[alsa-devel] SND_PCM_STREAM_CAPTURE and SND_PCM_NONBLOCK producing strange artifacts

[Trent Reed]

(Ping)

I'm afraid I still don't understand why this is failing. I found that a
call to sdn_pcm_wait() when -EAGAIN is returned fixes the problem because
it waits for samples to be ready, but I don't understand why it is a
problem in the first place. Much of the sample code is written in a way
that suggests that the call to snd_pcm_wait() is optional, and only
required when directly reading/writing from the device (MMAP, I'm not doing
this).

I'm just wondering what is wrong with basically looping over a call to
snd_pcm_readi() in non-blocking mode, reacting to errors, and capturing
frames. This produces awful static (which is basically garbage samples), is
it a requirement that I call snd_pcm_wait() when -EAGAIN is passed back to
wait for a full period to be ready?

Thanks,
- Trent Reed


On Tue, Jun 28, 2016 at 7:27 PM, Trent Reed <treed0803 at gmail.com> wrote:

> Hey All,
>
> I've been banging my head against the wall for about a week on this bug.
> The following gist shows my sample reproduction code:
> https://gist.github.com/TReed0803/985c5d5c3d295245e19006a27be447c3
>
> I'm simply opening up a non-blocking PCM capture stream and writing the
> contents of the reads to stdout.
> (Originally, I was writing to the playback stream, but I was hearing this
> strange static occasionally!)
>
> It's the static I'm trying to debug. It doesn't happen every time. In
> fact, sometimes I'll go a few consecutive executions without hearing it.
> I was able to capture some of the bad data, and I loaded it up in Audacity
> for visualization:
>
> https://drive.google.com/file/d/0B-1aumGKQcQTcUJoZzIwRWhYSFE/view?usp=sharing
>
> It looks like the internal buffer occasionally is sending me more data
> than it actually captured, and I end up either reading old PCM data from
> the internal ring buffer, or (at the very beginning) a bunch of zeros.
>
> Can anyone help me understand what is going on? What could cause these
> definitely incorrect samples to be recorded? (I get the same effect
> regardless of hardware, but I will list hardware just in case.)
>
> I hope I have all the information you might need:
> Hardware: Samson Meteor Mic (USB-Audio, USB Mixer) [Though, it even
> happens with my built-in microphone]
> ALSA version: Advanced Linux Sound Architecture Driver Version
> k4.4.0-28-generic.
> apt-cache policy (installed alsa-lib version): 1.1.0-0ubuntu1
>
> Thanks,
> - Trent Reed
>