[alsa-devel] [Portaudio] Recording devices opened mono still sending stereo causing corrupt audio

[RJ Ryan]

bcc: alsa-devel

Excellent -- glad to see it's as simple as a fix to the ALSA hostapi! Is
anyone from the PA side interested in picking this up or should we work on
a patch? We'd like to fix this for good in our next major release (early
2014?) so it would be good to try and get this into a PA release soon so we
can get it packaged in the various distros.

Best regards,
RJ


On Fri, Oct 11, 2013 at 9:05 PM, Ross Bencina <rossb-lists at audiomulch.com>wrote:

> On 11/10/2013 11:33 PM, Clemens Ladisch wrote:
>
>> Sean M. Pappalardo - D.J. Pegasus wrote:
>>
>>> the problem occurs when PA is asked to open just one channel of
>>> a sound input device yet the device seems to stream two anyway.
>>>
>>
>> Many devices do not support mono streams.
>>
>>  I'm suspecting there's a miscommunication between PA and ALSA
>>>
>>
>> The communication between ALSA and PA works just fine; ALSA reports the
>> minimum number of channels, and PA saves this value.
>>
>> What does not work is the communication between PA and the application;
>> PA has no mechanism to report the minimum number of channels.
>>
>
> Since PortAudio is designed to provide useful features to the client, it
> does not make sense to restrict the client from opening a stereo device as
> mono.
>
> This is by design.
>
> However obviously there is an implementation issue in PA/ALSA if it does
> not respect ALSA's interface requiring support for a minimum number of
> channels > 1.
>
> In this case PortAudio should either select the first (left) channel  or
> downmix to mono before passing to the client. I'd suggest the former. (or
> in general min(userRequestedChannelCount,**availableChannelCount)
>
>
>
>  Even worse, if some application tries to use a channel count that is too
>> low, PA cleverly replaces it with the minimum support channel count:
>>
>>    self->numHostChannels = PA_MAX( params->channelCount,
>> StreamDirection_In == streamDir ? devInfo->minInputChannels
>>            : devInfo->minOutputChannels );
>>
>> and then does not bother to tell anybody about this.
>>
>
> The implementation should be dropping the second channel. So it should be
> telling the PA buffer processor to do so.
>
>
>
>  This bug is especially egregious because PA does later check whether the
>> number of channels is actually supported by the device; the code above
>> ensures that this checks uses the wrong channel number.
>>
>
> Thanks for your input,
>
> Ross.
>
>
>
>
>> Regards,
>> Clemens
>>
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