Patent GB-841627-A (GB)
Title: Improvements in or relating to television systems
Inventor: MCMULLAN DENNIS
IPC: H04N7/167
Filed: 1956-12-20
Abstract: 841,627. Secret television. RANK ORGANISATION Ltd. Dec. 20, 1957 [Dec. 20, 1956], No. 38849/56. Drawings to Specification. Class 40(3). Relates to a secret television system in which signals are transmitted in two or more different modes so as to prevent unauthorized reception and a control signal indicating changes of mode is also transmitted to facilitate authorized reception. In accordance with the invention reception by an unauthorized receiver is made more difficult by arranging that the change from mode to mode is responsive both to a control signal and the characteristic of part at least of the signals forming the television programme. The characteristic selected may be the integrated value of the picture signal corresponding to a selected area of the picture and the control signal is made effective to produce a mode change only if the integrated value exceeds a predetermined value. In an alternative arrangement picture signals appropriate to two areas may be integrated and the two values compared with each other or a predetermined value to determine the mode change. According to a variation the integrated value from one or more areas may be compared with the mean brightness of the entire picture area. Another way of carrying out the invention is to determine the amplitude of selected frequency components in the picture signal from the whole picture or selected areas thereof and to employ the amplitudes to determine mode change in a manner similar to that described for the integrated values. The sound signal accompanying the picture signal may also be used to determine mode change in terms of its amplitude or frequency characteristic at selected times. The specification describes in detail, Figs. 1 and 2 (not shown), a system in which mode changes are effected when the integrated value of a selected area of the picture is greater than the means brightness of the entire picture area. At the transmitter suitably timed pulses derived from the line and frame synchronizing pulses gate out the picture signals corresponding to a selected area and these signals are integrated and compared with integrated signals from the whole area. A control signal is then transmitted whenever the integrated value of the selected area differs from the integrated value of the whole picture, but the control signal is allowed to effect a mode change at the transmitter only when the integrated value of the selected area exceeds that of the whole picture. To ensure positive operation despite variation in receiver circuits two comparitor circuits providing a degree of "overlap" are employed at the transmitter so that the control signal is transmitted and mode change effected only when there is a clearly distinguishable difference in the integrated values. At the receiver a similar arrangement of gate and integrators is provided and the control signal, which appears from the transmitter for every difference in the integrated values is allowed to actuate a mode change correction circuit only when the value for the selected area exceeds that of the whole picture. The receiver thus operates to effect mode changes in step with the transmitter. As described only two operating modes, corresponding to positive and negative signal modulation, are employed. The control signal is transmitted on a subcarrier in the picture signal channel, although a reference is also made to the use of pulses in the blanking interval.