Patent GB-1183496-A (GB)
Title: Colour Television Receiver
IPC: H04N9/68
Filed: 1967-07-12
Abstract: 1,183,496. Colour television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 12 July, 1967 [15 July, 1966; 1 July, 1967], No. 32015/67. Heading H4F. A colour television receiver for use with N.T.S.C. (or P.A.L.) type signals which includes an AGC circuit, the control voltage for which is derived from the burst signals, a device for adjusting the amplitude of the colour information (difference) signals, a transmission channel for the colour information and burst signals the transmission of which is switched periodically so that it is only influenced by the adjusting device during the occurrence of the colour information signals and not the burst signals, characterized in that the control voltage is supplied to the AGC circuit during the occurrence of the colour information and burst signals completely independently of the periodic switch so as to maintain constant the amplitude of the burst signal without affecting the adjustable amplitude ratio (i.e. colour saturation) between the burst signals and colour information signals caused by the switchable transmission and amplitude adjusting device. To obtain a combined adjustment of contrast and saturation the composite video signal (luminance and chrominance) is passed through the transmission channel with the switchable transmission. In a first embodiment, Fig. 1, the luminance signal Y and synchronization signal S are fed to the base of transistor 43 in an adjusting section 2 and the colour information and burst signals are supplied to an input 11 of section 2 and are made available at output 13 in such manner that the amplitude of the colour information signal is adjustable by means of an adjustable member 47 but the amplitude of the burst signal is not adjustable by means of member 47, i.e. the transmission of channel 11-13 is switched periodically. To effect this periodic switching a keying signal is applied to input 12 during the occurrence of burst signals to control a potentiometer comprising resistor 14 and voltage-dependent impedance 16 formed by parallel circuit 18 tuned to the subcarrier, diode 28 and capacitor 30. Due to the keying voltage which is positive and the adjustable negative voltage derived from transistor 43 and potentiometer 47, a voltage appears at the cathode of diode 28 so that it conducts during the occurrence of the colour information signal to an extent determined by adjustable member 47, but does not conduct during the occurrence of a burst signal. Thus the colour saturation is adjustable by means of member 47, i.e. the amplitude ratio between the burst signal and colour information signal is adjusted. An AGC (controlled by the burst signal) which is active during the whole signal, does not affect the ratio but keeps the burst constant and therefore the colour information signal constant, namely at a value determined by the adjustment of member 47. The luminance signal Y and sync. signal S appear across collector resistor 45 and a signal is obtained at output 20 which is not adjustable in amplitude. In addition a luminance signal which is adjustable in ampli. tude appears at the adjustable tapping of potentiometer 47. Thus adjustment of the saturation and the contrast of the displayed picture is possible by means of potentiometer 47. The adjustable and nonadjustable luminance signals are applied to channel 29 which also handles the sync. signal and includes conventional circuits for sync. separation and timebase generation &c. Outputs 35 and 56 supply keying signals to input 12 of the adjusting device 2 and to a gate 55 in the colour channel 22 which comprises a chrominance amplifier 61 and a demodulator-and-matrix device 63. In gate 55 the demodulated burst signals are keyed out, converted into a control voltage and supplied for AGC to amplifier 61. The AGC may be effected in the I.F. stage 1, i.e. before the switching device. The voltage-dependent impedance 16 may comprise an adjustable resistor of which the end remote from resistor 14 is connected to earth via a diode or transistor during the occurrence of the colour information signal and disconnected from earth during a burst signal. The contrast adjustment and saturation adjustment may be separate instead of being combined. In a second embodiment, Fig. 2, the output from R.F. and I.F. stages 1 is supplied to a transmission channel 11-13 in the contrast adjusting device 2 which includes a first signal path via gain control member 15 and a time-selective element 17 and a second signal path via path 19 and element 17. Device 2 also includes signal paths 20 and 21 (see later). The video section 3 comprises a channel 22 including a chrominance amplifier for amplifying the colour information and burst signals which is controlled by an AGC 2  voltage. Channel 22 also includes demodulator, matrix, colour killing &c. circuits for deriving the colour difference signals and a time-selection signal for device 2 on output 27. Channel 29 is fed with the luminance and sync. signals and produces the luminance signal for display 4, deflection currents for deflection device 34, and on outputs 35, 36 and 56 a time-selection signal to device 2, an AGC 1  voltage to channel 1, and a timeselection signal to channel 22. From signal paths 20 and 21 the timing signals including the AGC 1  voltage, and the contrast adjusted luminance signal are respectively derived in channel 29. The dual AGC is provided in order that the amplitudes of both the luminance and colour difference signals are independent of variations in the input voltages of the aerial. AGC 1  acts on the line sync. signals or black level signals and AGC 2  acts on the colour bursts. By means of contrast adjusting device 2 the amplitude of Y and the colour difference signals may be varied in such a manner that the ratio is kept constant so as to prevent variations in the saturation of the colours displayed. The control signals for the two AGC voltages are passed by device 2 without being influenced by control member 15, by switching-on the path 19 and by means of path 20. The contrast adjusting device 2 may be included in stage 1 or the control member 15 may be of the dual type, i.e. one part in channel 22 and the other part in channel 29. Elements 15 and 17 may be transposed with respect to the direction of the signals. The circuit of Fig. 2 is described in more detail with respect to Figs. 3 and 4 (not shown).