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

[Klaus Jaensch]

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.

Klaus

>> I think mixing  should be done by the applications, not by the audio
>> system.
> Surely it's better use of resources to implement mixing once in the
> audio server rather than in every single application separately? Also,
> if an application wants to be in charge of mixing the audio, it can do
> that. Apparently the developer of whatever recording application you're
> using didn't bother with that, if it doesn't allow you to pick the
> device channel you want to record from.
>
> -- 
> Tanu

-- 
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Klaus Jaensch
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Institut fuer Phonetik und Sprachverarbeitung
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