Default Sample rate for Pulse Audio/Jack etc.

David Henningsson david.henningsson at
Tue Jan 8 14:46:49 UTC 2013

On 01/08/2013 03:38 PM, Len Ovens wrote:
> On Mon, January 7, 2013 11:50 pm, David Henningsson wrote:
>> 2013-01-08 00:45, Len Ovens skrev:
>>> skype where the incoming audio may be at 44.1k but the mic runs at 48k
>>> (only)? This is not uncommon BTW. I have already had to change the
>>> default
>>> on my netbook in pulse so that the built in mic would work.
>> PulseAudio would not switch to something the backend/driver/hardware
>> would not support. In the case of Jack, PA's backend only supports the
>> sample rate that Jack is configured to.
> Assuming device support.... in the case of mixing two streams, which rate
> would be chosen? The rate of the stream to start (say 32k), or the highest
> rate?

Try and see. IIRC the sample rate is never changed if a stream is 
already running, so the first stream gets to decide (in order to avoid 
clicks etc)

>> In the case of your built in mic, this sounds like a bug in the driver
>> (or hardware), if the driver says that it supports 44100 where in fact
>> it does not.
> It is a part of the HDA spec that a device (an HDA controller) may have
> codecs of varying capabilities. I don't know how well ALSA deals with hw:0
> having one input at 44.1/48 and another input at 48 only.

If there is an input that handles 48 only, without the codec telling us 
so, we would add a quirk in the driver that forces the input to 48 kHz.

> I don't know if
> the mic is even a different sub device (hw:0,1 for example), Pulse shows
> the input as selectable line/mic, but the two codecs are not the same. In
> theory a single HDA device should be able to support a phone conversation
> at 32 or 24 k while at the same time streaming 5.1 audio at 48k. Different
> codecs but same HDA controller. This from having read the Intel HDA
> spec... I don't know where the bug should be considered in this case... or
> if it is just something ALSA doesn't support (yet? ever?)

As for multiple streams in parallel, that's something PulseAudio does 
not attempt to detect, but you have to set PA up for this manually.

You can read up at 
and the directory /usr/share/pulseaudio/alsa-mixer/ for how PulseAudio 
selects how and what of ALSA it tries to detect.

David Henningsson, Canonical Ltd.

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