TV signal out of sync.

[sigma]

When I switch from an analogue TV transmission to a digital TV transmission I find they are out of sync by about two seconds. If I time signal is broadcast, which one is correct and why are they out of sync.

[R1Geezer]

They are out of sync because they are tranmitted from 2 different systems. The digital one is slightly behind the analogue one. The analogue one is more accurate for the time on the screen eg in BBC news for example. Digital is slightly behind because because of various factors, like having to go through software etc but the delay is half a second max.

[neuron]

Eh? half a second? I checked this out a few weeks ago at two different properties. In both, the TV takes about two seconds to switch from analogue to digital when both are transmitting BBC News (Channel 503 on Sky) at night.
Even after that two second switchover delay, I still had to wait around a second or two for the zeroes on the hour on the digital channel when switching from analogue to digital.
Aren't theoretical figures great?

[R1Geezer]

If you have 2 TV's side by side one with analogue and one with digital, the digital 1 ids have a second behind the analogue. I'm not talking about switching from one to another, dunno what gave you that Idea Neuron.

[R1Geezer]

" is half" - bashed it out quick, doh!

[R1Geezer]

No the signal comes through the standard analogue ariel, I was assuming the questioner was referring to freeview

[Rollo]

The same thing happens with digital radio broadcasts. They are one to two seconds behind the analogue one.

[ChuckFickens]

the extra delay caused by the signal coming from the satellite for SKY tv is about 250ms (1/4second)

The rest of the delay in any digital broadcast is caused by three things..

1) encoding the signal into digital at the transmitter

2) decoding the signal back to analogue at the receiver

3) buffering. all digital receivers buffer an amount of time so they can deal with breaks in the data being received without effecting the picture on your TV, this buffering is the main cause of the delays you see. (although it a digital TV would never buffer more than a couple of seconds, in extreme circumstances, such as I work with! the buffering delay can be upto about 1minute!)

[neuron]

R1Geezer, you've clearly not seen the point I was making. The The switching part was irrelevant; what I was showing was that the switching was taking two seconds after which I still had to wait a couple of seconds for the digital transmission to catch up with the analogue.
To simplify, if you add those two seconds to the visual delay seen on the screen in the form of the countdown timer to zero seconds on the hour, the total delay is somewhere around four seconds.
It's certainly not half a second as you claimed in your first post nor one second as you claimed in your second.

[scotman]

I can't comment on television but on freeview radio the digital is approx one second behind the analogue for the time pips. I understand this is because of the encoding/decoding time.

[sigma]

Thanks for all your answers. I'm viewing on freeview and it would appear that a system that is ideal for broadcasting is far from ideal for receiving as any time signal would be inaccurate and the TV signal is often subject to interference from the weather resulting in loss of signal.
It seems that by 2015 we are going to be forced to use a system that is that is only of benefit to the TV companies and not the consumer.

[ChuckFickens]

sigma,

What on earth are you doing that you need the time on the screen of your television to be millisecond accurate?

Sure digital broadcasts may put the time out by a couple of seconds.... but does it matter??

As for your problems receiving the signal in bad weather, it is widely publicised that the current freeview transmissions are at reduced power so they don't interfere with the old analogue transmissions, it has also been very well advertised that some people may need to upgrade their TV aerials to receive digital signals, by switchover (2012) the power will be increased and you have plenty of time to get your antenna sorted out.

I take it your the sort of person that would rather we were all still riding about in horse and carts?

[Dusty Bin]

When the analogue signal gets switched off for radio, will there be any point in radio stations broadcasting the pips at the top of the hour as they're always going to be inaccurate?

[sigma]

Horse and cart for transport , you were lucky, when I was little we had to be pushed along in a wheelbarrow.

No, I don't need the time signal to be that accurate but read Dusty Bin's comment. Also, imagine the scenario, New Years Eve, your watching Big Ben strike midnight, you can then switch over to digital and hear it again.