[Steve in Pudsey]

Page 690 in Yorkshire. It was often incredibly specific, I remember one saying something like:

Emley Moor: loss of redundancy due to YE work on Jagger Lane incomer

Meaning that Yorkshire Electricity were doing some work on one of the two electricity feeds to the site, so there was a risk of disruption if an unrelated fault arose on the other one.

[Technologist]

Unlike the BBC where it was triggered by a piece of equipment at TV Centre being unplugged and plugged back in.

Unplugging the input to the Sound in Syncs encoder - that then output black level with no Field syncs ..
Detect Field syncs at the trasnmitter -
if they are there .....turn on with sequence of getting each stage up befre the next
if not after a time out ... turn off.

There was something always on the line so the PO did not disconnect it.

[Si-Co]

I’ve never really understood what “sync pulses” are (and similarly, the need to genlock between sources to achieve clean cuts between them). However, talking of whether or not the transmitter interpreted a period of black as a fault or not, the blue screen would also appear if vision was lost but sound continued - and one would assume that’s not a loss of signal or pulses?

[Technologist]

Sync Pulses are what are in the video signal to say that a line starts or finishes
or likewise ( but in analogue world more complexly ) that the field or frame has Started .
As TV is a sequence of frame /field made up of lines it is essentail
that if you are going to change from one source to another that these are aligned.
.... the picture jumps if they are not and the TV may take a frame to recover
(And I can think of some VT Machines which would take many seconds)

Sound is very different -it has no framing structure - execpt when carried digitally
where there is a need to align and /or process the audio over time periods.

Synchronistion is a complex topic - and all the more so when signal are carried (plesi)isochronously
in a Telco Multiplex....... but is the audio embeded on video or carried seperate but needs time synching
IP which has litle inherent timing is a lot easier- except that you need to be able to reconstrct the source clock at the far end over a link with variable transit time.... so some sync signal in content are useful....
and of course (source) clocks have a toleranace and they vary .....
and it may be that Sound and video clocks are linked (hard or softly) or not!!!

[Neil Jones]

I only know about sync pulses Technology Connections did on the presence of Macrovision on commercial VHS releases, as this not only screws up the VCR's ability to record from a Macrovision protected tape, but it screws up the TV's ability to "lock on" to that sync pulse as well. I'm assuming its a similar principle for what transmitters are looking for, but that being said there was a video on YouTube where some programme falls off the air but continues in sound but the ITV company in question were aware of it.

[Technologist]

Itv contractors mostly used a Video and an audio pair from the Post office
so one would work without the other ...
and an analogue transmitter had separate Video and audio modulators and amplifiers.

[Stooky Bill]

Page 690 in Yorkshire. It was often incredibly specific, I remember one saying something like:

Emley Moor: loss of redundancy due to YE work on Jagger Lane incomer

Meaning that Yorkshire Electricity were doing some work on one of the two electricity feeds to the site, so there was a risk of disruption if an unrelated fault arose on the other one.

Westcountry's 690 pages were very detailed compared to the others I saw, although not quite that specific. Useful stuff, except when there was an issue and you couldn't see teletext to check what the issue was

www.teletextarchive.com .

Details - Teletext Archive
The Teletext Archive Home Services About Contact Register Login Westcountry, Mon…
  Source: www.teletextarchive.com

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Itv contractors mostly used a Video and an audio pair from the Post office
so one would work without the other ...

This can be seen with this breakdown on LWT. AIUI the vision mixer in LWT TX died. The programme returned after they patched the VT straight to air


youtu.be 

[JAS84]

Loss of transmission (seemingly planned) during an episode of Woof on Yorkshire TV
youtu.be 

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?

[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.