[alsa-devel] Conversion to int16_t and resolution loss with rate converter plugins

[Pavel Hofman]

Dne 5.4.2018 v 05:05 Flavio Protasio Ribeiro napsal(a):
> Hi Folks,
> 
> 
> 
> The libsamplerate and speex rate converter plugins only implement the
> convert_s16 callback from snd_pcm_rate_ops_t. This has ALSA convert
> the buffer to int16_t before handing it over to the plugin for
> resampling. On devices with more than 16 effective bits, this implies
> words gets truncated and bits are lost.
> 
> 
> 
> Specifically for libsamplerate, the resampling is internally
> implemented with floats, so truncating to 16 bits does not offer a
> computational benefit.
> 
> 
> 
> I get why only convert_s16 is implemented for plugins that aren't
> part of the main alsa-lib package. The more generic convert callback
> comes with the burden having the plugin support conversion from/to
> any of the ALSA data types (or alternatively, having the plugin
> source include plugin_ops.h and its dependencies). So here's what I
> propose: replace convert_s16 with a convert_s32 callback which uses
> int32_t instead of int16_t. This would preserve all bits from the
> device and keep the plugin interface simple. Of course this means
> plugins have to be updated to use convert_s32 and rebuilt.
> 
> 

Hi,

Many years ago I tried to extend the rate API to float, the native 
formats of libsamplerate and speex.

I go stuck on float support of all platforms, perhaps you can get 
something from the discussion and patches.

http://mailman.alsa-project.org/pipermail/alsa-devel/2011-June/041059.html

http://mailman.alsa-project.org/pipermail/alsa-devel/2011-July/041503.html

If we succeed in adding >16 bit API to the rate plugin, perhaps 
supporting libsoxr would be the next step which would bring great value 
(performance vs. CPU load) to alsa-lib. Unfortunately that is not just 
about conversion, there are other quirks to iron out...

Anyway, thanks a lot and good luck to your very useful endeavour.

Best regards,

Pavel.

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