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

[Henrik Johansson <henjo@ieee.org>]

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.

What do you think?

Regards
Henrik Johansson




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