[FFmpeg-devel] [PATCH 5/5] avformat/mxfdec: rework MCA channel layout parsing

[Pierre-Anthony Lemieux]

On Mon, Nov 29, 2021 at 11:20 AM Marton Balint <cus at passwd.hu> wrote:
>
>
>
> On Sun, 28 Nov 2021, Pierre-Anthony Lemieux wrote:
>
> > On Sun, Nov 28, 2021 at 5:00 PM Marton Balint <cus at passwd.hu> wrote:
> >>
> >> Setting the channel layout was based on SoundfieldGroupLabelSubDescriptor, but
> >> this was wrong, because soundfield groups are not necessarily used and a file
> >> can also contain e.g. a single channel of a multichannel audio.
> >
> > For reference, below are MCA scenarios I am aware of:
> >
> > - single MXF file containing a single soundfield and all its
> > constituent channels (specified in the Interoperable Master Format -
> > IMF)
> > - single MXF file containing a single soundfield and all its
> > constituent channels, plus additional auxiliary channels, e.g.
> > visually-impaired narration (specified in Digital cinema)
> > - N MXF files, each containing one channel of a single soundfield
> > (specified in IMF, but I have personally never seen it in practice)
> >
> >>
> >> The new approach is to parse every AudioChannelLabelSubDescriptor and create a
> >> channel layout based on those. We should also make sure that we only provide a
> >> channel layout if every channel designation is recognized and is supported. If
> >> a designation is not supported, or if a channel designation is used more than
> >> once (e.g. multiple languages) we fall back to the unknown channel layout.
> >>
> >> MCA channel ID was also ignored previously, that should have been used to
> >> determine which AudioChannelLabelSubDescriptor references which audio channel.
> >> This is also fixed now.
> >>
> >> The new parsing code also detects ambigous language or service types, and only
> >> set these metadata if all channels have the same value.
> >>
> >>
> >> Soundfield groups are now only used for determinig the language of the tracks,
> >> if it is not provided in the AudioChannelLabelSubDescriptor. Track sub
> >> descriptors not necessarily contain the references to their soundfield groups,
> >
> > ST 377-4 states that "If the Audio Channel is part of a Soundfield
> > Group then the SoundfieldGroupLinkID shall be present in the
> > AudioChannelLabelSubDescriptor Set. The SoundfieldGroupLinkID shall be
> > the MCA Link ID of the SoundfieldGroupLabelSubDescriptor to which this
> > Audio Channel belongs."
> >
> > What do you mean by "Track sub descriptors not necessarily contain the
> > references to their soundfield groups,"?
>
> I mean that the SubDescriptors item of the WavePCMDescriptor not
> necessarily contains the references to the soundfield group. E.g. if a
> soundfield group consist of two channels, one channel is in audio track 1,
> another channel is in audio track 2. As far as I understand only one
> WavePCMDescriptor can reference the SoundfieldGroup in its SubDescriptors,
> because it is a strong reference.

As I recall, the intent is for all MCA subdescriptors relevant to a
track to be contained within the File Descriptor for that track, as
suggested by Figures 1 and A.2 of the MCA spec (see attached).

For example, if the L channel of a stereo soundfield is in track 1 and
the R channel of a stereo soundfield is in track 2, then the File
Descriptor for track 1 would contain one L MCA Audio Channel
SubDescriptor and one Stereo MCA Soundfield SubDescriptor, while the
the File Descriptor for track 2 would contain one R MCA Audio Channel
SubDescriptor and one Stereo MCA Soundfield SubDescriptor. The two MCA
Soundfield SubDescriptors would have identical MCA Link ID.

The MCA spec states "all MCALabelSubDescriptor instances associated
with the audio described by a File Package shall be contained within
the same File Package". This could be construed as allowing a MCA
Soundfield SubDescriptor in one child descriptor of a
MultipleDescriptor to be referenced by a MCA Audio Channel
SubDescriptor from another child descriptor of the same
MultipleDescriptor. Happy to check with other implementers and report.
In any case, this is somewhat academic at this point unless anyone is
aware of MCA-enabled MXF files that are not single track and/or that
have soundfields split across File Packages. [ed.: I am not aware of
such files.]

It might therefore make sense to focus, for now, on single track MXF
files that contain complete soundfields, which your patch addresses if
I am not mistaken.

Happy to generate sample files. Just let me know.

>
> Obviosly the soundfield group can be found by iterating over all the items
> and searching for the link ID, but not by iterating over the UID
> references of the SubDescriptors of the WavePCMDescriptor.
>
> Regards,
> Marton
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