Hello list,

I'm working on an application that requires a period time of 10ms.  The hardware at the other end of the bus is a ulaw codec running at 8kHz.  The application runs at 16kHz S16_LE and I was hoping to use the plughw device to do sample rate conversion and ulaw encode/decode.  This way the application doesn't need to change to accommodate this ulaw codec.  However, it's not working and I don't know how to go about debugging.  I'm hoping I can get some pointers here.

The problem is best illustrated by this output, first using plughw: alsa_read: 10 alsa_read: 20 alsa_read: 0 alsa_read: 20 alsa_read: 0 alsa_read: 20 alsa_read: 0 alsa_read: 20 alsa_read: 0 alsa_read: 20

It would appear that plughw gives the correct period time on the first call, but then subsequently buffers 2 periods before returning.   Using hw with matching sample rate, sample format, and number of samples: alsa_read: 10 alsa_read: 10 alsa_read: 10 alsa_read: 10 alsa_read: 10 alsa_r ead: 10 alsa_read: 10 alsa_read: 10 alsa_read: 10 alsa_read: 10

This code snippet generated the above output:

for (i = 0; i < 10; ++i) {         gettimeofday(&time1, NULL); err = snd_pcm_readi (capture_handle, buf, NSMP);          if (err != NSMP) {                 printf ("read from audio interface failed "                         " (%s)\n", snd_strerror (err));                 exit (1);         }         gettimeofday(&time2, NULL);         printf("alsa_read: %d\n", getTimeDiff(time1, time2)); }

I have tried using the patch from here (without the environment variable switch):

http://mailman.alsa-project.org/pipermail/alsa-devel/2008-October/01170 3.html

to print out the configuration of plughw's plugin chain, but it generates no output.  I've verified the md5sum of the library after compilation and it matches the one I compiled.  And I ran ldconfig after replacing libasound.so.  So I don't know what's going on there.  I definitely call snd_pcm_hw_params() when setting up the capture device.  This is using alsa-lib-1.0.25.

Does anyone know why plughw might be acting the way it is and how I can get it to deliver 10ms periods?  Equally appreciated would be any tips on how to debug this problem.

Thanks, Stef