[pulseaudio-discuss] Recording from USB devices without channel mixing

Tanu Kaskinen tanuk at iki.fi
Tue Mar 1 14:44:50 UTC 2016

On Tue, 2016-03-01 at 14:40 +0100, Klaus Jaensch wrote:
> Am 01.03.2016 um 11:19 schrieb Tanu Kaskinen:
> > On Mon, 2016-02-29 at 17:33 +0100, Klaus Jaensch wrote:
> > > I'm wondering why it is the default to mix the channels.
> > If there's a device with channel map "front-left,front-right", and a
> > capture stream appears that has channel map "mono", do you wish that
> > pulseaudio would by default take audio only from the left channel? Why
> > would that be more likely what the user wants, rather than taking audio
> > from all channels the device has, or only from the right channel?
> Yes. If the user wants only the left channel he has no chance to do so. 
> (Without changing the configuration.)
> You can't split the mixed channels later. The only way is to record 
> stereo and split the channels later.
> If you want a mix of both channels  it is always possible to record 
> stereo and mix it later.
> Another problem with mixing is that the level meter of applications like 
> audacity show the amplitude of the mix. If you do not know that the
> level meter shows only half of the dynamic range (-6dB) if only one 
> microphone is plugged to the left channel it is likely that you overmodulate
> the recording.
> And as far as I know it is the default on Windows and Mac OS X to record 
> only the left channel. I've checked this with some of our USB audio devices.

It could be also argued that if the recording application wants to pick
just one specific channel, it should set the stream channel map
accordingly. But after pondering this for a bit, I think I agree it
would probably good to map mono recording streams only to the first
channel (usually left) of the device by default. This is, however, one
of those "patches welcome" situations - I don't think I'll have time to
implement this myself.


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