Delay between transmitters

As explained on another thread on these forums, I found the other day that I could receive an analogue signal of BBC1, ITV1 and Channel 4 from the Bluebell Hill transmitter...

I flicked over to Channel 4 from Bluebell Hill earlier, and had another TV on that was tuned to the analogue version of Channel 4 from Crystal Palace... and I noticed there was a delay between the two of them... I switched both TV's to BBC1 and there was no delay there....

I thought the delay might be to do with each region opting out for local commercials but when Channel 4 went to an ad break, both versions showed the same adverts...

Can anyone explain the delay and has anyone who can pick up more than one analogue transmitter noticed this before?

I think Bluebell hill rebroadcasts Crystal Palace, that might introduce a delay

Nope - Bluebell Hill is one of the three main TXs that broadcast BBC1 South East (the other two being Heathfield and Dover). South East Today would be the regional programme broadcast from it.

Bluebell Hill Coverage Area Map

It wouldn't be that as channel 4 doesn't have any opting out, all the adverts come from London. Not sure if Bluebell and CP are in the same C4 ad regions - just because the ads are the same doesn't mean that they are the same.

Not in the case of BBC1 (it is fed from Tunbridge Wells), but maybe Channel 4 is. That might be the answer, although the signals would travel roughly the same distance. Perhaps there is something digital in the way?

Are you sure, because I'm sure I've seen a few local adverts on Channel 4. Unless you mean all the adverts, including any local ones, are played out from London.

Yes, there are regional adverts (about 5 or 6 regions IIRC) but they all come from Ch4 in London. there are no opt-outs

Ah, OK. Thanks for clearing that up!

I'm not CERTAIN on this, as Channel 4 isn't my specialist subject, so don't put TOO big a bomb under me if I'm wrong.

But aren't Crystal Palace and the satellite uplink both driven by a direct feed from Channel 4, and all the other transmitters driven by a feed from the satellite?

That would make CP ahead of ALL the other transmitters, but all those other transmitters would be in sync with each other.

That's certainly how Five works, and I'm pretty sure Four now do the same.

On DSat because all of the macro regions are on the same transponder they introduce a differnet amount of delay to each one, so that they aren't identical so that the statmuxing has a fighting chance of working properly. Whether or not that delay is also present on analogue I don't know.

Don't think C4 distribute to their analogue (or digital) transmitters via satellite - think they still use their 34Mbs (?) compressed digital circuits that were introduced around the same time they adopted D5 (uncompressed) VTRs as their delivery format. (They wanted to avoid compression in the VT chain as they had compression in the transmission chain)

I certainly don't know of any C4 broadcast quality distribution links in the same way that Five have theirs. (Five don't broadcast their Astra 2 service on analogue - they uplink to their analogue transmitters separately). C4 certainly aren't using their Astra feeds to link to their transmitters - the picture quality of C4 on Astra is appalling at times...

I suspect that the analogue transmissions are late either as a result of the 34Mbs compression used to distribute (which may not be used for the CP feed - as CP is often fed "differently" for London resilience reasons) or a result of being in a different sub-region (for ad reasons) ISTR that the various C4 regions on DSat are delayed relative to each other to aid compression via statmuxing on satellite - maybe the delay is introduced early enough in the chain to influence analogue as well as a digital?

Thanks for that, noggin.

Makes a lot of sense.