Patent GB-1381903-A (GB)
Title: Encoding and decoding system for catv
IPC: H04N7/171
Filed: 1972-01-25
Abstract: 1381903 Wired broadcasting, secret transmission OPTICAL SYSTEMS CORP 25 Jan 1972 [8 Feb 1971] 3476/72 Headings H4R and H4L  In a coding system for television video signals modulated on a carrier, particularly for a cable T.V. system, the carrier video signals are amplitude modulated by a signal of a frequency and phase related to the line scan frequency of the video information before transmission, and modulated by a signal of similar frequency but opposite phase at the receiver. The overall system leaves amplitude modulation of the signal at twice the frequency of the coding modulation, which can be removed by a further amplitude modulation by an appropriately phased signal of twice the coding modulation frequency, leaving only an insignificant amount of modulation at four times coding frequency. The modulation at twice the coding frequency is preferably introduced at the transmitter rather than duplicated at each receiver. The coding modulation can be at line scan frequency, phased to reduce sync pulse amplitude and increase a portion of the picture signal, to which the receivers will tend to try to lock, or preferably at twice the line scan frequency so that the receivers will have two positions along each line of picture signal at which they tend to lock. To further increase the security of transmission in the latter case the transmitted signal can be further amplitude modulated by a relatively small signal of the line scan frequency which is phase modulated 90 at about video signal frame frequency, which modulation emphasises first one portion of the line of video signal, then another, at the phase modulating frequency, so that receiver line time base tends to lock first in one incorrect position then in a different incorrect position.  Preferred embodiment Fig. 7 has the video signal from 18 modulated at 14 by the carrier signal from oscillator 10, combined with a sound signal, also modulated on the appropriate carrier, in circuit 22 and fed to the distribution network via modulators 46, and 64. A sync separator 48, amplifier and filter 52 derive a line frequency signal from the video signal to provide, after phase and amplitude adjustment in 56, a modulating signal for the coding modulator 46. In addition the line frequency from filter 52 is doubled filtered and its phase and amplitude appropriately adjusted to provide the modulating signal for the modulator 64, to provide the residual distortion compensating modulation of the transmitted signal. The apparatus can be switched, at SW1 and SW2, to provide either coded or clear transmission. In coded transmission the sound signal is frequency modulated on a carier 1 MHz below the video carrier, preventing its reception by convention receivers while on uncoded transmission the sound is frequency modulated on a carrier 4À5 MHz below vision carrier. Fig. 8 shows the receiver comprising a conventional frequency converter with the addition of an adder 86, and a modulator 86. The decoder comprises a plug in unit which picks off the audio signal, which is amplitude modulated by the coding modulation, shifts the f.m. audio carrier to the correct frequency spacing of 4À5 MHz from the video carrier, and feeds it back to the adder 86 in the converter. The amplitude modulation on the f.m. carrier is detected at 108 and the decoding signal of 15À75 kHz selected and appropriated adjusted in amplitude and phase for application to modulator 88. In a modification of the system, Figs. 12 and 13 (not shown), the coding frequency is twice line frequency and a supplementary modulation using 90 phase modulated line frequency signal is provided to give extra security, with corresponding decoding at the receiver.