Patent GB-1475743-A (GB)
Title: Method and apparatus for scrambling and unscrambling communi cation signals
IPC: H04N7/167
Filed: 1974-08-15
Abstract: 1475743 Secret transmission; television SYSTEM DEVELOPMENT CORP 15 Aug 1974 [15 Aug 1973] 35892/74 Headings H4R and H4F In a secret transmission system communication signals are transmitted in a scrambled form together with control signals, to assist unscrambling, and identification signals, which are compared at the receiving stations with the receiving station identity and if a match is determined the unscrambling is enabled, using the received control signals, which are preferably continuously changing during the course of transmission. As described the system is applied to a television distribution system in which subscribers request a particular programme from a central station which transmits the programme in scrambled form together with control signals and signals identifying the appropriate subscriber, which are transmitted in the vertical blanking interval of the programme signal. The programme video signals are scrambled by inverting selected lines of the video signal, either according to a predetermined sequence, in accordance with a pseudo random series, or in accordance with a control signal preceding each line. Audio signals can be scrambled by moving them to a non-standard frequency spacing from the video signals. Fig. 2 shows a preferred embodiment of a receiving apparatus in which the received signals are frequency changed, IF amplified and detected in the normal manner at 26, 31, to provide audio signals on line 33 and video signals on line 32. Synchronizing signals are separated from the video signals at 42, to control the recovery of the identification signals, and control signals are separated from the video signals at 45, the control signals being used to switch the video inverter 36 into or out of circuit when the identification signals have been correctly received, and also to feed the audio signals through the unscrambler 56 to restore the audio signal to the appropriate frequency range relative the video signal. The unscrambled video and audio signals are mixed and frequency changed at 48 to bring them to a standard channel frequency receivable by a standard television receiver. The unscrambler control and address recognition logic 45 is shown in Fig. 3 in which a horizontal synch pulse counter 92 and decoder 95 are reset by the vertical synch pulse so as to open gate 97 to pass only the horizontal synch pulses immediately preceding those lines, during the vertical blanking interval, which carry the identification and control signals, the pulses being coded digitally at a 1 MHz bit rate. The horizontal synchronizing pulses passing through gate 97 enable the output of an 8 MHz clock 104 which, via a divide by 8 circuit 107, centre samples at 112 the digital identification and control signals in the video signal applied to terminal 85, and feeds them into a serial shift register 114. At the appropriate time, when all the identification and control bits are present in the counter the identification bits are compared with a stored address in comparator 119 and, if the address is correct, the control register 88 is enabled to receive the control signals. Included in the control signals is an unscrambling synchronizing pulse which is enabled by the horizontal synch counter at the appropriate time to cause the unscrambling operation to commence. In one embodiment of the decoder Fig. 7 (not shown) control signals from the register 88 cause the output of a horizontal synch pulse counter (141) to invert the lines of video information at selected counting positions, e.g. inverting alternate lines, pairs of lines, groups of four lines &c. In a second decoder embodiment Fig. 8 (not shown) a control pulse transmitted at the commencement of each horizontal line controls the inversion or non-inversion of that line: while in a further decoder Fig. 9 (not shown) the control signals are fed into a shift register (162) as a starting pattern and the register is connected to recirculate as a pseudo random binary digit generator which controls inversion of a pseudo random pattern of lines.