[linux-dvb] Re: Fw: DVB audio/video decoder API

[Johannes Stezenbach <js@convergence.de>]

On Thu, Oct 31, 2002 at 04:57:20PM +0200, Dennis Noordsij wrote:
> 
> > One thing the driver does which I think it shouldn't is emulating
> > demux features in software that the hardware doesn't have.
> > E.g. the AV7110 does not give us the TS, thus the output for
> > the dvr device must be recreated in software from PES packets.
> > IOW, there's a complete TS multiplexer implementation in the
> > driver.
> 
> No wonder 30-40% of CPU is spent in system (TS multiplexing) on my VDR machine 
> :-)

This could just as well be a high IRQ load due to the small
dual ported RAM which the AV7110 uses to pass data to the PC.

> Would it speed up things to not create a TS stream in the driver, but simply 
> allow the app (VDR is a popular example :) to receive all the PES streams it 
> wants (as long as they are available on that transponder of course) and then 
> save them however it likes?

It is not faster to do things in userspace or in the kernel. But if the
application can find out the hardware capabilites it could chose
the fastest way to do the job. Also, userpace software is easier to
debug, and bugs won't crash your machine.

> Is it already possible to just ask for a(ny) number of PES streams?

Yes (but currently broken).

> And if you are receiving say an audio and video PES stream, independantly, can 
> they be combined into a synchronized audio/video stream? (i.e. do these 
> streams have the proper timestamps or markers to re-sync them later?).

PES packets may carry a PTS (presentation time stamp) and a DTS (decoding
time stamp) in their header.

TS packets may carry a PCR (program clock reference) and other timing
information in its adaptaion field.

AFAIK the PTS is enough to correctly multiplex two PES streams together.

However, I'm not shure if it is supposed to work to cat two
PES streams into /dev/dvb/a*0/audio0 and video0 and have the
MPEG decoder produce correctly synced output...


Regards,
Johannes


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