[Stooky Bill]

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?

How would they reschedule a networked programme, in a networked block of children's programmes? Also why disrupt the schedules for all the kids who weren't watching TV from the transmitter that had the interruption?

They were obviously expecting the interruption to take place going by the quickness of the apology, but probably not knowing exactly when. I'd have thought they'd have specified that they break shouldn't be during the adverts

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If the problem was with LWT pres, would that cause their clean feed to other regions to be lost (because in this occasion that happened)? Wouldn’t that feed normally bypass the pres vision mixer unless they needed to do an announcement over or after the end credits, or network a promo or similar - and even then they would be unlikely to flick the switch to “go dirty” so far in advance?

Someone, I think on a predecessor to this site did give the full story as to what happened. AIUI it was the pres mixer that went so yes presumably the programme being sent out to everyone else was fine. It looks like they put that to air in London manually and then the ACR or whatever they used for ads, then the next programme.

[Neil Jones]

The IBA Engineering Announcements were often full of transmitters being off air, on reduced power or liable to interruption for one reason or another, and for 1989 I dare say that interruption would have been known about and probably made it to a bulletin, though there aren't many 1989 examples of those announcements online.

[Si-Co]

Someone, I think on a predecessor to this site did give the full story as to what happened. AIUI it was the pres mixer that went so yes presumably the programme being sent out to everyone else was fine. It looks like they put that to air in London manually and then the ACR or whatever they used for ads, then the next programme.

You may have misread or misinterpreted what I wrote above, but the feed to the other regions was NOT fine. I remember watching this episode on TTT and we lost picture AND sound at the same time as LWT lost the picture. Local pres filled with a caption and music and we rejoined the programme a few seconds later than LWT (just in time to hear Brucie saying goodbye). That would make sense if the show had just reappeared on their feed from the South Bank and they took a few seconds to react - though I’m not sure why they seemingly lost sound when LWT didn’t. I’m not suggesting the explanation given is incorrect, but the breakdown wasn’t restricted to just the London area.

[James2001]

Guess it was doubly unfortunate that it happened not only while LWT were networking a show, but when they were networking the next one as well.

Not sure why whoever uploaded that video dubbed the sound from the first 2 minutes over the last 2 minutes...

[Steve in Pudsey]

I think the explanation of the LWT epic was that the same equipment handled local pres and a separate output for the network, with two separate control services for pres and MCR.

When it failed, both functions were lost.

[TIGHazard]

Guess it was doubly unfortunate that it happened not only while LWT were networking a show, but when they were networking the next one as well.

Not sure why whoever uploaded that video dubbed the sound from the first 2 minutes over the last 2 minutes...

"the last part of the clip i had to mute due to been blocked by copyright its after the problem had been fixed." according to the description

[Stooky Bill]

You may have misread or misinterpreted what I wrote above, but the feed to the other regions was NOT fine. I remember watching this episode on TTT and we lost picture AND sound at the same time as LWT lost the picture. Local pres filled with a caption and music and we rejoined the programme a few seconds later than LWT (just in time to hear Brucie saying goodbye). That would make sense if the show had just reappeared on their feed from the South Bank and they took a few seconds to react - though I’m not sure why they seemingly lost sound when LWT didn’t. I’m not suggesting the explanation given is incorrect, but the breakdown wasn’t restricted to just the London area.

Sorry I missed that bit of your post.

I imagine they thought there was little point staying with the audio, it was only left on LWT by chance rather than choice.

I remember the person who posted with the background to the incident saying that it was a legendary breakdown at LWT,

[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.

[TakeTwo]

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.

In Northern Ireland, right up until closedowns stopped in the late 1990s, BBC Scotland was sometimes up later than when BBC-1 and BBC-2 Northern Ireland closed. In those situations, RBS kicked in, and BBC Scotland was rebroadcast on NI transmitters.

In 1994, when the power supply to Divis went down (on a weekday afternoon), BBC-1 and BBC-2 was still available in at least some parts of Northern Ireland because BBC Scotland was rebroadcast. UTV and Channel 4 had no such arrangement and were off air completely.

[AvroVulcanXH607]

Oh that's interesting, perhaps not quite so clear cut then. I know Limavady could RBS Scotland - maybe it was deliberately set to accept their code as 'better than nothing'?

I did read a memo from an early-2000s re-engineer in BBC NI which noted that Divis was refusing to accept London during RBS tests and had to have an NI ITS added in Belfast. It doesn't detail the cause or fix though.