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Sky/Terrestrial Delay

(October 2004)

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IS
Inspector Sands
jay posted:
five via Sky is at the same speed as my terrestrial, strange!


Same speed?.... I don't think you mean that!

Analogue Channel 5 (except in London) is distributed to the transmitters by satellite, hence it arrives at you set at roughly the same time as the Digital satellite version
JA
jay Founding member
Inspector Sands posted:
jay posted:
five via Sky is at the same speed as my terrestrial, strange!


Same speed?.... I don't think you mean that!

Analogue Channel 5 (except in London) is distributed to the transmitters by satellite, hence it arrives at you set at roughly the same time as the Digital satellite version

You know what I mean!
IS
Inspector Sands
Sput posted:
Every channel (bar News 24 I believe?) will do this. The difference is the time it takes for the feed to be compressed, maybe encrypted, then transmitted and decompressed and of course maybe decrypted by the set top boxes.


Yes, the delay is caused by encoding and decoding the digital signals as well as other processes such as Aspect Ratio Conversion. And the obvious delay occurs when the signal has to go into space and back.

Quote:
If broadcasters were nitpicky enough they could delay the analogue output by the 2-ish seconds it takes though, but there still might be a difference between digital satellite and digital terrestrial.


That still wouldn't synchronise the digital and analogue services - each platform (analogue, cable, satellite, DTT) has a diffrent delay between broadcaster and home. The delay also depends on the reciever, some decode digital signals faster than others.
SP
Sput
Uncle Bruce posted:

The satellite is 35,784 km away, and the signal has to do that trip twice (once up, once down).


That accounts for 0.23 seconds, what about the rest? Smile

Also, to the News 24 question - I seem to recall that because it's digital only, it times to perfection when they simulcast.
NH
Nick Harvey Founding member
It would seem sense for all this information to be put in the FAQ section, so all the new children could ignore it in there and we could then send them off to read it every time.
SP
Sput
Nick Harvey posted:
It would seem sense for all this information to be put in the FAQ section, so all the new children could ignore it in there and we could then send them off to read it every time.


I can picture it now!

"Why is there a delay on digital television?"

Look it up in the FAQ, it's under "Who the feck is this Nick Harvey Weirdo?" Wink

Seriously though, it's nice to flex my technical knowhow, don't take it away from me Smile
FA
fanoftv
Here, five used to be a second or two out as did BBC One & Two, with ITV1 and Channel 4 around 3 seconds out.

Now though since Sky have done something, not sure what they've changed, but five has no delay to the terrestrial, BBC One, Two & Channel 4 have about 3-4 seconds and ITV1 has 5-6 seconds, though it does make the delay less annoying.

When BBC One, Two and five were just a fraction out and you had the same channel on in two rooms, it became annoying!
IS
Inspector Sands
fanoftv posted:
Here, five used to be a second or two out as did BBC One & Two, with ITV1 and Channel 4 around 3 seconds out.

Now though since Sky have done something, not sure what they've changed, but five has no delay to the terrestrial,!


It does have a delay, but not one you'd notice as you have nothing without a satellite or digital delay to compare it with!
NG
noggin Founding member
Steve in Pudsey posted:
An artificial delay is introduced on BBC1, C4 and ITV because there are multiple versions of the same channel on the same transponder, ie all the regional variants. The delay is slight but is to do with the statmuxing (dynamically changing how much bandwidth each channel takes up) so that they're all slightly different.

It should be the case that 5Live on DSat is the same as MW - the MW transmission goes through a delay stage so that it's in synch with the satellite transmission so that if a transmitter loses its feed it can fail to the digibox backup without causing problems in the overlap areas where there are two transmitters on the same frequency.


Err - there were rumours that ITV1 was going to statmux, but apparently they don't - they are broadcast at CBR instead (with the varying delays related more to the routing and coding structures required to get the right regional variant back to London)

All of the BBC One English regions are also CBR as they are fully encoded in each region - so can't be statmuxed as this requires coding to be co-sited.

Not sure if the BBC One and Two NATIONAL variants are coded in London or the Nations - they, along with BBC News 24, Three/CBBC and Four/CBeebies, along with the BBCi channels, COULD (not saying they are) be statmuxed. However I don't think the Beeb do any delaying to help out.

The reason for the difference in BBC One and Two feeds is the number of coding chains they have been through, the number of satellite hops, and also (in the Nations) whether the analogue transmitter feed is being derived from the analogue or digital network feed. (The digital network feed is "behind" analogue)
UB
Uncle Bruce
Sput posted:
Nick Harvey posted:
It would seem sense for all this information to be put in the FAQ section, so all the new children could ignore it in there and we could then send them off to read it every time.


I can picture it now!

"Why is there a delay on digital television?"

Look it up in the FAQ, it's under "Who the feck is this Nick Harvey Weirdo?" Wink

Seriously though, it's nice to flex my technical knowhow, don't take it away from me Smile


<sigh> You have to wonder about this place right now, eh Nick?
UB
Uncle Bruce
Sput posted:
Uncle Bruce posted:

The satellite is 35,784 km away, and the signal has to do that trip twice (once up, once down).


That accounts for 0.23 seconds, what about the rest? Smile


I'd have thought that was obvious.

Half a second to transmit, another half a second to encode into MPEG.

IT'S NOT ROCKET SCIENCE!

Ask your science teacher! I'm sure between telling you about how to blow things up, he'll explain.
SD
Steve D
TVDragon posted:


The delay on ITV and S4C is about 3 or 4 seconds, but BBC1 is only about one or two.

The feed is a second later on BBC Wales mind you as it routes through Cardiff -- until the last local junction [if not at N24/ceefax handover] when you notice a click to network and you lose a second's worth of programme.

So the difference between BBC Wales analogue and BBC network digital is nothing really.


No, the 'click' - which is only on the analogue chain - is because after closedown the analogue chain is switched to sustain on the analogue network feed from London. This is the only time BBC Wales viewers get a feed of analogue network, as from 6am until closedown both transmission chains sustain on the digital network.

Both the analogue and digital signal paths ALWAYS go via Cardiff. Even after closedown the network feeds of both digital and analogue networks come from TVC to BH Cardiff, from where they're distributed to the transmitter chain via Wenvoe, or in the case of DSat back to TVC and on to the uplink.

During the day, because digital and analogue chains are sustained from the same network, you don't get a 'click' on the analogue chain when the Cardiff pres suite goes out of circuit - in fact it's seamless.

For the record, the difference between the analogue and digital feed when it hits BH Cardiff is around one second (which is why you lose that 1" of programme during the switch), but by the time the DSat feed has gone through the rest of the chain and hits your STB it will have introduced another second of delay. The DTT delay is about half of this.

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