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Pete8,939 posts since 18 Jun 2001
Is it me or is the gap between analogue and digital getting smaller.

Channel 4's delay doesn't exist and BBC1 is now only 1 second off. I havn't checked the other channels as Football ofr other sporting events with clocks are the best indiactors.

Hopefully this will mean the clock coming back but it is nice to get rid of the horrible delays.
Earlie0 post since 24 Mar 2017
Well its not the transmitters fault. Its your box. For some reason they do vary in the delay. Not sure why.

The signal for both digital and analogue is sent out at the same time. The delay is caused by your box decoding the data to show the picture and sound.

As the boxes get better this delay should be reduced.
cwathen3,377 posts since 27 Dec 2001
The clock has gone forever because design houses have convinced broadcasters that they have no place in modern presentation.

Digital delay was not a reason for getting rid of clocks; if that was the case all clocks would have disappeared in autumn 1998. People were never timing the beeb's clock to the second for accuracy, I didn't catch anyone writing into POV saying 'your clock is 2 seconds out for DTT viewers, get rid of it' it was just a presentation device used to say 'look, this programme is so important that we'd like to clarify the time that it's on'.

In any case, I think the old clock did get (another) revamp with the new BBC1 presentation, it's just not being used. Although BBC1 NI made a point of saying during their balloons closedown that it was the last time you'd be seeing 'this particular clock' implying that maybe clocks will be making a comeback eventually. In terms of link time it's quicker to have a clock into the news than an ident anyway, if lorraine finds a way to use those few seconds for promotion, you can bet the clock will be back.
Chris Kelly0 post since 24 Mar 2017
Earlie posted:


The signal for both digital and analogue is sent out at the same time. The delay is caused by your box decoding the data to show the picture and sound.

As the boxes get better this delay should be reduced.


Not nessasarily ...

ISTR when Virgin Radio first went on satellite in the mid-ninties, they had the same problem, so they put a half-second delay on their MW feed.

How do you not know if BBC1 has done something similar?
Steve in Pudsey7,872 posts since 4 Jan 2003
I believe 5 Live is delayed on AM so that it matches satellite, but that's to do with the transmitters using satellite as the back-up programme feed and not causing echos and delays in overlap areas
Larry Scutta0 post since 13 Oct 2001
The delay exsists because the box buffers the data before displaying it on the screen - this is because of the way that MPEG works (by reflecting changes in the picture) and because of error correction.

The delay varies depending on the complexity of the picture, the signal quality and the performance of the hardware/software in the reciever.

There is more of a delay for DTT than DSAT - despite the sateliite hop - because DTT has a slower data rate
Larry Scutta0 post since 13 Oct 2001
Chris Kelly posted:

Not nessasarily ...

ISTR when Virgin Radio first went on satellite in the mid-ninties, they had the same problem, so they put a half-second delay on their MW feed.


That was probably because MW stations which have more than one transmitter on the same frequency work as a 'Synchronus Network' so that the signals don't clash with neighbouring transmitters.

Some of Virgins transmitters are/were fed by satellite
Steve in Pudsey7,872 posts since 4 Jan 2003
I believe 5 Live is delayed on AM so that it matches satellite, but that's to do with the transmitters using satellite as the back-up programme feed and not causing echos and delays in overlap areas
Pete8,939 posts since 18 Jun 2001
Larry Scutta posted:
The delay exsists because the box buffers the data before displaying it on the screen - this is because of the way that MPEG works (by reflecting changes in the picture) and because of error correction.

The delay varies depending on the complexity of the picture, the signal quality and the performance of the hardware/software in the reciever.

There is more of a delay for DTT than DSAT - despite the sateliite hop - because DTT has a slower data rate


so how come the delays are longer on Sky? Or is it just rubbishy boxes.

I shall have to get a new Freeview box and compare.
noggin12,061 posts since 26 Jun 2001
Larry Scutta posted:
The delay exsists because the box buffers the data before displaying it on the screen - this is because of the way that MPEG works (by reflecting changes in the picture) and because of error correction.

The delay varies depending on the complexity of the picture, the signal quality and the performance of the hardware/software in the reciever.

There is more of a delay for DTT than DSAT - despite the sateliite hop - because DTT has a slower data rate


No - the data rates on DTT and DSat are comparable Larry - though BBC One is not statmuxed on DTT in England and is a fixed data rate service to allow for regional opts.

The extra delay on DTT is probably a result of a second MPEG decode/code process.

BBC One Digital Network leaves TV Centre as a 9Mbs MPEG2 distribution link, is then permanently decoded to SDI in the English regional centres (and the National centres as well), passed through an SDI opt switch and then permanently recoded to approx 4-5Mbs MPEG2 for broadcast (emission rate)

There are thus two MPEG2 code/decode processes for the DTT feed (even for BBC London who still have an opt-switch permanently in circuit I believe), whereas there is only a single MPEG 2 code/decode in the BBC One England DSat chain (the decode being at the domestic receiver)

Given that MPEG2 for broadcast normally runs on a GOP of about 12-18 frames (I think) - there is at least that delay introduced by both the coder and decoder? That would be a second minimum per code/decode process?
noggin12,061 posts since 26 Jun 2001
Hymagumba posted:
Larry Scutta posted:
The delay exsists because the box buffers the data before displaying it on the screen - this is because of the way that MPEG works (by reflecting changes in the picture) and because of error correction.

The delay varies depending on the complexity of the picture, the signal quality and the performance of the hardware/software in the reciever.

There is more of a delay for DTT than DSAT - despite the sateliite hop - because DTT has a slower data rate


so how come the delays are longer on Sky? Or is it just rubbishy boxes.

I shall have to get a new Freeview box and compare.


Are you watching BBC One Scotland or BBC One England on DSat?

BBC One Scotland will have been derived from a 9Mbs MPEG2 BBC One Network feed coded to MPEG2 in London, it will have been decoded to SDI in Glasgow, to create BBC One Scotland, then recoded back to MPEG2 to be fed back down to London to be muxed with other services and then uplinked to satellite.

I don't know if the coding in Scotland is at 9Mbs for distribution or at a lower rate for broadcast. If BBC One Scotland is statmuxed then it will need to be decoded again in London and re-coded. This could add a 3rd MPEG code/decode process - so I expect it isn't statmuxed though I might be wrong.
Pete8,939 posts since 18 Jun 2001
I'm watching BBC One Scotland. I know the analogue is on a second delay from england as I hear the English sound on the phone just before i hear the scottish.