Photographs of some of our work

There was an unusual requirement for this job. Not only were the five off-air analogue channels to be made individually switchable on and off, but when a channel was off it had to be really well and truly off, with no stray signal whatsoever available at the outlets. To achieve this we designed a head end that replaced an ‘off’ channel with noise having the correct bandwidth to obliterate the video and mono audio carriers, without affecting other channels. The noise came from a wideband noise generator, the output of which was amplified and passed through three-stage passive channel filters. The filter response was tailored for this application. The switches selected between the signal and the blanking noise. The noise was carried at a level 10dB below the corresponding video carrier (when ‘on’), which was more than enough to prevent reception from signal picked up on flyleads, etc.

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