... somebody will know about this.
Spouse gave me a personal DAB radio.
If I listen to it at the same time as I listen to a normal wireless set the DAB sound is 1 - 2 seconds after the other signal.
This even applies to the Greenwich time signal.
Is this normal?
Mary
I don't have one, but I would think it is normal...
DAB is digital radio, it uses mpeg compression (as far as I am aware) the audio is compressed on the fly then transmitted - in order to transmit the frames, information about the next frame is needed - this is why there is a delay
It is the same on cable TV, there is a short delay compared to terrestrial analogue TV
Sparks...
We are not talking satellite transmitted digital radio here; are we? If terrestial DAB I guess it takes time to 'pack it' and send? :-) Seasons greetings Mary.
In message , Mary Fisher wrote
Yes, the same is true for digital radio on Freeview.
On 26 Dec 2003, Mary Fisher wrote
Yes, it is. (I think it has to do with coding/decoding the digital signal.)
I understand the lag on the time pips is *really* annoying to some people. (It doesn't bother me, as I don't use the pips any more for setting clocks: the best invention in years was the invention of clocks that set themselves automatically...)
Yup.
Yes this is the case with our TVs If I'm quick shout down the stairs hubby can catch whatever of interest. It's about 3 seconds here.
I got hubby one of those clocks a few years ago and we think it's great. Only once has it been in error. One night the alarm went at 5 instead of 7, and the clock was displaying 7. It was one of those gut wrenching feelings of being woken early and then confusion as to why it was dark etc.
Nearly. DAB is satellite distributed so there will inevitably be a delay. Nothing to do with it being digital.
And to you, Terry.
Ah - so the signals are bounced off something a long way away - that makes sense in my little mind, thank you.
Makes a bit of a nonsense of the pips though, for those who are picky. Doesn't bother me, we have no watches and few clocks, those we have aren't in synch and the computer clock doesn't accord with any of them. The sun's a pretty good indicator of time - as is my stomach :-)
Thanks to everyone who replied for your interesting and patient replies.
Mary
The satellite delay is probably the largest single delay in the chain but there will coding/decoding/propergation(*) delays as well. The cumulative effect of all those may well be more than the single hop sat delay of about 525mS.
(*) Time it takes the signal to pass through a single bit of kit, frequently with digital kit this will be at least one "frame" in duration.
No it isn't.
Yes, it is. It's *entirely* to do with it being digital.
No they aren't.
I'm pretty certain DAB is line fed, since the landline structure exists to all the transmitters. Delay is intrinsic with any codec. IIRC, the only satellite fed terrestrial system is CH5.
There was discussion at one point of putting a deliberate digital delay into the feeds for analogue transmissions as well so that the various services were equalised, and then to have the time signal early so that at the user's receiver the time would be right to at least within about 100mS.
As I understood it, this gets quite complicated because some analogue transmitters are landline fed with satellite as backup so it becomes pretty complex and expensive to implement.
Streaming transmissions over the public internet can incur even longer delays - tens of seconds - because of the buffering that has to be used to cope with the lack of QoS (quality of service) mechanisms to deal with latency and jitter.
.andy
To email, substitute .nospam with .gl
Running the entire operation early was thought about at one point, but then it was realised that some delays are variable in real time.
Operational nightmare, they find it hard enough not to talk over the pips ATM let alone if they were happening early!
Which leads to the question of how far in advance do you run the network? 1s, 5s, a minute?
An interesting post which I shan't remember - except for the expression I was looking for regarding 'regular' transmissions: analogue.
Thanks, Andy.
Mary
Realistically, if you want to have accurate time for all but high precision scientific purposes there are a few ways:
- Clock with receiver synchronised to the MSF station at Rugby. These clocks are now pretty cheap.
- GPS receiver
- Using a computer connected to the Internet and synchronised using NTP (Network Time Protocol.) There are numerous bits of software around for doing this,
e.g. Tardis
.andy
To email, substitute .nospam with .gl
Lots of replies have explained this in terms of digital processing delays.
A similar problem exists with synchronisation between radio and TV. In the past, with analog simulcasts you could watch programmes such as the Last Night of the Proms on TV and listen to the sound in higher quality on Radio 3.
Can't do that any more - because sound and vision are not synchronised when transmitted by two different digital media.
There's progress for you!
Did you notice the queens speach was around a minute behind on the TV compaired to the radio!
Sparks...
In message , Andy Hall wrote
Many of the cheap clocks are synchronised to a German source.
Does this have to be accurate? Isn't it the time _difference_ in receiving signals from different satellites that is important.
Not very accurate but okay for Internet connections. You request a time from a time server that may be locked accurately to one of the time references but the journey time back to you is variable. It's much the same as pinging a machine over a network. It may take 200ms or many seconds to respond depending on network (ISP) loading..
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