[AvroVulcanXH607]

I think I asked the question before, did the system know the difference between black as in "oh crap everything's gone off" and black as in "we're transmitting but we're deliberately not showing anything". And the answer was yes.

I think it was whether the feed itself was present was the test, as the transmitter would only show the blue screen if it had lost said feed - otherwise you'd just have static.

I wouldn't normally reply to something so far back, but in this case speculation continues later in the thread, which will also be addressed...

As already stated, yes, the test wasn't "is the screen blank" but "are there sync pulses". In BBC transmitters, loss of syncs would cause the transmitter to failover into "Re-Broadcast Standby" (RBS), and if that also yielded no usuable signal the transmitter would shut down.

However, when nations/regions started transmitting their own programmes, and therefore didn't necessarily closedown at the same times as one another, a problem could arise: on loss of input syncs, a transmitter may try to RBS from another region which hadn't yet closed down! (or indeed, and worse still, from a completely unrelated TV channel! - but that's another story...)

The BBC therefore used a special data signal which was mixed in with the "Insertion Test Signal" (basically a one-line test card in the Vertical Blanking Interval). This signal identified what region each video feed originated from.

From then on, the transmitters didn't check for syncs, they checked for this address - and if it wasn't either it's own regions or London's, it would not radiate it - essentially treating it as no signal at all.

This also had the useful side-effect of preventing Max Headroom style attacks, as an intruder would need to produce a correct ITS address for major transmitters to re-radiate their feed. (whether this was ever attempted I have no idea).

I'm not sure, but it seems likely that ITV employed a similar system? There are of course multiple ways of identifying feeds that could have been in use.