Patent GB-781537-A (GB)
Title: Color-saturation control apparatus
IPC: H04N9/68
Filed: 1955-10-03
Abstract: 781,537. Television. HAZELTINE CORPORATION. Oct. 3, 1955 [Oct. 29, 1954], No. 28131/55. Class 40 (3). Colour saturation of images reproduced from received signals of the NTSC type is controlled by controlling the relative gains of the luminance and chrominance channels in accordance with the departure from a predetermined ratio of the amplitude of the deflection control synchronizing pulses and the peak amplitude of the colour " burst " synchronizing signal. With this arrangement the colour saturation is made independent of variations in the gains of the two channels. In the described embodiments the amplitudes of the deflection and " burst " synchronizing signals in that part of the chrominance channel which conveys the Blue colour-difference signal are compared and any departure from equality in these amplitudes is employed to reduce the gain of the chrominance channel which, initially, is adjusted to provide slightly more gain than is required. Since the Blue colour difference channel is arranged to have a gain of about twice (2.03) that of the luminance channel and the relative amplitudes of the deflection and " burst " synchronizing signal, as determined at the transmitter, and as applied to the input circuits of the luminance and chrominance channels at the receiver, are in the ratio of two to one, departure from equality is indicative of incorrect relative gains of the two channels and consequently of incorrect colour saturation. Fig. 1 shows one embodiment in which the comparison of the amplitudes is effected by means of a pair of peak detectors 35, 36, the common cathodes of which are supplied from the " Blue output " of the matrix amplifier 29. In operation detector 35 is gated on during line retrace periods (by means of a positive pulse derived from the line frequency generator 17) and clamps the tips of the (negatively-going) line synchronizing pulses to ground potential. The colour " burst " signal which immediately follows each line synchronizing pulse is then effective on the common cathodes and, if the peak (negative) amplitude of the " burst " exceeds that of the line synchronizing pulses (i.e. is more negative than ground potential), diode 36 conducts and the potential of its anode is reduced by an amount related to the difference between the amplitudes of the line and colour " burst " signals. The negative potential thus developed across resistor 44 is then averaged by filter 45 and fed to the chrominance signal amplifier 27 to reduce the gain to such a value that the line and colour "burst" signals applied to the common cathodes are of equal amplitude. To ensure that some correction is always developed the gain of the chrominance channel is, initially, made higher than it should be so that the correction signal always acts to reduce the gain of this channel. In a modification of the latter embodiment the D.C. restoring diode 35 is normally conductive and is rendered non-conductive during the occurrence of the colour " burst " signal by means of a negative pulse derived from the colour sub-carrier amplifier 21 (Fig. 2, not shown). In a further modification the pair of diodes are replaced by a single triode 70 (Fig. 3) the cathode and control grid of which act as a peak detector to clamp the negatively going line synchronizing pulses to ground potential, the anode being cut-off by means of a negative pulse derived by differentiating the leading edge of a line synchronizing pulse in circuit 73. During the occurrence of the immediately following colour " burst " signal the anode is supplied with a positive pulse (derived in circuit 73 from the trailing edge of the line synchronizing pulse and a potential dependent on the difference in peak amplitudes of the colour " burst " and line synchronizing signals is developed across resistors 72 and applied via terminals 46 to reduce the gain of the chrominance amplifier. Fig. 4 illustrates a further embodiment in which the degree of colour saturation desired is manually controlled and automatically maintained. In this arrangement the circuit 422 in which comparison of the amplitudes of the line synchronizing pulses and the colour "burst" signal is effected is similar to circuit 322 of Fig. 3, differing therefrom in that the grid/ cathode circuit is arranged to be supplied with positively-going signals. These latter signals are derived from the matrix amplifier 29 and comprise a luminance component with positively going line synchronizing pulses (derived via phase inverter 29d) and a positively going colour " burst " signal (derived in the matrix 29c) the relative amplitudes of which (as applied to the grid of 85) may be varied by variation of the potential divider. Variation of the latter is then effective to vary the colour saturation of the reproduced image and the operation of circuit 422 maintains the pre-set saturation by varying the gain of the chrominance channel until the amplitudes of the line synchronizing and colour "burst" signals at the grid of valve 85 are equal. Specification 765,855, [Group XXXV], is referred to.