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

Henning Thielemann alsa at henning-thielemann.de
Tue Nov 29 09:39:08 CET 2011

On Mon, 28 Nov 2011, Clemens Ladisch wrote:

> 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.

That's something I worry about, too. However I must somehow map timestamps 
of ALSA sequencer events to sample counts in order to let notes start 
within a sample block. JACK claims to synchronize everything and it runs 
on top of ALSA - how does it manage synchronization between MIDI and PCM?

> 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.

Ok, I am still uncertain whether this solves my problem. I think about how 
I can decrease processor load if I detect a buffer underrun. My idea was 
the following: Say, I have a blocksize of 10ms and got some events A, B, 
C, ... from the ALSA sequencer:

  1ms: A, 7ms: B, 13ms: C, 19ms: D, 25ms: E, 31ms: F

Now, when I encounter a buffer underrun, I want instead to render 
according to:

  1ms: A, 7ms: B, 10ms: C, 10ms: D, 15ms: E, 21ms: F

and play a 10ms pause at timestamp 10ms. That is I do not want to drop 
events (this might introduce inconsistencies) but I could save rendering a 
block and this way catch up with the emission of sample blocks.
  How can I detect a buffer underrun when I disabled underrun detection?


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