[linux-dvb] How to convert MPEG-TS to MPEG-PS on the fly?

Josef Wolf jw at raven.inka.de
Thu Aug 21 19:45:12 CEST 2008


Thank you for the extensive answer, Barry!

On Thu, Aug 21, 2008 at 05:10:10AM -0700, barry bouwsma wrote:
> --- On Wed, 8/20/08, Josef Wolf <jw at raven.inka.de> wrote:
> 
> > > >I'd like to convert live mpeg-ts streams from DVB-S on the fly into
> > > >a mpeg-ps stream.  I know that (for example)
> > 
> > In principle, yes.  But there is a big drawback to such a solution:
> > the pipes (and demuxing/muxing in a different process) will introduce
> > lots of context switches.  Since I want to convert four full
> > transponders at the same time (about 25 channels), this will certainly
> > kill my 450MHz PII machine.  Let alone the 25 additional mencoder
> > processes all running in parallel.
> 
> Can I ask for more details?  As I'm using a 200MHz and similar
> machines for full- and partial-TS work from 4 DVB cards, I have
> some concerns, that may or may not be a problem.
> 
> What sort of cards are you using -- internal PCI or external USB?

Technotrends internal PCI budget cards.

> When I'm handling a high bandwidth (BBC-HD) program on my internal
> PCI card, not even a full transport stream, I start to feel the
> CPU pinch, which will be far worse for USB streams.

I am not interested in HD (yet).  But surely this will change at
some point in time.

> Given about
> 36Mbit/sec per transponder, you'll be schlepping quite a bit of
> data, which may give you concern.  Keep an eye on idle time.

Grabbing 18 TV TS streams from 3 transponders gives 60% idle at the
moment. (my fourth card has died and I have not bought a replacement
yet.  AFAIK they have stopped manufacturing the cards :-(( )

> Of course, my machine is only an MMX Pentium, and only 32MB RAM,
> so will by far reach its capacity well before yours; mine seems to
> max out with a 15Mbit/sec HD stream (internal PCI), a full 16Mbit/
> sec transport stream via USB of DVB-T, and two filtered USB1 partial
> radio streams, doing nothing but writing files to internal disks.

Watch out for a catch when writing to internal (ext3) disks: When
the commit-interval is reached and the journal is flushed, write(2)
blocks for a significant time.  You risk buffer overruns on the
incoming TS if you are reading in the same thread.  I had this problem
a long time ago when I did my first experiments with DVB drivers.

> Are you intending to use the PSen in real-time like it seems you
> describe, or will you/can you be recording for later use?

Both.  But the recording would probably be by grabbing the already
converted real-time stream via

  wget http://dvb.local:1234/zdf.ps

or something.  Decoupling recording from demuxing saves me from the
above mentioned catch.  In addition, recording can be done on every
host in my network.  I could even roll a script based on LWP to get
the start/end time of the recording correct.

> It sounds like you may, given your example of ZDF, be streaming
> the oeffis from 10744 (arte & Co), 11836 (ARD & Co), ZDF, and some
> Dritte programs at 12110.

11836 + 11954 + 12188 + 12545.  Unfortunately, they have moved arte
from 12188 and a fifth card is not supported by the drivers :-(

> If I'm not mistaken, your program stream should include the video PID
> data, plus an audio PID (only one, I'll assume the primary mp2 audio,
> though you may choose the AC3 where present) from each channel, so no
> worry about second/alternative audio, teletext, or additional program
> tables sent in the full stream.

No, I want to get all the streams so I can select language on the client
(vlc or something).

> The program `ts2ps', part of the dvb-mpegtools suite, or something
> similar from those programs, can be used to repack the data into
> PS, and should be a lot more lightweight than mencoder.

This would still need the pipes.  Introducing pipes would introduce
significant context switching since pipes are (AFAIR) only 8kbytes.
So, assuming 500kbytes/sec, I would get 240 context switches per
second for every program.  This gives a total of 6000 context switches
every second.  You need _really_ big iron to cope with this.

> Timing data is partially within each PID, so you should be able to
> get a usable PS from just the two PIDs.

Yes, timing is within the PID.  But there are lots of times there:
 - PCR, OPCR, DTS_next_AU from the adaptation field
 - PTS, DTS, ESCR_base, ECSR_extension, ES_rate from the PES header
 - there's the possibility that the PCR is carried in a different
   PID (indicated by the PCR_PID field in the PMT)

Which one do I have to use to create the PS header?  I guess I have to
use the PTS from the PES, but I fail to deduce this from the iso-13818-1.

> Given the amount of data you'll be handling on your 450MHz machine,
> you may see lost packets and thus corruption at full load, so test
> by working your way from a single functioning transponder up to the
> full workload.

I don't have lost packets, but still artefacts in the video.  Looks like
the additional stream_id's on the video-PID disturbs vlc's decoder.



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