[alsa-devel] how to handle buffer underrun in a softsynth

Clemens Ladisch clemens at ladisch.de
Mon Nov 28 21:16:28 CET 2011

Henning Thielemann wrote:
> I have written a software synthesizer in Haskell that fetches MIDI events
> from ALSA sequencer and plays the rendered signal via ALSA pcm. However it
> cannot cope correctly with buffer underruns.
> ...
>   Now when I detect a buffer underrun on writing a block to an ALSA pcm
> sink, I like to know the time gap in order to adjust the timestamps of the
> incoming MIDI events.

The sample clock is usually not synchronized with any other clock, so
you won't be able to get a very exact measurement.

However, there is a different way to approach this problem: just disable
underruns.  :)  Set the stop_threshold to the same value as the boundary,
and ALSA will no longer stop the device when an underrun happens.¹  This
means that you'll have to write some data more quickly than normally to
catch up², but the overall timing of the samples will not be affected.

¹ The device will still stop on fatal errors such as an unplugged USB
² Or, if you detect this case, just use snd_pcm_forward(); the samples
  that are 'too late' won't be played anyway.


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