Patent GB-726030-A (GB)
Title: Color-television receiver signal-modifying system
IPC: H04N11/14
Filed: 1953-06-18
Abstract: 726,030. Colour television. HAZELTINE CORPORATION. June 18, 1953 [July 24, 1952], No. 16861/53. Drawings to Specification. Class 40 (3). The invention relates to colour television receivers utilizing a constant luminance type of signal and is concerned with the generation of a correcting signal to compensate for changes in the image brightness caused by the chromaticity signal due to non-linearity in the image-reproducing device. The type of signal considered is one made up of luminance signal together with a chromaticity signal comprising a sub-carrier modulated at 0 degrees and 90 degrees phase points by colour difference signals. In such a signal the intensities of the components of the luninance and chromaticity signals are proportioned relative to each other such that ideally the luminance signal determines the brightness of the image and the chromaticity signal contributes only colour. This is effective, however, only where the system is linear. In practice the cathode-ray image-reproducing device is non- linear such that second and higher order effects result in the chromaticity signal affecting the image brightness. Where the two modulation components p and q of the sub-carrier are of form p = Eb<SP>1/γ</SP> - Ey<SP>1</SP> and q = Er<SP>1/γ</SP> - Ey<SP>1</SP>, (Eb<SP>1/γ</SP> and Er<SP>1/γ</SP> being gamma connected colour signals, and Ey<SP>1</SP> the luminance signal made up of Eg<SP>1/γ</SP>, Er<SP>1/γ</SP> and Eb<SP>1/γ</SP> terms proportioned in accordance with their relative luminosities) the luminance error introduced by the chromaticity signal may be dependent on the phase angle of the subcarrier (so-called elliptical sub-carrier) and therefore variable in magnitude with image colour. In order to be able to apply correction it is essential that the error should be independent of the phase angle (so-called circular subcarrier) and in the first instance it is necessary to modify the components of the sub-carrier to achieve this. This operation may be carried out either at the transmitter or at the receiver. With the signal in this modified form it is then shown that the correction signal which should be combined with the luminosity signal is proportional to the square of the intensity of the sub-carrier signal divided the luminance signal. In a first embodiment, Fig. 1 (not shown), the modification of the sub-carrier is carried out at the transmitter. The two components are then of the form p<SP>1</SP> = Eb<SP>1/γ</SP> = Ey<SP>1</SP> and q<SP>1</SP> = At the receiver the correction signal is derived by passing the sub-carrier signal through an amplifier having a gain related to the reciprocal of the average value of the square root of the luminance signal and then squaring the output signal in a modulator stage. The resultant signal is combined with the luminosity signal in an adder. The remainder of the receiver is conventional apart from a cross coupling from the blue to the red channel to cancel out the Eb<SP>1/γ</SP> - Ey term added to the q component at the transmitter. The phase at which the subcarrier is demodulated is alternated as described in Specification 702,182. In a modification, Fig. 2 (not shown), the division of the subcarrier signal is carried in a dividing stage receiving inputs from the sub-carrier and luminosity channels. A second embodiment, Fig. 3 (not shown) operates with a signal which has not been specially modified and converts it to the circular sub-carrier form by matrixing stages before deriving the correction signal as in the first embodiment. The additions and subtractions to be carried out in the matrixing stages are analysed. In a third embodiment, also operating with a normal signal, the conversion to circular sub-carrier form is effected by modulating the sub-carrier with a local signal of twice the frequency. By choice of appropriate gain and phase it is shown that the desired form of sub-carrier is obtainable. The modulator output is squared and then divided by the luminosity signal in a divider stage to derive the correcting signal. A modification of this embodiment employs two modulator channels with a switching circuit to permit operation with a signal the modulation sequence of which is alternating. Specification 689,356 also is referred to.