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

[Fernando Rodriguez]

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.

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