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

[David Henningsson]

On 2016-03-01 15:44, Tanu Kaskinen wrote:
> 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.

But - now every mono recording of, e g, line in, will only record the 
left channel instead of both.

Also, what about dual-channel internal laptop microphones with high 
background noise? By not mixing the channels, won't your S/N ratio 
decrease by 6 dB?

This sounds like fixing one problem but causing other problems.

-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic