[FFmpeg-devel] Set channel_layout in mpegaudio

[Justin Ruggles]

On 02/10/2011 01:10 PM, Ronald S. Bultje wrote:

> Hi,
> 
> On Mon, Feb 7, 2011 at 6:10 PM, Justin Ruggles <justin.ruggles at gmail.com> wrote:
>> On 02/07/2011 06:01 PM, Baptiste Coudurier wrote:
>>> On 02/05/2011 10:01 AM, Kieran Kunhya wrote:
>>>> $subj
>>>>
>>>> 0001-Set-channel_layout-for-mpegaudio.patch
>>>>
>>>>  From 112c79fe16957a6badc20eb69cdfdb0b5eed270b Mon Sep 17 00:00:00 2001
>>>> From: Kieran Kunhya<kieran at kunhya.com>
>>>> Date: Sat, 5 Feb 2011 17:59:48 +0000
>>>> Subject: [PATCH] Set channel_layout for mpegaudio
>>>>
>>>> ---
>>>>   libavcodec/mpegaudiodec.c |    1 +
>>>>   1 files changed, 1 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/libavcodec/mpegaudiodec.c b/libavcodec/mpegaudiodec.c
>>>> index 0cd0b68..6fa3d85 100644
>>>> --- a/libavcodec/mpegaudiodec.c
>>>> +++ b/libavcodec/mpegaudiodec.c
>>>> @@ -2054,6 +2054,7 @@ static int decode_frame(AVCodecContext * avctx,
>>>>       }
>>>>       /* update codec info */
>>>>       avctx->channels = s->nb_channels;
>>>> +    avctx->channel_layout = s->nb_channels == 1 ? AV_CH_LAYOUT_MONO : AV_CH_LAYOUT_STEREO;
>>>
>>> Do we have dual mono layout as well ? mpegaudio can specify that as well
>>> IIRC, to carry CENTER+LFE as a pair for example.
>>
>>
>> We do not have "dual mono" per-se but we can do:
>> channel_layout = AV_CH_FRONT_CENTER | AV_CH_LOW_FREQUENCY
> 
> How do I detect that in mp3? Grep shows no hit for any such a term, so
> I don't really know when to set it.
> 
> I guess I could look it up in the spec.


I think it's the mode field in the header.

'00' stereo
'01' joint_stereo (intensity_stereo and/or ms_stereo)
'10' dual_channel
'11' single_channel

But there is nothing I can find that says what the dual channels are
other than 2 mono channels put together in 1 file.  Like in AC-3 it
appears to just mean that the 2 channels are not meant to be mixed as
left and right.

Maybe some containers use this and have some format-specific way of
saying what the 2 channels are?

-Justin