[linux-dvb] relating PTS to 'real' time

John Birch Johnb at screen.subtitling.com
Tue Aug 23 15:09:46 CEST 2005


Stef,

Why do you want to map them against 'wall time'?
If you want to break it into chinks of the same size - use the pcr values
for chunking.
The PCR is continuous (apart from the wrap at 0xFFFFFFFF...) so you can
choose a sensible binary fraction of the max PCR value to chunk up the
stream.

There really is no way of synchronising the wall clock against PCR because
of the fact that PCR maximum value occurs at a longer interval than 24
hours, so the PCR value drifts against wall time.... so on day 1 the PCR
value at 12:00 midday might be X, on day two the PCR value at 12:00 midday
will be X - n, where n is the difference in the 24hr 'day' wrap interval and
the PCR wrap interval (working the wrap out is left as an exercise for the
reader :-) - basically MAX 32 bit value * tick value.

If you want the same sized chunks - which effectively means the same
duration of play time for each chunk - use the PCR values and chunk on
that....

If you want to be able to determine when a chunk was transmitted - just
record the PC time (or GPS time) at the start of each chunk.....

Note: PCR values are not guaranteed to be continuous... broadcasters do
sometimes reset equipment - which will cause a PCR change.... also some
broadcasters change the PCR value periodically for other operational
reasons... there is a flag that is used to indicate a discontinuity in PCR
values....

regards

John Birch
Senior Software Engineer,
Screen Subtitling Systems  
The Old Rectory, Church Lane
Claydon, Ipswich, Suffolk
IP6 OEQ
 
Tel: +44 1473 831700
Fax: +44 1473 830078
www.screen.subtitling.com

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-----Original Message-----
From: linux-dvb-bounces at linuxtv.org
[mailto:linux-dvb-bounces at linuxtv.org]On Behalf Of Stefan Magdalinski
Sent: 23 August 2005 13:57
To: linux-dvb at linuxtv.org
Subject: Re: [linux-dvb] relating PTS to 'real' time


John Birch wrote:
> Stef,
> 
> There is no process relating PCR /PTS to real time (assuming by real 
> time you mean GMT or UTC etc)
> PCR values wrap at an interval greater than 24 hours.
> There is rarely any relationship between the PCR value and 'wall clock 
> time'.
> 
> The PTS relates to the PCR - that is the PTS is the presentation time 
> stamp for the content it is embedded in (video, audio or subtitles). 
> When the PCR value in the stream matches the PTS, the content should be 
> presented (i.e. the video frame should be visible, audio heard, or 
> subtitle displayed).
> 

ok. That makes things a lot clearer.

What I'm trying to do is break the stream into small chunks, and save 
them mapped against 'wall clock time' - If the PTS and PCR don't carry a 
reliable form of 'wall clock time', should I just make sure the PC's 
clock is accurate and use that?

I don't need subsecond accuracy, or even second accuracy. +/- a minute 
is probably ok.

> Hope this helps...
> 
> regards
> 
> John Birch
> Senior Software Engineer,
> Screen Subtitling Systems 
> The Old Rectory, Church Lane
> Claydon, Ipswich, Suffolk
> IP6 OEQ
>  
> www.screen.subtitling.com


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