[alsa-devel] Sound streaming application: which device to use?

Lex Wassenberg lex.wassenberg at gmail.com
Thu Jun 4 13:28:53 CEST 2009

Hi all,

I'm completely new to ALSA, so forgive me if these are newby questions.

I'm working on an application which receives sound chunks via an IP
connection, and it should use the local sound card to make this sound
audible. Up to now it used OSS, but there were some problems with
that, so I'm busy with rewriting the application to ALSA. The sound is
16-bit stereo, sampled at 8000 Hz, and it comes in chunks of 1024
frames. I managed to get things more or less working, but there are
still some things unclear to me.

1. If I open the soundcard with device "default" or device "plughw" I
get good sound, but the device "default" only accepts 1020 frames of
every 1024 that I offer it (as can be seen from the return value of
snd_pcm_writei() ). The device "plughw" nicely accepts all 1024
frames. Why is this? In both cases I have set the sample rate to 8000
via a call to snd_pcm_hw_params_set_rate_near() and the buffer size to
2048 frames via snd_pcm_hw_params_set_buffer_size_near().

2. If I use device "default", there seems to be no obligation to set
the buffer size nor the period size, I still get good sound at the
output. If I use "plughw", I have to set either the buffer size (2048
frames) or the period size (1024 frames), otherwise I get distorted
sound. If I set both (the buffer size being 2048 and the period size
being 1024) I also get distorted sound. Why?

3. If I set the buffer size twice the period size (as above), and ask
the sizes back, ALSA nicely reports the buffer size as being twice the
period size. But if I ask the buffer and period TIMES, they are
reported as being the same: 128000 us!  Huh????

4. The return value of snd_pcm_hw_params_set_rate_near() is sometimes
8000, sometimes 7999, from the very same program. I would expect ALSA
to always return the same value, be it what I request or not, but at
least being deterministic. How can this be?

5. There is another application running on the same target system,
which will sometimes produce audible alarms. However, if my
application is running that other application cannot generate the
alarms, it produces a "resource busy" error. It is clear to me that my
application is using the sound card exclusively, but how can I get it
to allow other applications access to it too? I tried using the device
"dmix", but then my sound is again very distorted.

Hope that somebody can help me out.
Thanks in advance.


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