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

[Massimo Eynard <eynard.massimo@gmail.com>]

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.
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?

> 
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