[FFmpeg-devel] [PATCH] libavcodec/mpegaudio_parser.c: differentiate MPEG audio dual mono

[Scott Theisen]

On 12/12/24 16:50, James Almer wrote:
> On 12/12/2024 3:47 AM, Anton Khirnov wrote:
>> Quoting James Almer (2024-11-30 14:41:20)
>>> with no speaker location implied
>>
>> Also, is it really the case that no speaker location is implied? I'd
>> think mono (as opposed to just "1 channel") does carry the implication
>> is is the center channel.
>
> Mono, as i understand it, implies it's a single channel with the 
> entire audio signal, opposed to simply "1 channel" which can be 
> anything, even LFE.
> Apple has separate values for FC and mono, and Quicktime files with 
> one stream per audio channel will signal the channel meant for the 
> center speaker as FC and not mono (something that affected lavf 
> parsing of such files, and forced us to do some heuristics).
>
> In practice yes, you can simplify it as mapping to FC, and in the 
> absence of said speaker, just play it on FL. But there are scenarios 
> where the distinction is needed, and we currently lack a way to 
> properly signal this.

I agree monophonic implies it is the entire audio signal.

There appears to be still room in the enum AVChannel to add AV_CHAN_MONO 
if desired (and also AV_CHAN_MONO_2 if it is desired to avoid 
AV_CHANNEL_ORDER_CUSTOM for dual mono).

Regards,

Scott Theisen