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 6:47 PM, Hendrik Leppkes wrote:
> On Sun, Mar 23, 2025 at 9:35 PM James Almer <jamrial@gmail.com> 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.
>>> 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?
>>
>> Possibly, but it may make the stream undecodable unless you remap the
>> channels (probably with a filter in the filterchain).
>>
>> Is there no better representation for the output? What are these 12
>> channels the sample exports? 16 channels (as you say the MLP substream
>> contains) would match Ambisonics 3rd order, but i assume that doesn't
>> apply here, unless you should also be outputting something else.
>>
> 
> Its object-based audio. Every extra "channel" represents an audio
> object at any arbitrary position in space, as defined by separate
> metadata, which you are then supposed to mix together for your final
> speaker configuration.
> Typically, the "bed" channels (eg. the base 7.1) will contain audio
> that doesn't require much localization information, music, background
> noises, and the objects will contain audio which is more relevant to
> have full spatial localization. A mixer is then tasked based on the
> spatial metadata and knowledge of the physical speaker configuration
> to mix the objects for ideal spatial representation.
> 
> We don't have a channel layout that would identify this sort of setup
> as of yet, nevermind a mixer that could actually deal with it, or even
> exporting the metadata from the TrueHD stream, but baby steps I
> suppose.

So we'd need a new layout (or pseudo-channel) where you set arbitrary 
coordinates? Sort of like what Apple defined in 
https://developer.apple.com/documentation/coreaudiotypes/audio-channel-coordinates

> 
> FWIW, taking all this into account, I fully agree that it should by
> default output the 7.1 representation that everyone can actually
> process, because the bed+objects representation is rather unexpected
> and unhandleable at this time.
Agree.


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