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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

Time delay - Big Ben slow?



 
 
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  #21  
Old January 3rd 12, 12:12 AM posted to uk.tech.digital-tv
Steve Thackery[_2_]
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Posts: 2,552
Default Time delay - Big Ben slow?

Steve Thackery wrote:

I'm only reporting what the programme said (sorry, I can't remember what
it was called or the channel). They distinctly mentioned delay, and how
the newer sats are much better.


Incidentally, they also didn't mention whether they were talking about
comms satellites or broadcast satellites. Clearly telephone calls are
far less tolerant of delay than broadcasts.

Like you, I also understand that most of the delay is in the encoding
and decoding, especially at the receiver.

--
SteveT


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  #22  
Old January 3rd 12, 12:16 AM posted to uk.tech.digital-tv
Bill Wright[_2_]
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Default Time delay - Big Ben slow?

Steve Thackery wrote:
Steve Thackery wrote:

I'm only reporting what the programme said (sorry, I can't remember
what it was called or the channel). They distinctly mentioned delay,
and how the newer sats are much better.


The fact is, most of what the bloke in that programme said was OK, but
the bit about time delay was wrong. He just had the wrong end of the
stick for some strange reason. Just because something's on the telly
doesn't mean it's true.

Bill
  #23  
Old January 3rd 12, 07:03 AM posted to uk.tech.digital-tv
Kennedy McEwen
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Default Time delay - Big Ben slow?

In article , Paul Ratcliffe
writes
On Mon, 2 Jan 2012 22:55:01 +0000, Kennedy McEwen
wrote:

The Beeb may well have been using a satellite link for the OB back to
base, so that's going to add latency too ?

Where's the satellite then?


Well, aren't you up yourself?


No, it isn't there - but your gob seems large enough.

It only takes 2.7sec for a relay on the
moon, half that for Neil Armstrong's one way Apollo broadcasts in 1969,
and I am pretty sure the satellite isn't that far away. Relay to
geostationary orbit is less than 1/10th of that time. Sure, it adds to
the latency, but only 5%.


It's bloody obvious that most of it is MPEG coding and decoding not
the trip to/from the satellite FFS.


Sure its so bloody obvious that its MPEG, NOT, since DAB has similar
delays!
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
  #24  
Old January 3rd 12, 07:51 AM posted to uk.tech.digital-tv
Silk
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Posts: 565
Default Time delay - Big Ben slow?

On 02/01/2012 13:19, John Aldred wrote:
I was watching the countdown to the New Year on BBC (Freeview).
As Big Ben struck the hour, my time pieces were indicating about 5 secs past
midnight.

Our clock is synchronised with the broadcast time signal (NPL MSF), and the
computer is synchronised with the internet time servers.
Both indicated that Big Ben was slow.

So my question is does it really take about 5 seconds for the TV signal to
be processed and reach my eyes/ears? Or am I living in a time warp.


I hope that one day, if I work hard and keep myself out of trouble, that
I'll wake up contented in the knowledge that all I have to worry about
is having to wait 5 seconds to hear the sound of Big Ben.

How wonderful your lives must be.
  #25  
Old January 3rd 12, 09:11 AM posted to uk.tech.digital-tv
Mark Carver
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Default Time delay - Big Ben slow?

Kennedy McEwen wrote:
In article , Paul Ratcliffe


It's bloody obvious that most of it is MPEG coding and decoding not
the trip to/from the satellite FFS.


Sure its so bloody obvious that its MPEG, NOT, since DAB has similar
delays!


As Paul explained in his own unique way, the code/decode iteration on
satellite links adds significant latency. There are so many codecs in
the Tx path these days whatever route is employed, the delays are now
seconds rather than hundreds of milliseconds.

Also BBC DAB is delivered to the transmitters by satellite too (the
major DAB sites are terrestrially fed, but obviously delayed to match
the remote satellite fed ones)

I saw that satellite programme too. It's a shame, because the business
about satellite processing delay was ********. 25 years ago when
analogue communication was used the latency introduced on a satellite
link was only governed by the 44,000 mile round trip, so if anything
things today are worse than then !
  #26  
Old January 3rd 12, 09:24 AM posted to uk.tech.digital-tv
Richard Tobin
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Default Time delay - Big Ben slow?

In article ,
Mark Carver wrote:

Also BBC DAB is delivered to the transmitters by satellite too (the
major DAB sites are terrestrially fed, but obviously delayed to match
the remote satellite fed ones)


Obviously? Not to me... Why are they delayed?

-- Richard
  #27  
Old January 3rd 12, 09:33 AM posted to uk.tech.digital-tv
Andy Burns[_7_]
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Posts: 1,278
Default Time delay - Big Ben slow?

Richard Tobin wrote:

Mark wrote:

Also BBC DAB is delivered to the transmitters by satellite too (the
major DAB sites are terrestrially fed, but obviously delayed to match
the remote satellite fed ones)


Obviously? Not to me... Why are they delayed?


Because it's an SFN, so needs to be synchronised ...

  #28  
Old January 3rd 12, 09:35 AM posted to uk.tech.digital-tv
Mark Carver
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Default Time delay - Big Ben slow?

Richard Tobin wrote:
In article ,
Mark Carver wrote:

Also BBC DAB is delivered to the transmitters by satellite too (the
major DAB sites are terrestrially fed, but obviously delayed to match
the remote satellite fed ones)


Obviously? Not to me... Why are they delayed?


Because DAB muxes operate as SFNs (Single Frequency Networks). The
transport stream as transmitted has to be precisely synchronised with
adjacent transmitters, otherwise reception of more than one DAB
transmitter will result in an overall destructive effect on the received
data. For the same reason, DAB networks are designed so reception of
another transmitter on the same SFN is not normally possible if the
separation is in excess of about 80km (this equates to the guard
interval ).

That's why during lift conditions reception of DAB can be destroyed,
despite receiving a distant transmitter carrying the same mux.

The synchronisation is achieved through GPS reference at each site.
  #29  
Old January 3rd 12, 09:50 AM posted to uk.tech.digital-tv
Dave Farrance
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Posts: 1,551
Default Time delay - Big Ben slow?

Steve Thackery wrote:

Richard Tobin wrote:

Dunno. It was one of that short series of programmes about engineering
in the UK. There was one about the Airbus wing, the satellite one, and
a couple of others, I think. Sorry, I can't remember the programme
name or channel. Late 2011, they were broadcast.

Like you, I was surprised. However, this issue with delay was
discussed, and they talked about how much it had been improved in the
later generations of comms satellites.


There's a half second delay in the double roundtrip (120ms x 4) from
geostationary orbit, so that's the minimum time to get a reply in a
conversation. I understand that phone calls go by undersea fibreoptic
cable these days so there's little delay. I also saw that program and I
wonder if they simply got the explanation wrong -- wouldn't cable be used
for studio-to-studio TV links as well now?

BTW, the reports about the removal of the dishes from the BT Tower say
that they were prompted to do that by the rusting of the support brackets,
but that fibreoptic cable had made them redundant many years ago. So it
seems to me that a huge network of high-bandwidth cable has long rendered
fixed radio links redundant.
  #30  
Old January 3rd 12, 10:09 AM posted to uk.tech.digital-tv
Peter Duncanson
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Posts: 4,272
Default Time delay - Big Ben slow?

On Tue, 03 Jan 2012 00:44:53 GMT, Paul Ratcliffe
wrote:

On Mon, 2 Jan 2012 22:55:01 +0000, Kennedy McEwen
wrote:

The Beeb may well have been using a satellite link for the OB back to
base, so that's going to add latency too ?

Where's the satellite then?


Well, aren't you up yourself? It's in the usual place?

It only takes 2.7sec for a relay on the
moon, half that for Neil Armstrong's one way Apollo broadcasts in 1969,
and I am pretty sure the satellite isn't that far away. Relay to
geostationary orbit is less than 1/10th of that time. Sure, it adds to
the latency, but only 5%.


It's bloody obvious that most of it is MPEG coding and decoding not
the trip to/from the satellite FFS.


Except:
http://en.wikipedia.org/wiki/Transpo...munications%29

Most communication satellites are radio relay stations in orbit, and
carry dozens of transponders, each with a bandwidth of tens of
megahertz. Most transponders operate on a "bent pipe" principle,
sending back to earth of what goes into the conduit with only
amplification and a shift from uplink to downlink frequency.
* However, some modern satellites use on board processing, where the
* signal is demodulated, decoded, and then re-encoded and modulated on
* board the satellite. This type of transponder , a "regenerative"
* transponder, has many advantages but it is much more complex.

Google finds lots of hits for "regenerative transponder" but almost all
in Google Books, which makes quoting difficult.

This webpage is dated 2008:
http://www.satellitetvref.com/Spacecraft.html

At present, transparent (or 'bent pipe') satellite transponders are
the norm, but regenerative transponders with onboard computers are
expected gradually to supersede them because of several advantages
offered by the latter, The chapter includes: a brief overview of
transparent transponders but with the emphasis on regenerative
transponders, as the technology of the former is well documented in
several textbooks; ...

I haven't found any indication of the relative proportions of
transparent and regenerative transponders currently in use.

This IEEE paper from 1984 describes an experimental "regenerative
transponder" which is not the same as described above. It does not seem
to included de- and re-coding.
http://ieeexplore.ieee.org/Xplore/lo...hDecision=-203
or
http://tinyurl.com/7kr9hv9

--
Peter Duncanson
(in uk.tech.digital-tv)
 




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