Patent GB-766619-A (GB)
Title: Improvements in or relating to subscription color television system
Inventor: DRUZ WALTER STANLEY
IPC: H04N7/169
Filed: 1955-02-22
Abstract: 766,619. Television. ZENITH RADIO CORPORATION. Feb. 22, 1955, No. 5275/55. Drawings to Specification. Class 40 (3). Unauthorized satisfactory reception of colour television signals of the type employing a luminance signal together with a colour modulated sub-carrier wave signal is prevented by means of a coding arrangement which periodically delays some portion or portions of the signals with respect to the remainder at the transmitter, a compensating decoding delay being introduced at authorized receivers and the arrangement being such that a precise phase relationship is maintained between the colour modualted sub-carrier wave and the colour reference sub-carrier wave. In one embodiment (Figs. 1 and 2,. not shown), in which coding and decoding is effected by periodically delaying the luminance and the colour modulated sub-carrier wave with respect to the colour reference sub-carrier wave (synchronizing " bursts " of which are periodically transmitted) and the scanning control signals (line and frame synchronizing pulses) maintenance of the required phase relationship is ensured either by constructing the delay devices incorporated in the coding and decoding arrangements such that the delay introduced is equal to an integral number of cycles of the colour sub-carrier frequency (to within five electrical degrees) or, since such accuracy is difficult to realize in practice, by including in either the transmitter or receiver, or in both, phase correction circuits which continuously modify the phase of the colour sub-carrier reference wave so as to compensate for the departure from an integral number of cycles of sub-carrier frequency the delay actually introduced. In a second embodiment (Figs. 3 and 4, not shown), the coding delay is introduced between the composite colour signal (inlcouding the luminance signal, the colour modulated subcarrier wave and the colour reference wave " bursts ") and the scanning control synchronizing signals and a similar compensating decoding delay introduced at the receiver. With this arrangement the required phase relationship is always maintained constant since the modulated sub-carrier and the reference wave " bursts " are jointly affected by the delay device. In a third embodiment (Figs. 5 and 6, not shown), in which the phase relationship between the colour modulated sub-carrier and the sub-carrier reference wave signal is also always maintained constant, the coding delay at the transmitter is introduced into the component signal channels, specifically the luminance channel and the red and blue colour difference channels, before those signals are modulated on to the sub-carrier wave and the decoding delay at the receiver introduced into luminance and colour channels immediately prior to the image reproducer. Since this latter embodiment employs three separate delay devices in the coding and decoding arrangement the " secrecy " of the system may be improved (over those embodiments utilizing only one delay device) by arranging that the three delay devices operate in various combinations instead of simultaneously. In all of the embodiments described, control of the decoding apparatus at authorized receivers is effected by means of a " key " signal which is sent to selected receivers by any suitable means, e.g. separate telephone links, punched card code schedules or by radio transmission of a coded " key " signal.