[Technologist]

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.

Not quite ...... the line fed BBC transmitter will switch off if it got Line syncs only ....
But the Main sites would go to RBS on lack of input syncs
So if England had closed down the English Tranmitters would turn off
so they would not rebroadcast a say Welsh Transmitter

One line ITS was rather Clever
- the National/Regional unit would pass through "London" signal if it was there ....
Thus auto equalisers etc could look at it and adjust and thus a better picture was obtained .
But if there was no ITS it would put out its own ITS
(as the source must be local and thus may need differnt equalisation)
and this had its own ID which a decoder at the trasnmitter decoded and decided if the feed was OK to Broadcast - i.e London or the "right" region.....
Priorr to this there had been some Signalling in ITS - and lot of strange arramgements
e.g. one small regional site locally agreed that only its output or London should be transmitted
so RBS was disabled during opts!!!! Other places had bits of clockwork which delayed switching
But i was never too sure it worked !!! .
But as part of the rationalisation and standardisation of the Network systems In Regions (and Nations)
in the early 1990s all this strangeness was replaced so everwhere was the same.
There were a few interesting aspects of the project -
Like the new New (commercail) ITS generators output ITS to the BBC Spec,
But that was not the waveform the Legacy BBC Made ITS generators did -
so the Auto Equalisers at remoter Scotland transmitters had all to be modified......
Or the feed to Douglas (IoM) was normally from Caldbeck but switched to WInter Hill for the Opt
as this was being done during NICAM Roll out and the switch at Douglas was done at IF
there was a time when the IoM got NICAM only for the Opt which was in Mono!!!
Or for DR purposes there was a set of Plugs for the back of the ITS generators to be swapped
in Birmingham when its output chains became Net 1 and Net 2 as if from London

One line ITS also carried notionally frame accurate count down as part of Presfax....
and signalling when Symbol was on air and the Auto Opt back box which connected
the PO/BT line incoming to the Region/ Nation to the Feed to the transmitters
- thus totally bypassing the entire station!