[alsa-devel] Question about the buffer length in the transfer callback of the PCM External Plugin
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?
Kind regards
Dr. A. Bach, Software-Entwicklung Simulatoren DFS Deutsche Flugsicherung GmbH Forschungszentrum Am DFS-Campus 5 63225 Langen 06103-707-5709
DFS Deutsche Flugsicherung GmbH Am DFS-Campus D - 63225 Langen
Tel.: +49-(0)6103-707-0
Sitz der Gesellschaft: Langen/Hessen Zuständiges Registergericht: AG Offenbach am Main, HRB 34977 Vorsitzende des Aufsichtsrats: Dr. Martina Hinricher Geschäftsführer: Prof. Klaus-Dieter Scheurle (Vors.), Robert Schickling, Dr. Michael Hann
Internet: http://www.dfs.de Public-Key der DFS: http://www.dfs.de/dfs/public_key.asc
Bach, Andrea wrote:
how can I change the size that is provided inside the „transfer“- callback of an external PCM plugin?
You cannot.
ALSA assumes that there is a ring buffer that has the configured buffer size, and that when N frames are reported as available, any number of frames between 1 and N can be read/written.
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
Handling less bytes than requested would imply that an error occured.
You could add your own buffer to keep any remaining frames of the 128-frame chunk until the next transfer call.
Regards, Clemens
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 DFS Deutsche Flugsicherung GmbH Forschungszentrum Am DFS-Campus 5 63225 Langen 06103-707-5709
DFS Deutsche Flugsicherung GmbH Am DFS-Campus D - 63225 Langen
Tel.: +49-(0)6103-707-0
Sitz der Gesellschaft: Langen/Hessen Zuständiges Registergericht: AG Offenbach am Main, HRB 34977 Vorsitzende des Aufsichtsrats: Dr. Martina Hinricher Geschäftsführer: Prof. Klaus-Dieter Scheurle (Vors.), Robert Schickling, Dr. Michael Hann
Internet: http://www.dfs.de Public-Key der DFS: http://www.dfs.de/dfs/public_key.asc _______________________________________________ Alsa-devel mailing list Alsa-devel@alsa-project.org http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
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 DFS Deutsche Flugsicherung GmbH Forschungszentrum Am DFS-Campus 5 63225 Langen 06103-707-5709
DFS Deutsche Flugsicherung GmbH Am DFS-Campus D - 63225 Langen
Tel.: +49-(0)6103-707-0
Sitz der Gesellschaft: Langen/Hessen Zuständiges Registergericht: AG Offenbach am Main, HRB 34977 Vorsitzende des Aufsichtsrats: Dr. Martina Hinricher Geschäftsführer: Prof. Klaus-Dieter Scheurle (Vors.), Robert Schickling, Dr. Michael Hann
Internet: http://www.dfs.de Public-Key der DFS: http://www.dfs.de/dfs/public_key.asc _______________________________________________ Alsa-devel mailing list Alsa-devel@alsa-project.org http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
-- Fernando Rodriguez
participants (3)
-
Bach, Andrea -
Clemens Ladisch -
Fernando Rodriguez