Re: A suggestion for better resamplers in alsa.

[Takashi Iwai <tiwai@suse.de>]

At Thu, 7 Apr 2005 09:56:48 +0200 (CEST),
Jaroslav wrote:
> 
> On Tue, 5 Apr 2005, James Courtier-Dutton wrote:
> 
> > I have yet another possible idea. What about reporting to the
> > application whether resampling needs to be done at the rate requested by
> > the application, together with reporting the resulting hardware rate.
> > The application could then detect when it should maybe try resampling
> > itself to improve quality. We could then have resampling helper
> > functions to alsa-lib, that the application could call to do the actual
> > resampling before it send the samples to the card. In this way the
> > application will get visibility of the fixed hardware period size, and
> > make more educated descisions regarding which output rate to use.
> > This could probably be supported by the addition of a new function call
> > "snd_pcm_prevent_resampling(...)". This call would be called by the
> > application just before it calls snd_pcm_set_rate(), and this would
> > modify the behaviour of the snd_pcm_set_rate() function or just
> > implement a new function "snd_pcm_set_rate_no_resample()".
> > 
> > We would still want alsa-lib pcm layer to do the sample format
> > conversion and all the other plugins, just not the problematic resampling.
> > alsa-lib would then provide a selection of resampling functions that the
> > application could then use before snd_pcm_writei() or any other form of
> > mmap transfer.
> > 
> > For example, the application xine has a feature to force a particular
> > output sample rate, but at the moment, the user has to manually
> > configure the output sample rate if they wish it to be anything apart
> > from the sample rate of the media stream. With this new alsa function,
> > xine could be informed when to use it's own internal resamplers, and
> > when not to bother, removing the need for the user having to manually
> > configure the "force_output_sample_rate" setting. But xine would still
> > wish to send floating point samples to alsa-lib, knowing that alsa-lib
> > will convert them to 24bit or 16bit depending on the sound card.
> 
> I like this idea. This setup belongs to:
> 
> snd_pcm_hw_params_set_rate_noresample()

This feature would be nice especially also for the sound servers which
do resampling by themselves.


Takashi


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