[heero_yuy]

IIRC there wasn't anything more sophisticated on the NTL cable network as regards encryption.

I do know that Sky went to a more cunning encrytion that at an apparently random point along the TV line the video gets inverted, synch and all. Different point on every line.

The exact point was controlled by a key card and a pseudo random generator and so if you had the key you could decrypt the signal.

Naturally there was a busy black market in cloned and pseudo key cards.

One anti-copying trick that was used on film channels was to insert several lines of peak white during the frame retrace fairly randomly. This is where VCR's set their reference recording levels, as the video here is normally at black level, this would ruin any attempt to record off air.

Most CRT TV's blank this retrace set of lines so the effect wasn't visible on screen.

Incidentally this is why older TV material is 576i and DVDs generally in 576i or 576p rather than 625i. The "missing" lines are the frame retrace period (24.5 lines) needed by the older CRT TVs and other than a few lines being "stolen" to provide the Teletext service contain no video.