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

[Klaus Jaensch]

Am 01.03.2016 um 15:44 schrieb Tanu Kaskinen:
> 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.
I will see if I can do that. Could you point me to the code location 
where the channel mixing is done?

Klaus
>
> -- 
> Tanu

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Klaus Jaensch
Muenchen
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Institut fuer Phonetik und Sprachverarbeitung
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