[vlc-devel] [PATCH] aout: Add a 'mono' option to the stereo-mode parameter

[Martin Storsjö]

On Wed, 10 Jul 2013, Rémi Denis-Courmont wrote:

> On Tue,  9 Jul 2013 16:42:04 +0300, Martin Storsjö <martin at martin.st>
> wrote:
>> This parameter value needs to be dealt with before starting the
>> actual aout, which is why the aout->Start call is moved down
>> to after the stereo-mode parameter is created.
>> 
>> ---
>> Tested to work properly with the android_audiotrack output
>> (with a patch that adds a gui preference for enabling this mode).
>> 
>> Also tested on linux with the qt4 gui, where the new option appeared
>> properly in the gui and seemed to work as intended.
>> 
>> (However, the stereo mode submenu appeared grayed out when playing an
>> 8-channel audio file, while it appeared normally and worked fine when
>> playing a video with 2 audio channels.)
>> ---
>>  include/vlc_aout.h        |    1 +
>>  src/audio_output/output.c |   19 +++++++++++++------
>>  src/libvlc-module.c       |    6 ++++--
>>  3 files changed, 18 insertions(+), 8 deletions(-)
>> 
>> diff --git a/include/vlc_aout.h b/include/vlc_aout.h
>> index 49c0ec6..62a1e83 100644
>> --- a/include/vlc_aout.h
>> +++ b/include/vlc_aout.h
>> @@ -83,6 +83,7 @@
>>  #define AOUT_VAR_CHAN_LEFT          3
>>  #define AOUT_VAR_CHAN_RIGHT         4
>>  #define AOUT_VAR_CHAN_DOLBYS        5
>> +#define AOUT_VAR_CHAN_MONO          6
>> 
>> 
> /*****************************************************************************
>>   * Main audio output structures
>> diff --git a/src/audio_output/output.c b/src/audio_output/output.c
>> index 988d200..d0e5afa 100644
>> --- a/src/audio_output/output.c
>> +++ b/src/audio_output/output.c
>> @@ -362,12 +362,6 @@ int aout_OutputNew (audio_output_t *aout,
>> audio_sample_format_t *restrict fmt)
>>          aout_FormatPrepare (fmt);
>>      }
>> 
>> -    if (aout->start (aout, fmt))
>> -    {
>> -        msg_Err (aout, "module not functional");
>> -        return -1;
>> -    }
>> -
>>      if (!var_Type (aout, "stereo-mode"))
>>      {
>>          var_Create (aout, "stereo-mode",
>> @@ -378,6 +372,15 @@ int aout_OutputNew (audio_output_t *aout,
>> audio_sample_format_t *restrict fmt)
>>          var_Change (aout, "stereo-mode", VLC_VAR_SETTEXT, &txt, NULL);
>>      }
>> 
>> +    if (var_GetInteger (aout, "stereo-mode") == AOUT_VAR_CHAN_MONO)
>> +        fmt->i_original_channels = fmt->i_physical_channels =
>> AOUT_CHAN_CENTER;
>
> This won't work if the audio output insists on stereo (e.g. ALSA). It will
> only work if the audio output accepts mono and the back-end transparently
> upmixes back to stereo (e.g. PulseAudio).

That's correct, I noticed it during testing as well. Of the 3 current 
android aouts, only one accepted mono, the other ones forced stereo, 
losing all the downmixing.

Any better suggestion on where/how to handle it, to make it work 
regardless of the aout implementation?

// Martin