[Steve in Pudsey]

I’ve never really understood what “sync pulses” are (and similarly, the need to genlock between sources to achieve clean cuts between them).

This is a huge simplification, but we know that a frame of an analogue picture is made up of 625 lines. For simplicity, we'll assume that the lines are transmitted one at a time in order.

The sync pulses can be thought of as markers that identify the start of a new line and a new frame.

If, at a given point in time, source A is on line 5 and source B is on line 429 and you try to cut between them, it's going to jump - instead of line 6 you get line 429 so you don't get a full frame and it jumps. So genlocking will gently force one source to be in sync with the other by adding or subtracting a couple of lines to each frame until they are both at the same point.

The more modern technique is a digital frame store which holds the last frame of source B in memory and plays it out in sync with source A


(Again, I fully acknowledgte that's is a massive simplification for the purposes of illustration)

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This strikes me as unprofessional. If there's a planned outage, don't schedule the programme in the first place. They should've postponed that programme and shown it at the weekend, maybe after the Saturday morning kids magazine or after the ITV Chart Show (if that aired immediately after like it's successor CD:UK did). How many kids must've had tantrums that day because CITV got interrupted mid-programme?

Realistically it's likely to have been a very brief switching break at a single transmitter, probably to switch between the reserve aerial on low power back to the main one after a period of maintenance. You can't predict when the work will be complete.