GB871597A - Improvements in or relating to television systems - Google Patents

Abstract

871,597. Secret transmission. RANK ORGANISATION Ltd. May 7, 1959 [May 8, 1958], No. 14752/58. Class 40 (4). [Also in Group XL (b)] In a transmitter for a subscription television system, Fig. 1, the signals are rendered secret by separating the video signals into a low- and a high-frequency band, modulating the lowfrequency band on a sub-carrier from a source 8 in a modulator 7, developing the high-frequency band in opposed polarities by a phase-splitter 9 and then selecting one or the other polarity by a switch 10 controlled by coding signals applied from source 12 via a bi-stable circuit 11, and finally combining the resulting signals from 7 and 10 in an adder 13 for transmission. The separation of the video signals into bands is effected by first selecting the low-frequency band by means of a channel including an inverter 2 and suitable low-pass filter 3 and then subtracting the low frequencies from the whole band of the original signals by adding the inverted low frequencies to the original signals in an adder 6. The channel for the whole band of the original signals includes a circuit 5 to equalize the delay of filter 3. The preferred lowfrequency band is 0-200 kc/s., and the preferred frequency for the sub-carrier is of the order of 1.25 mc/s. and also an odd multiple of half line frequency. The coding signals cause the polarity selection of the high-frequency band to be effected in a random manner and may be derived and communicated to receivers by the methods described in Specification 841,627 and Specification 850,824. The lowfrequency band is preferably modulated on the sub-carrier in negative polarity. At a cooperating receiver, Fig. 2, the sub-carrier is selected by a selective amplifier 2 and demodulated at 25 to obtain the low-frequency band, whilst the received signals are also derived in opposed polarities by a circuit 23 and the outputs applied to a switch 24, which is controlled by decoding signals applied from 28 via a bistable circuit 27, so as to re-establish correctly the polarity of the high-frequency band. A network 29 equalizes the delay via amplifier 22. The re-established low- and high-frequency bands are combined at 26 and applied to the television display.