[alsa-devel] Question about the buffer length in the transfer callback of the PCM External Plugin

[Fernando Rodriguez]

On Mon, Oct 31, 2016 at 02:11:17PM -0400, Fernando Rodriguez wrote:
> On Mon, Oct 31, 2016 at 09:01:02AM +0000, Bach, Andrea wrote:
> > Hello,
> > 
> > how can I change the size that is provided inside the „transfer“-callback of an external PCM plugin?
> > 
> > Background:
> > I need to write a filter plugin and I have to use for processing a chunk size of power of two (32, 64, or 128 bytes … that’s because of the use of FFT). But I get inside the transfer callback a buffer of length 1000. That would be ok if I could just return let's say  960 bytes – but after some struggling I have some doubt about it because I got the impression that ALSA cannot change the bitrate inside such a plugin – the documentation states it could not change the sample rate – but actually how can I return less or more bytes – so maybe the bitrate should also be constant? Maybe I misunderstood the concepts of ALSA completely?
> > 
> 
> Yes.
> You're working with raw PCM data, so the bitrate (bits per second) is
> constant ((bit_depth/8) * samplerate * channels). The term usually
> refers to compressed audio streams.

Oops, that was in bytes. It's bitdepth * samplerate * channels.

> 
> I haven't done this but by looking at the definition for the transfer
> callback it looks pretty standard. It returns snd_pcm_sframes_t so I
> suppose you can write less than *size* frames and return what was
> actually written. It is signed so a negative value would indicate an
> error. So you'll probably have to buffer any leftovers and write them on
> the next call. That's not the same as changing the framerate because
> at the end of the pipeline alsa will take care of playing it at the
> right rate).
> 
> That should clear some of the confussion. You'll probably have to look
> at alsa-lib or some plugin's source code for more details.
> 
> 
> > Kind regards
> > 
> > Dr. A. Bach, Software-Entwicklung Simulatoren
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> 
> -- 
> Fernando Rodriguez

-- 
Fernando Rodriguez