In the TV department in John Lewis last night, a mix of digital and analogue screens on display were all showing BBC ONE. The time delay between the old technology and new was incredible. I timed it at around 4 seconds.
I knew there is a delay but thought it was sub-second or one second at the most.
Anyone know more about this and why last night's delay was so long?
Depends on the distance around the country the signal travels, (from London to Belfast/Cardiff/Glasgow and back again), then the time it takes to get to the satellite and back again (22000 miles each way). Plus statistical multiplexing works best if all the channels are showing different things, so rather than have 6 versions of, say ITV, all exactly in sync, they will delay each one by a slightly different amount in order to make the stat mux work better. DTT processing is done at each regional centre and adds almost as much delay as a satellite uplink and return path.
The delay varies depending upon the channel - as they have different analogue and digital distribution and coding systems, and can have different amounts of satellite and MPEG2 encoding/decoding links in their chain.
For example most BBC One feeds on DTT will have two MPEG2 encode/decodes in their path - one encode in London, one decode and re-encode in the national/regional centre and then the final decode in your digital receiver. If you are watching a BBC One region on satellite then there is also the uplink/downlink delay caused by satellite path distance (even at the speed of light it takes time for the signal to go to and from the satellite)
In the future it is possible that the Nations will get uncompressed feeds of Network, and the English regions will be encoded in London rather than locally, which would remove an MPEG2 encode/decode process.
For info (on most channels) each MPEG2 encode/decode will add around a second to the delay on most channels - as MPEG2 is used with approx 12-18 frames of GOP encoding - so encoding buffers by at least that amount, as does decoding. Channel Five and the other channels on the SDN mux (now owned by ITV I believe) use a much longer GOP (up to 60 frames?) which may mean a much longer delay between analogue and digital on C5?
Depends on the distance around the country the signal travels, (from London to Belfast/Cardiff/Glasgow and back again), then the time it takes to get to the satellite and back again (22000 miles each way). Plus statistical multiplexing works best if all the channels are showing different things, so rather than have 6 versions of, say ITV, all exactly in sync, they will delay each one by a slightly different amount in order to make the stat mux work better. DTT processing is done at each regional centre and adds almost as much delay as a satellite uplink and return path.
AIUI NTL aren't using delays with ITV - though it was widely rumoured at the time. However I believe C4 DO add different amounts of delays into their regional variations to aid the stat mux (after all only the adverts benefit otherwise...)
Here in the channel isles, the BBC and Channel 4 difference is about a second, but the ITV one appears to be about 4 to 5 seconds!
The Channel Islands are a very diffrent case. BBC is fed via an off-air feed via Digital Satellite (Sky) so the diffrence between Sky and analogue is virtually nothing as they're both coming from exactly the same place (if there was an analogue service like in the UK you'd notice a big diffrence)
ITV comes via a terrestrial link (a wire IIRC) whereas Channel TV via Sky goes from St Helier to London, then into space and then back to your dish. Hence the bigger delay
Here in the channel isles, the BBC and Channel 4 difference is about a second, but the ITV one appears to be about 4 to 5 seconds!
The BBC in the Channel Islands is different to the BBC elsewhere.
Unlike anywhere else in the UK, the BBC ANALOGUE transmitters on the Islands are fed via an off-air digital satellite feed. There should be almost no difference between BBC One Channel Islands satellite (which is coded in Plymouth - with the local news studio from Jersey fed back to Plymouth for opt-out switching), and BBC One Channel Islands analogue off-air (as the latter is fed from the former)
BBC One Channel Islands analogue AND digital satellite should both be a second or two behind BBC One London digital satellite.
Channel TV get their feed of ITV1 Meridian via a fibre circuit, rather than satellite, so Channel TV analogue and Channel TV via digital satellite (which is fed back to NTL in London via fibre and then uplinked to satellite) should show a bigger delay AIUI.
Not sure how Channel Four is distributed to the Islands.
I'd therefore expect BBC One and Two to show almost no delay when switching between analogue and digital satellite (Channel Islands version) but a significant delay beteen analogue and digital satellite (London version)
I would also expect a significant delay between ITV1 Channel TV off-air analogue and ITV1 Channel TV digital satellite.