Patent GB-1072708-A (GB)
Title: Improvements relating to television apparatus
IPC: H04N11/12
Filed: 1964-08-20
Abstract: 1,072,708. Colour television. INTERNATIONAL POLAROID CORPORATION. Aug. 20, 1964 (Aug. 21, 1963], No. 34098/64. Heading H4F. In a colour television system based on the " red-white " theory of colour reproduction described in Specification 985,622, equal bandwidth red and green colour component signals are derived at the transmitter and transmitted in such a manner that the " green " signal is modulated directly on the main carrier whilst the "red" signal is modulated on a subcarrier, the system being made compatible with the normal black-and-white and N.T.S.C. type colour systems by selecting the sub-carrier to have the normal N.T.S.C. frequency at an add multiple of half line frequency and a phase which corresponds to that allotted to red in the N.T.S.C. system, i.e. - 76À5 degrees with respect to a reference " burst," which is also included in the transmitted signal in the customary manner. A black-and-white receiver reproduces a picture in response to the " green signal, and the " red " signal cancels visually due to the choice of sub-carrier frequency. An N.T.S.C. colour receiver reproduces the signal in colour by the " red-white " method since the " green " signal is treated as the luminance signal and causes the production of white light whilst the subcarrier, due to the choice of phase, appears as an N.T.S.C. " red signal and causes the production of red light. A receiver constructed specially for the system utilizes a twocolour cathode-ray tube (see Specification 1,072,707) in which the target is formed of superposed red and blue-green (i.e. minus red) phosphors and beam penetration of the target is controlled by switching a high voltage applied to the target between two values, the lower value resulting in the penetration of only the red phosphor and the higher value resulting in the penetration of both phosphors and the production of white light. The tube is operated to produce red and white in alternate fields under the control of switching circuits driven by vertical deflection signals. The receiver is made compatible with a normal black-and-white transmission by applying the vertical deflection signals to the switching circuits via a gate which is controlled by a circuit responsive to the presence of a sub-carrier " burst." Thus in the absence of a " burst " the tube is switched to produce white light continuously. The " green " and " red " signals are separated in the receiver by the use of a first detector which responds to the whole signal, but, since the subcarrier is at a lower level (- 6 db.) than the main carrier, mainly produces the " green " signal, and a second detector preceded by a notch filter for the main carrier which thus produces the " red " signal. A transmitter for the system is described which apart from the use of two cameras to derive the " red " and " green " signals employs conventional practices. The " green " and " red " signals are transmitted with equal bandwidth on their respective carriers, the upper side-band of the " green " and the lower side-band of the "red" overlapping and the remaining sidebands being vestigial.