From mboxrd@z Thu Jan  1 00:00:00 1970
From: Clemens Ladisch <clemens@ladisch.de>
Subject: Re: how to handle buffer underrun in a softsynth
Date: Mon, 28 Nov 2011 21:16:28 +0100
Message-ID: <4ED3EC1C.70100@ladisch.de>
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To: Henning Thielemann <alsa@henning-thielemann.de>
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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.=B9  This
means that you'll have to write some data more quickly than normally to
catch up=B2, but the overall timing of the samples will not be affected.

=B9 The device will still stop on fatal errors such as an unplugged USB
  device.
=B2 Or, if you detect this case, just use snd_pcm_forward(); the samples
  that are 'too late' won't be played anyway.


Regards,
Clemens