Re: [FFmpeg-devel] [PATCH] avformat/mxfdec: support MCA audio information

[Pierre-Anthony Lemieux <pal@sandflow.com>]

On Sat, Jan 1, 2022 at 5:11 PM Marton Balint <cus@passwd.hu> wrote:
>
>
>
> On Sat, 1 Jan 2022, Pierre-Anthony Lemieux wrote:
>
> > Minor suggestion below.
> >
> > In addition, will sample file(s) be added to FATE? Below are two examples:
> >
> > http://ffmpeg-imf-samples-public.s3.us-west-1.amazonaws.com/callout_51_l_r_c_lfe_ls_rs.mxf
> > http://ffmpeg-imf-samples-public.s3.us-west-1.amazonaws.com/callout_51_l_c_r_ls_rs_lfe.mxf
>
> Good idea, can someone with access please upload these to the fate mxf
> folder?
>
> [...]
>
> > I suggest adding the following audio channels, which are specified in
> > SMPTE ST 2067-8 [1] and have a direct mapping to ffmpeg audio
> > channels:AV_CH_STEREO_LEFT
> >
> > urn:smpte:ul:060e2b34.0401010d.03020120.03000000 (Left Total) ->
> > AV_CH_STEREO_LEFT
> > urn:smpte:ul:060e2b34.0401010d.03020120.04000000 (RightTotal) ->
> > AV_CH_STEREO_RIGHT
> >
> > [1] https://registry.smpte-ra.org/view/published/labels_view.html
>
> Unfortunately this is a Dolby encoded stereo mix, not an ordinary LeftOnly
> / RightOnly mix. So I am not how this should be signalled.

My interpretation below.

In "mov_chan.c", AV_CH_LAYOUT_STEREO_DOWNMIX means
MOV_CH_LAYOUT_MATRIXSTEREO [1], which is defined in Core Audio [2].

LtRt, i.e. matrix-encoded audio, is not generally Dolby-specific [3].

[1] https://github.com/FFmpeg/FFmpeg/blob/d6b2357eddca392ee137cb2a92ff178a0a7d0cce/libavformat/mov_chan.c#L168
[2] https://developer.apple.com/documentation/coreaudiotypes/1572101-audio_channel_layout_tags/kaudiochannellayouttag_matrixstereo?changes=_8_7&language=objc
[3] https://en.wikipedia.org/wiki/Matrix_decoder

>
> Regards,
> Marton
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