Re: Sample Rate Conversion Quality in ALSA

[James Courtier-Dutton <James@superbug.co.uk>]

Marc Brooker wrote:
> Hello,
>
> I am new to the ALSA development, and would be interested in
> contributing in future.
>
> After installing the latest Ubuntu on my home machine, I noticed that
> dmix is enabled by default. This is a very good move in my opinion, as
> it makes Linux sound much more transparent to the end user. However, I
> was concerned with the quality of the mixing algorithm and sample rate
> conversion used by dmix.
>
>
> Cheers
>
> Marc Brooker
>   
The problem is actually far more complicated than you described. A fair 
amount of ALSA core will have to be modified to really fix the problem.
The main problem is the buffer and period sized as they pass through dmix.
For example, if the sound card hardware is running with 1024 samples per 
period at 48000, and an application wished to use a sample rate of 
44100, the application should really get 940.8 samples per period. That 
is nor possible, so the application gets 940. dmix then tries to convert 
940 samples to 1024 samples ready for the hardware. So, even if the dmix 
algorithm used the super high quality sample rate conversion function, 
the actual rate change applied would still be slightly off.

James


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