Re: [FFmpeg-devel] [PATCH] avcodec/mlpdec: Add decoding of object audio data

[Marton Balint <cus@passwd.hu>]

On Sun, 23 Mar 2025, James Almer wrote:

> On 3/23/2025 4:33 PM, Massimo Eynard wrote:
>>  On 23/03/2025 20:01, James Almer wrote:
>>>  On 3/22/2025 2:49 PM, Massimo Eynard wrote:
>>>>  This patch adds support for decoding the fourth MLP substream
>>>>  which contains the 16-channel presentation used for Atmos
>>>>  audio objects.
>>>>
>>>>  By default only the first three substreams are decoded
>>>>  unless the new extract_objects flag is enabled as the resulting
>>>>  presentation contains audio object feeds instead of classic
>>>>  loudspeaker feeds.
>>>>
>>>>  As this introduces interpolation of primitive matrices, precision
>>>>  has been increased to 2.18 fixed point. Therefore this requires
>>>>  DSP code upgrade which has been done for C and x86 implementations
>>>>  but not the ARM implementation.
>>>>
>>>>  Adds two FATE tests using existing atmos.thd sample to reflect
>>>>  changes.
>>>>
>>>>  Signed-off-by: Massimo Eynard <eynard.massimo@gmail.com>
>>>>  ---
>>>>     libavcodec/arm/mlpdsp_armv5te.S  |   2 +-
>>>>     libavcodec/arm/mlpdsp_init_arm.c |   3 +-
>>>>     libavcodec/mlp.h                 |  10 +-
>>>>     libavcodec/mlp_parse.c           |  31 ++-
>>>>     libavcodec/mlp_parse.h           |   1 +
>>>>     libavcodec/mlp_parser.c          |  11 +-
>>>>     libavcodec/mlpdec.c              | 389
>>>>   +++++++++++++++++++++++++++----
>>>>     libavcodec/mlpdsp.c              |  50 +++-
>>>>     libavcodec/mlpdsp.h              |  25 ++
>>>>     libavcodec/x86/mlpdsp.asm        |  19 +-
>>>>     tests/fate/truehd.mak            |  10 +
>>>>     11 files changed, 476 insertions(+), 75 deletions(-)
>>>
>>>  With atmos.thd i get:
>>>
>>>>  [aist#0:0/truehd @ 00000209caf3ee00] Guessed Channel Layout: 7.1.4
>>>>  Input #0, truehd, from '../samples/truehd/atmos.thd':
>>>>     Duration: N/A, start: 0.000000, bitrate: N/A
>>>>     Stream #0:0: Audio: truehd (Dolby TrueHD + Dolby Atmos), 48000 Hz,
>>>>   7.1.4, s32 (24 bit)
>>>
>>>  Which is unlikely to be correct. The file has 11 (or 12) objects, which
>>>  is exported as 12 channels in an unspecified layout, and automatically
>>>  assumed to be a 7.1.4 fixed layout.
>>>
>>
>>  This is caused by `guess_input_channel_layout` (in `ffmpeg_demux.c`) which
>>  tries to assume a layout.

I wonder if the default value for guess_layout_max should be set to 8. An 
unspecified layout with more than 8 channels seems unlikely. (unless it is 
indeed unspecified/unknown)

>>  Would using `AV_CHANNEL_ORDER_CUSTOM` with all channels set to
>>  `AV_CHAN_UNKNOWN` (for unknown position, except LFE if present) be a
>>  better solution?
>

AV_CHANNEL_ORDER_CUSTOM with all channels being AV_CHAN_UNKNOWN is 
equvivalent to AV_CHANNEL_ORDER_UNSPEC. So unless there is a channel which 
is not AV_CHAN_UNKNOWN, there is no reason to use AV_CHANNEL_ORDER_CUSTOM.

Regards,
Marton
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