Digital delay - here to stay?

Discussion of television and radio technology - professional and domestic.
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Private Frazer
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Digital delay - here to stay?

Post by Private Frazer »

This is about digital TV and Radio. With my Freeview radio the delay compared to the analogue FM station signal is maybe one or two seconds. So it's no loger live (or as live as analogue radio is), and the time beeps are presumably less accurate. I'm tempted to ask if it really is progress. I read that "Your digital TV or radio receiver is receiving the encoded and compressed signals, and using an onboard microprocessor to decode the incoming data and convert to video & audio. This encoding and decoding process takes a little processing time, hence the delay." My question is - does anyone know if this delay will reduce much in future as technology gets faster? In a way it doesn't really work yet.
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DMatthews
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Re: Digital delay - here to stay?

Post by DMatthews »

I guess there is also a delay at the broadcast end whereby they do the encoding/compression.

In terms of decoding/decompressing, there's no techncial reason why processors and on-board memory can't be improved but I guess some manufacturers might choose to stick with slower/outdated components simply to keep the costs down and improve profits.

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Brian F
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Re: Digital delay - here to stay?

Post by Brian F »

I often wonder if on the Freeview HD channels (BBC at least) there isn't an extra delay do that if you get the notification "Do you want to watch in High Definition" and switch, you don't actually miss a word.

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Scary
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Re: Digital delay - here to stay?

Post by Scary »

The delay is mainly due to the need to compress the signal for broadcast and the subsequent process to decode at the other end. That's mainly in the last bit between broadcaster and home though, within a broadcaster the video and audio is non-compressed and the latency is very low, although any processing such as standards conversion adds a bit of a delay.

It's not just digits of course, there also can be a satellite hop even with terrestrial broadcasting like DAB or cable. Although for TV it can take longer for a terrestrial channel to get to your set than the equivalent satellite one just due to the differing encoding methods and speed of the reciever. There will always be some sort of delay

I'd imagine it could improve purely because of Moore's Law but I cant imagine it being a priority for broadcasters or manufacturers

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Don Satchley
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Re: Digital delay - here to stay?

Post by Don Satchley »

When looking at satellite transmission as opposed to terrestrial masts, much of the delay is due to distance travelled. Microwaves used in satellite transmission travel at pretty much the speed of light but have to go 72000km from A tosatellite to B. Whereas terrestrial masts using radiowaves travelling at around the speed of light usually only have to relay signals 100-200 miles. This results in a delay of around 0.25 seconds.
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Scary
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Re: Digital delay - here to stay?

Post by Scary »

Don Satchley wrote:When looking at satellite transmission as opposed to terrestrial masts, much of the delay is due to distance travelled. Microwaves used in satellite transmission travel at pretty much the speed of light but have to go 72000km from A tosatellite to B. Whereas terrestrial masts using radiowaves travelling at around the speed of light usually only have to relay signals 100-200 miles. This results in a delay of around 0.25 seconds.
Yes, although in the early days of digital TV I remember seeing the two systems side by side (showing a BBC channel) and the terrestrial was lagging behind satellite which is counter-intuitive for the reasons you mention.

I've just checked it now and it's no longer the case, but then this would have been on an old OnDigital era box with older slower encoders and with a different TX chain.

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Re: Digital delay - here to stay?

Post by brigham »

Still nowhere near the lag in Intermediate Film days, but a backward step nonetheless.

bbctim
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Re: Digital delay - here to stay?

Post by bbctim »

And of course, hence the sad demise of any BBC clock...

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Re: Digital delay - here to stay?

Post by GarethR »

Even without digital delay, I'd bet a pound to a bucket of ferrets that the clocks would still be long gone.

Does the delay actually matter enough to be considered a backwards step? Does it impact broadcasting in any meaningful way?

I suppose it means that the GTS pips are no longer accurate, even on FM, but if you need your clocks or watches to be *that* precise, you really want to be using ones that pick up the time signal from Anthorn.

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Private Frazer
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Re: Digital delay - here to stay?

Post by Private Frazer »

Given that we've used radio waves to communicate since Marconi, and that even using a satellite link the delay due to the distance they travel is only 0.25s, the delay in encoding/decoding etc. that was introduced is currently relatively big (about 1.5s at a guess from FM to Freeview Radio). There's just a slight feeling that it's not quite as live; something else was on top of that of course at the Doctor Who 50th Anniversary party with the chaotically delayed link up to the States!

P.S. Just tried to listen to the analogue Radio 2 FM and Radio 4 LW delays on two different (not-too-old) digital Freeview boxes. On one box (a Philips) the delay of both was about 1sec and on the other (a DV3T) again both the same, but about 1.5sec.
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Scary
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Re: Digital delay - here to stay?

Post by Scary »

Private Frazer wrote:There's just a slight feeling that it's not quite as live
It is still live though, I could understand the problem if the delay was artificial because of editing or some sort of Big Brother style profanity dump system. It is coming to you directly with nothing in between. Unless you're somehow at a live event watching it on TV or radio* and comparing the two it makes no difference to the viewing experience.

....with one exception. betting on sports events. Online betting sites have had to put in ways to prevent people betting on an event from the venue before the goal is seen on TV at the HQ of the betting company
something else was on top of that of course at the Doctor Who 50th Anniversary party with the chaotically delayed link up to the States!
That wasn't really the fault of digital delay, just bad planning be whoever organised the OB at the production company. 2-ways between London and the US happen every day on TV without that problem. I suspect the delay was caused by something IP based - either the video from LA-London or the audio from London to LA. Delays in 'traditional' broadcast circuits aren't that long compared with what they can be when IP based stuff is in use


*in fact without a digital delay - on FM radio for example - the problem can be the opposite. You can be at a concert which is live on radio and hear it through the air after you do on a radio. A few years ago I was in Hyde Park while the Radio 2 Proms in the Park was taking place. I could hear the concert on my phone FM receiver at least 2 seconds before I could across the park. Technically those at the back of a concert aren't actually hearing it live!

perov
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Re: Digital delay - here to stay?

Post by perov »

Listening to TMS cricket on the radio, and watching it on Sky, there is 5 second delay.

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Private Frazer
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Re: Digital delay - here to stay?

Post by Private Frazer »

Just another bit of Radio information:
Last night, Radio Scotland on MW was about one second later than on FM (Bryan Burnett: Get it On programme). So, all sorts of delays these days.
"Now listen you guys, I don't wish to alarm you but there's some pretty weird things going on out here..."

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Re: Digital delay - here to stay?

Post by brigham »

Private Frazer wrote:Just another bit of Radio information:
Last night, Radio Scotland on MW was about one second later than on FM (Bryan Burnett: Get it On programme). So, all sorts of delays these days.
That's interesting. More imaginative folk could read something into that. It could prove that scientists are tampering with the ionosphere, or that Paul McCartney really is dead.

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