[vdr] Re: Softdevice + EPIA-M / Sound resampling

[Holger Waechtler <holger@convergence.de>]

Henrik Johansson wrote:
> Colin Paton wrote:
>
> > Hi,
> >
> > I have glued some CLE266 hardware mpeg2 decoding code into softdevice. 
> > I've
> > now got this working with my Nehemiah M10000 and CVS DirectFB. Video is
> > working well - smooth pictures on the display.
> >  
> Great news.
>
> > However, the sound is still a bit iffy and AV sync isn't perfect. I 
> > suspect
> > that the sound card (I'm using ALSA) isn't quite playing back at the same
> > rate as the video.
> >
> > I wait for the vertical blanking pulse before flipping the video
> > framebuffer. This works well - get smooth pictures, but causes 
> > problems with
> > the sound.
> >
> > I would therefore like to change the following:
> >
> > - Make video the primary sync source, and match audio to video.
> > - Resample the audio in order to alter the frame rate slightly if 
> > necessary.
> > This should hopefully avoid the gaps and clicks that I've been 
> > experiencing.
> >
> > I think that xine resamples the audio so I'll have a look at that.
> >
> > I have two questions:
> >
> > - Is audio resampling really necessary? I'm not an MPEG A/V sync expert -
> > perhaps someone more qualified could point me to a source of information?
> > - Is anyone already doing this? I don't want to reinvent any wheels!
> >
> >  
> I'm not an MPEG A/V sync expert either but isn't the problem even worse. 
> When you're waiting for the vertical blanking pulses which are not in 
> perfectly sync with the DVB stream clock reference (PCR) you will 
> eventually get dropped frames. To do this right (which probably is not 
> possible) I think you need to sync both the frame rate of the video mode 
> and the audio clock to the PCR. The latter should be possible by 
> resampling the audio stream before sending it to the soundcard as you 
> suggest. For the video I think it could be done by implementing a PLL in 
> software which measures the VBL frequency and compares this with the PCR 
> clock and use the correction signals to program the videomode timing 
> parameters.
You are correct. The VideoLAN people have a pretty good introduction 
into this topic in their development introduction paper: 
http://developers.videolan.org/vls/vls/doc/developer/vls-devel.pdf (page 
12), the referenced code is accessible in their CVS: 
http://cnedra.via.ecp.fr/cgi-bin/cvsweb/vlc/src/input/input_clock.c?rev=1.45&content-type=text/x-cvsweb-markup

To resolve the sync issue cleanly you need to do something similiary in 
the softdevice plugin.

Holger



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