Patent GB-689356-A (GB)
Title: Color-television system
IPC: H04N11/14
Filed: 1951-03-27
Abstract: 689,356. Colour television. HAZELTINE CORPORATION. March 27, 1951 [May 1, 1950], No. 7075/51. Drawings to Specification. Class 40 (iii). In a dot-sequential colour television system, the three output signals from the colour camera at the transmitter, corresponding to the red, green and blue components of the televised image, are combined in predetermined proportions to derive a plurality of further signals all but one of which are successively and cyclically sampled at a high rate (e.g. 3.8 Mc/s.) and are then added to the remaining signal (called the " monochrome " signal), the resulting signal being radiated as a modulation of a highfrequency carrier wave. At the receiver, the received demodulated signal is applied to a sampling device operating in synchronism with that at the transmitter to separate the sampled signals and also to a filter for extracting the monochrome signal; these signals are subsequently combined in such a manner as to obtain signals equivalent to those generated by the television camera. In the described embodiments these signals are applied to respective cathode-ray tubes each associated with an appropriately-coloured filter, the images on the cathode-ray tubes being combined to form a single coloured image by an optical combination employing dichroic mirrors. By suitably choosing the manner in which the further signals are derived from the primarily generated signals, interference, especially interference produced as a result of high-frequency components heterodyning with the sampling frequency, is caused to produce variations of colour values in the reproduced picture instead of variations in the intensity thereof, the eye being less sensitive to variation of this nature. Details of operation. In one embodiment, which may be considered as an example, the monochrome signal M is defined by the equation where G, R and B respectively represent the green, red and blue outputs of the colour camera. Suitable fractional proportions of the signals G, R and B are obtained from potentiometers and then combined to derive the monochrome signal M. Similarly the signals g, r, b, which are successively and cyclically sampled for transmission together with signal M are defined by phase-inverting circuits, as well as potentiometers, being used to obtain the negative coefficients. At the receiver, signals g, r and b are extracted from the combined signal by means of a sampling device operating in synchronism with that at the transmitter. Signals rand b are then applied to amplifiers having amplification factors 2.23 and 22.3 respectively. these factors representing by their reciprocals the sensitivity of the eye to red and blue images produced by the cathode-ray display unit with respect to green images. These resultant signals are then combined with the monochrome signal to obtain signals G, Rand B since, as can be seen from consideration of the equations given above, These signals G, R and B are then applied in the appropriate fashion to the cathode-ray display unit. It can be shown that by virtue of the multiplication in the red and blue channels any interference produces equal intensity luminous effects phased 120 degrees apart which consequently cancel one another out visually. In an alternative embodiment the monochrome signal is accompanied by two sampled signals S1' S2 which are so chosen that It will be seen that although a low amplification of 5 is employed in the blue channel, the relative amplitudes of signal S2 in the green and blue channels are as 1 is to 22.3, thus maintaining the relationship obtained in the previously described embodiment. Other modifications are described, particularly ones wherein the sampling is effected at phases 0, 180 and 270 degrees, and at 0 degrees, (180 - 12.5 degrees) and 270 - 12.5 degrees).