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

[James Almer <jamrial@gmail.com>]

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On 3/23/2025 4:01 PM, 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.

Actually, it may be ok to let the guess code select that layout. I see 
Apple defines kAudioChannelLayoutTag_Atmos_7_1_4 for 12 channel Atmos 
presentations.


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