[FFmpeg-devel] [PATCH 0/5 v2] 22.2 channel layout support for AAC decoding

[Jan Ekström]

On Wed, Jul 29, 2020 at 2:51 PM Lynne <dev at lynne.ee> wrote:
>
> Jul 29, 2020, 13:39 by jeebjp at gmail.com:
>
> > Now that we actually have sample(s) for channel_config 13, it was possible
> > to see at which various points the AAC decoder fails attempting to parse
> > the bit stream.
> >
> > As the feature `-request_channel_layout 9223372036854775808` (AV_CH_LAYOUT_NATIVE)
> > seems to work, the channel order has been validated both visually with
> > Audacity, as well as by logging the post-reorder element ordering:
> >
> > (what specific CPE/SCE/LFE elements mean is documented in the
> > `avcodec/aacdectab: add mapping for 22.2` commit):
> >
> > Setting layout map list after reorder...
> > tag 0 = { pos: (null) (3), syn_elem: CPE, elem_id: 1 }
> > tag 1 = { pos: FC (4), syn_elem: SCE, elem_id: 0 }
> > tag 2 = { pos: LFE (8), syn_elem: LFE, elem_id: 0 }
> > tag 3 = { pos: (null) (30), syn_elem: CPE, elem_id: 3 }
> > tag 4 = { pos: (null) (c0), syn_elem: CPE, elem_id: 0 }
> > tag 5 = { pos: BC (100), syn_elem: SCE, elem_id: 1 }
> > tag 6 = { pos: LFE2 (800000000), syn_elem: LFE, elem_id: 1 }
> > tag 7 = { pos: (null) (600), syn_elem: CPE, elem_id: 2 }
> > tag 8 = { pos: (null) (5000), syn_elem: CPE, elem_id: 4 }
> > tag 9 = { pos: TFC (2000), syn_elem: SCE, elem_id: 2 }
> > tag 10 = { pos: TC (800), syn_elem: SCE, elem_id: 3 }
> > tag 11 = { pos: (null) (28000), syn_elem: CPE, elem_id: 6 }
> > tag 12 = { pos: (null) (3000000000), syn_elem: CPE, elem_id: 5 }
> > tag 13 = { pos: TBC (10000), syn_elem: SCE, elem_id: 4 }
> > tag 14 = { pos: BFC (8000000000), syn_elem: SCE, elem_id: 5 }
> > tag 15 = { pos: (null) (14000000000), syn_elem: CPE, elem_id: 7 }
> >
> > swresample seems to give out the correct channels for 5.1 and stereo
> > downmix (checked with ffmpeg.c by utilizing `-channel_layout` 5.1 and stereo),
> > although the default downmix logic seems to lower the volume somewhat.
> >
> > Fixes #8714
> >
> > Jan Ekström (5):
> >  avutil/channel_layout: add 22.2 layout
> >  avcodec/mpeg4audio: add newer channel_coding mappings
> >  avcodec/aacdectab: add mapping for 22.2
> >  avcodec/aacdec_template: mark second LFE element as LFE2
> >  avcodec/aacdec_template: add support for 22.2 / channel_config 13
> >
> >  doc/APIchanges               |  5 ++
> >  libavcodec/aacdec_template.c | 98 ++++++++++++++++++++++++++++++++----
> >  libavcodec/aacdectab.h       | 23 ++++++++-
> >  libavcodec/mpeg4audio.c      | 17 ++++++-
> >  libavcodec/mpeg4audio.h      |  2 +-
> >  libavutil/channel_layout.c   |  6 +++
> >  libavutil/channel_layout.h   |  6 +++
> >  libavutil/version.h          |  2 +-
> >  8 files changed, 142 insertions(+), 17 deletions(-)
> >
>
> Patchset looks good to me.

Thanks for having a look.

Took quite a bit of time to gather confidence that the channel order
was correct, but given the logging of coding elements as well as
visual comparisons with Audacity I would note that the decoder is now
pushing the correct channel order as specified by the relevant
specifications.

Unless there are additional comments, I will push this a bit later in
the evening, and then proceed to seeing if I can get some FATE
samples/tests made as well as finishing up the debug logging utilized
in the cover letter as I think logging the reordered coding elements
is quite useful when debugging (although it seems to get logged quite
often with LATM AAC, so probably needs some kind of "log only when
things change" kind of thing).

Best regards,
Jan