[alsa-devel] Dummy module and sample rates
Hi Clemens,
I fiddled around with the Dummy module and it seems to be the thing we're looking for. I observed a rather strange behavior though:
I set the hw params sample rate to 44k1 without error, but a subsequent call of snd_pcm_start returns EPIPE. When setting hw params sample rate to 4k this does not happen. Did I miss anything?
Regards Uli
____ uli@ankerklause:~/.../dlna/wdmr$ modinfo snd-dummy filename: /lib/modules/3.16.0-46-generic/kernel/sound/drivers/snd-dummy.ko license: GPL description: Dummy soundcard (/dev/null) author: Jaroslav Kysela perex@perex.cz srcversion: BB2C5716F580F3D7D0F53D1 depends: snd-pcm,snd intree: Y vermagic: 3.16.0-46-generic SMP mod_unload modversions 686 signer: Magrathea: Glacier signing key sig_key: 6C:3B:5C:5E:34:A0:6D:7D:5F:7D:A3:D0:50:98:75:D9:6C:6D:E6:C4 sig_hashalgo: sha512 parm: index:Index value for dummy soundcard. (array of int) parm: id:ID string for dummy soundcard. (array of charp) parm: enable:Enable this dummy soundcard. (array of bool) parm: model:Soundcard model. (array of charp) parm: pcm_devs:PCM devices # (0-4) for dummy driver. (array of int) parm: pcm_substreams:PCM substreams # (1-128) for dummy driver. (array of int) parm: fake_buffer:Fake buffer allocations. (bool) parm: hrtimer:Use hrtimer as the timer source. (bool)
Uli Franke wrote:
I fiddled around with the Dummy module and it seems to be the thing we're looking for. I observed a rather strange behavior though:
I set the hw params sample rate to 44k1 without error, but a subsequent call of snd_pcm_start returns EPIPE. When setting hw params sample rate to 4k this does not happen.
In theory, this error code is returned if you didn't write any samples into the buffer before starting.
An underrun is possible too, but should be reported later AFAIK. Could you check if a larger buffer size helps?
Regards, Clemens
I fiddled around with the Dummy module and it seems to be the thing we're looking for. I observed a rather strange behavior though:
I set the hw params sample rate to 44k1 without error, but a subsequent call of snd_pcm_start returns EPIPE. When setting hw params sample rate to 4k this does not happen.
In theory, this error code is returned if you didn't write any samples into the buffer before starting.
An underrun is possible too, but should be reported later AFAIK. Could you check if a larger buffer size helps?
I checked if this is actually an XRun, as this is the official error code. But even recover didn't have any effect.
When I write samples before starting the stream, snd_pcm_start returns
File descriptor in bad state (-77)
which is obvious as the state of the stream changes from PREPARED to RUNNING.
I tried to increase the buffer but with no effect.
No error occurs when I just write samples and omit the snd_pcm_start. I just read some examples and got the impression that this seems to be common practice anyhow. But up to now all my ALSA interfacing programs actively started the stream and I never had any similar effect.
You can find a small test program here:
http://460c9235de50fe3b.paste.se/
Regards, Uli
Uli Franke wrote:
I set the hw params sample rate to 44k1 without error, but a subsequent call of snd_pcm_start returns EPIPE.
In theory, this error code is returned if you didn't write any samples into the buffer before starting.
T(snd_pcm_start(pcm));
n = snd_pcm_writei(pcm, buffer, ARRAY_SIZE(buffer) / channels);
And indeed this code tries to start the device without any samples in the buffer.
When setting hw params sample rate to 4k this does not happen.
This appears to be an artifact of the resampler.
When I write samples before starting the stream, snd_pcm_start returns
File descriptor in bad state (-77)
which is obvious as the state of the stream changes from PREPARED to RUNNING.
To prevent the device from starting automatically, change the start_threshold.
Regards, Clemens
I fiddled around with the Dummy module and it seems to be the thing we're looking for. I observed a rather strange behavior though:
I set the hw params sample rate to 44k1 without error, but a subsequent call of snd_pcm_start returns EPIPE. When setting hw params sample rate to 4k this does not happen.
In theory, this error code is returned if you didn't write any samples into the buffer before starting.
An underrun is possible too, but should be reported later AFAIK. Could you check if a larger buffer size helps?
I checked if this is actually an XRun, as this is the official error code. But even recover didn't have any effect.
When I write samples before starting the stream, snd_pcm_start returns
File descriptor in bad state (-77)
which is obvious as the state of the stream changes from PREPARED to RUNNING.
I tried to increase the buffer but with no effect.
No error occurs when I just write samples and omit the snd_pcm_start. I just read some examples and got the impression that this seems to be common practice anyhow. But up to now all my ALSA interfacing programs actively started the stream and I never had any similar effect.
You can find a small test program here:
Your program did not setup sw_params but start pcm stream by snd_pcm_start() without snd_pcm_write() any data
participants (3)
-
Clemens Ladisch -
Raymond Yau -
Uli Franke