Patent GB-723698-A (GB)
Title: Improvements in or relating to colour television systems
IPC: H04N11/14
Filed: 1952-07-31
Abstract: 723,698. Colour television. PHILCO CORPORATION. July 31, 1952 [Sept. 10, 1951], No. 19394/52. Class 40 (3). The invention relates to colour television systems of the type in which the transmitted signal comprises a first component representative of image brightness and a second component, the phase and amplitude of which are representative respectively of image hue and saturation. According to the invention, the effects of interference are made more tolerable by reducing the saturation of the image colours in the presence of interference without correspondingly varying the hue or brightness. This is brought about by controlling the amplitude of the second component in dependence upon (1) the received signal-tonoise ratio or (2) the level of the received signal. Fig. 1 shows a receiver intended to operate with a transmission of the type described in Specification 715,506 in which the image to be televised is resolved into three colour components X, Y and Z, the taking characteristics being weighted so that the signal Y represents the image brightness. The Y signal is transmitted directly within a band of 0-3 Mc. and the X and Z signals are utilized to form two difference signals X-Y and Z-Y which are restricted in band-width to 0.6 Mc. and modulated in phase quadrature on a 3.6 Mc. subcarrier. The phase of this sub-carrier is determined by the relative values of the difference signals and hence the image hue whilst the amplitude is determined by the absolute values of the difference signals and hence the saturation. At the receiver, the brightness component Y is selected by low-pass filter 14 and applied as a first signal to the reproducing tubes R, G, B through amplifiers 16, 18, 20, 22. The sub-carrier is selected by band-pass filter 30 and applied through attenuator 32 and phase splitter 38 to balanced demodulators 34, 36 to derive the original X-Y and Z-Y difference signals under the control of appropriately phased demodulating signals from generator 42. These signals are then added to the original Y signal in proper proportions by means of controlled amplifiers 56, 60, 64, 68, 72, 76 to derive three further colour signals appropriate to the primaries represented by the cathode-ray tube phosphors. According to the invention, the subjective effect of interference is reduced by controlling attenuator 32 so as to reduce the sub-carrier amplitude and hence the colour saturation. This is effected by circuit 80 which is connected to the intermediate frequency amplifier and produces an output proportional to the received signal-to-noise ratio. A first band-pass filter 81 and detector 83 selects the range 25-29.2 Mc. occupied by the video signal and produces an output across resistor 85 equal to the signal plus superposed noise. A second filter 82 and detector 84 selects the noise in the adjacent band of 29.2-30 Mc. and produces an output across resistor 86 in opposition to that across resistor 85. The resultant voltage across both resistors is accordingly indicative of the signal intensity, and the voltage at the junction is indicative of the noise. The two voltages so generated are the applied through amplifiers 87 and 94 to bridge networks which produce outputs equal to the logarithm of the input voltages. By appropriate phasing, the resultant output, which is applied through amplifier 101 to control the attenuator, is made equal to the difference of the logarithms and hence the ratio of signal to noise. Fig. 2 shows a modification for controlling the subcarrier amplitude directly as a function of received signal intensity. Attenuator 32 comprises a triode with the grid supplied from filter 30 and the anode circuit connected to phase splitter 38. The grid bias is determined by the impedance of triode 114 and is controlled in accordance with the intensity of the received signal by returning the grid of triode 114 to the source of receiver A.G.C. potential. A threshold effect may be obtained so that the amplification of triode 100 is only reduced for low signal levels by arranging that the A.G.C. voltage is sufficient to cut off triode 114 at normal levels. In alternative arrangements the attenuator 32 may control the amplitude of the demodulating signal from generator 42 or two simultaneously variable attenuators may control the outputs from demodulators 34, 36.