 90 degrees from that in which the burst signal is transmitted thus producing the R-Y colour difference signal. The output signal from demodulator 6 also acts as a phase comparator- and produces a control signal when compared with black level in comparator 7 during the line fly-back period, which signal has a value which is dependent on the amplitude of the burst signal and on the difference in phase between a regenerated sub-carrier produced by a local oscillator 9 and the burst signal, and is used to readjust the frequency of oscillator 9 when applied to reactance circuit 8. This regenerated sub-carrier is fed via 90 degrees phase-shifting network 11 to synchronous demodulator 6 and via lead 13 to synchronous demodulator 14 from which the B-Y colourdifference signal is obtained. In a black level comparator stage 15 which is also caused to operate only during the line fly-back period, a negative control voltage proportional to the amplitude of the burst signal is derived, -V ASC , and applied as an automatic burst control voltage through leak resistor 17 to the control grid of triode 18 which, together with its associated circuitry, comprises means for applying the gain control signal through capacitor 23 to the control grid 5 of tube 2. Gating pulses 20, e.g. line fly-back pulses, are applied to the control grid of triode 18 through capacitor 19 with negative polarity so that positive-going fly-back pulses 22 appear across the anode load resistor 21, and triode 18 acts as a variable clipping device causing the amplitude of pulses 22 to be smaller with more negative values of control voltage -V ASC  (explained with reference to Fig. 2, not shown). The D.C. level of the control pulses 22 which occurs during the line scan periods is controlled by the negative voltage at the anode of diode 27 which forms with capacitor 28 and resistors 29 and 32 a clamp circuit, the level being varied by tapping 31. As explained with reference to Fig. 3, not shown, the D.C. voltage level at the control grid 5 during the line scan period is dependent on the voltage V k  across the resistor 34 and the variable voltage -V<SP>1</SP> 3 , and independent of the magnitude of the amplitudes of the pulse signals 22. The burst signals are amplified more in tube 2 the larger the amplitudes of signals 22, i.e. with less negative values of control voltage -V ASC . Thus, the amplitude of the burst signal obtained at the output of tube 2 is constant because it is amplified less when its amplitude tries to increase because of the corresponding increase in the bias. In order that the tube 2 is operative during the line fly-back period but inoperative during the line scan periods during reception of a black-white picture signal, so that any burst signal may still be transmitted through the colour channel, resistor 32 is included in the clamp circuit, which with the high values of the amplitudes of pulses 22 due to -V ASC  disappearing causes the tube 2 to cut-off during the line scan periods.