Time differences on BBC1/2, ITV, C4 and C5 on Digi

I recently was watching something on BBC1 then switched Digital on. It was on BBC1 digital and it had a time lapse of about 1-2 seconds!

I mean that something said on Analogue BBC1 (like a bit of speech) was seen on Digital BBC1 a few seconds later)

This also happens with News 24 - especially with Breakfast - as this is run in conjunction with BBC1!

yes, the News 24 lag happens with analogue cable too.

I watched Digital a while ago and kept flicking over and watching the time lapse. It was quite substancial (sp?) by TV standards. It is the same - but not as much lag when watching the screens behind the newsreaders on news 24 - the middle one is BBC1

I know! I do the same! I thought digital TV was supposed to be faster than Analogue! Or is that just me?

I don't think it's meant to be *faster* just better quality

Oh right, but it's not - well I don't think so anyway!

Sometimes there are picture freezes on certain channels that I didn't have on Analogue!

The reason for the delay is because as it's digital and compressed (there are numerous TV channels using the same frequency), it takes computing time to compress all the channels on that particular frequency, multiplex them together and transmit them. Then time is taken by your set-top-box to seperate the channels and uncompress them. Lots of computer time is being used!

Oh, and another addition to the delay is if you are watching DSat (Sky Digital etc). It takes time for the signal to reach the satellite and then to come back down to your dish.

BTW, DTT and DSat are not actually better quality, they just appear to be because you dont get the artifacts of analogue transmissions, like ghosting or white noise. If you look closely at a digital transmission, you will see that it is made up of blocks. The way it works is virtually the same as MPG files on your computer but they use better algorithyms. Now you've seen MPG files & their quality!


(Edited by techy peep at 1:51 am on July 13, 2001)

digital tv isn't always better quality - if you are watching News 24 in our region (kent), whenever it fades from the credits to the studio at the hour the screen goes all pixelated. i have been told (by the BBC) that this is because so many channels are using the frequency the signal has to be compressed. hence, when the screen fades there isn't enough space to transmit or receive all of the pixels changing colour and tone at once, so it does it in large clumps, resulting in pixelation. This is something that never happens with analogue. there has been such little uptake of digital that the government are considering keeping analogue on the air for at least another 20 years.

james

I kno what you mean, with News 24, they do that yellow/red wipe effect with the headlines, it looks impressive on analogue, but on digital, it looks like a pixelated blob!

The delay on regionalised channels such as BBC1 and ITV1 is greater as the signal has to pass via each regional centre. I think the additional delay is caused because the digital signal has to be decompressed at the region, converted to analogue, regional output inserted if necessary, then converted back to digital and compressed again. This is also why BBC1 is transmitted at twice the bit rate of non-regionalised channels such as BBC Choice or BBC News 24, to compensate for the loss of quality when converting to analogue and back to digital.

Of course I could be talking a lot of rubbish here.

I don't think this is strictly true. The only time BBC Regions interupt the network feed is when they opt-out to do their own programming. Don't forget that networked BBC1 is actually two seperate networks, run by seperate presentation suites. One is Analogue and the other is Digital.

Also, I think the ITV networks just convert what they are broadcasting on analogue into digits. I don't think ITV have the infrastructure for Carlton, the network broadcaster,  to send digits to all the other companies. I might have this bit wrong though!


(Edited by techy peep at 11:13 am on July 13, 2001)

The Nations do their own presentation on BBC ONE and opt in when the programme actually begins. Often they do this (well NI anyway) a split second early and we see the end of the network ident.