TV Broadcast Delay (Analogue TV days)

Poking about on YouTube I found this (for our purposes the thumbnail is irrelevant as we're only interested in the first 15 seconds or so):


There might be a simple explanation for what I want to highlight but this is a TV-am close and regional handover. On this occasion for whatever reason TV-am left their onscreen clock up over the eggcups, normally it's long gone by this point.

You'll notice at around 14 seconds into the video the minute hand on the clock moves, seemingly to 9:25 on the dot, but the transmitter switchover doesn't occur for another second afterwards.

Now of course its most likely that the clock isn't accurate to the second (as it didn't need to be) but some days of the service output is on YouTube with VT clocks burnt in and where there is the original clock sometimes it is accurate to the timestamp, not that that usually means anything so presumably the graphic generator drifted on occasion and here it just happens to be a second ahead. And of course for the legal reasons it can be safe to assume the handover at the regions wasn't delayed by one second.

I suppose it boils down: TV-am came from Camden Lock in London. This video is taken from the Granada area. Would there have been at this point any delay in the signal from London the further out it went via however this was done (as we know TV-am didn't go through the ITV companies like it did after 1993)? Ie somebody in the South West would have seen the exact same thing at the exact same second as somebody in the STV area would have.

I don't remember that Ed Welch theme to Crosswits

There was definitely delays on the network feed - if you lived in an area where you could flick between two ITV stations, they weren’t exactly in sync even on networked content.

I'm fairly sure that, back in the day, the switch between TV-am and the local ITV franchises wasn't handled by computers accurate to the second... it was done by a team of engineers (BT I think) who would literally/physically pull out the "TV-am cable" that was feeding the transmitter, and quickly plug in the "ITV region" cable. That was probably updated to 2 by 1 switches, to make it easier, you'd like to think. Hence the non-sync frame rolls at the point the switch occurred.

I'm sure that there is someone on here who remembers exactly how it all worked.

I think it's been said that on screen clocks were often a second fast anyway, otherwise when the news starts at 18:00:00 you wouldn't see the clock hit the top of the hour.

I had to do that once for another broadcaster, it takes an agonisingly long time, way more than the splat
that we saw on those ITV Tx changovers. Also, back then ITV didn't use sound in syncs, and this was way before embedded audio in SDI etc, so someone else would have had to have performed the same thing in the audio patch bay, and at exactly the same time.



It must have been co-ax relays, that were fired by a physical switch ?

Remember, for the first few months of TV-am there was a 10 min gap between TV-am finishing and ITV starting.
I strongly suspect BT hadn't upgraded all of their regional switching sites to allow instant switching in time for Feb 1 83. There was an IBA caption, saying 'ITV progs continue shortly' probably from the Tx site, to mask BT's manual replugging work.

Thames (but not LWT) were about 200 milliseconds behind TVS in the early 90s, (yes, I did get that round the right way !) I know Thames built a new Pres suite 89/90ish, they must have had some digital gizzmo in the chain that introduced that sort of delay !

I wonder if Thames synchronised their incoming network feed to station syncs, and used a PAL composite synchroniser (which had to respect the PAL 4/8-field sequence) and potentially introduced more delay than a synchroniser that decoded to component would, or otherwise locking yourself to network incoming?

Something in the back of my mind was that the facility was composite 177 Mb/s SDI (not the 270 Mb component SDI we have all come to know and love) ?

An absolutely accurate TV-am clock wasn't really necessary, as they weren't part of a network... essentially it could work in its own time zone... It wouldn't really have mattered if was a little slow or fast.

The BBC Nations clocks were deliberately set a second or so late, to allow for clean cutting in to the dirty network feed... often manually cutting on the mixer.

That said... in the digital Pres suites you had to also join the feed late, to account for the delay in the feed from London making its way around the UK. The delay not only caused by the physical distance, but also the various encoding / decoding of the feeds from London to the Nations.

Most of that time is the codec delays.

If you say the distance from London to Glasgow is 600km.
Speed of light is 3 x 10^8 metres/second, so light travels 300km in one millisecond.

Radio signals travel at almost the same speed

Electrical signals travel through most cable at about 2/3rds the speed of light (same is true for light in fibre optic cables)

So, if you assume a cable route (with no electronics) all the way from London to Glasgow that's 600km, a signal will take about 3 milliseconds to arrive. One TV frame is 40 milliseconds in duration.

Distance is small fry, as you say the vast majority of the delay are digital codecs along the way.

And it's only digital based kit that causes a delay. Analogue amps etc introduce no measurable delay

When digital TV was introduced to the UK, the BBC distributed Network 1 and 2 feeds to the Nations and the English regions using 9Mbs MPEG2 ISTR. That will have a codec delay.