[FFmpeg-devel] [PATCH 1/2] avcodec/s302m: enable non-PCM decoding

[Gyan Doshi]

On 2024-02-15 04:17 pm, Anton Khirnov wrote:
> Hi,
> sorry for the delay, I've been busy fixing things for the release
> Quoting Gyan Doshi via ffmpeg-devel (2024-01-29 05:00:33)
>> On 2024-01-28 04:24 pm, Anton Khirnov wrote:
>>>> a) it would mean essentially inlining this decoder in the demuxer.
>>> Why is that a problem? This decoder seems like it shouldn't be a
>>> decoder.
>>>
>>> I agree with Andreas that this seems like it's a demuxer pretending to
>>> be a decoder.
>> This module transforms the entire raw payload data to generate its
>> output, even if the syntax is simple which
>> essentially makes it a de-coder. The de-multiplexer aspect of multiple
>> streams is an academic possibility allowed
>> by the standard but not seen in any sample which makes me suspect it's
>> used for carriage between broadcast
>> facilities rather than something ever sent to an OTT provider, let alone
>> an end user.
> If it dynamically generates nested decoders, then it's not a proper
> codec in our model. It should be either a part of the demuxer, or a
> bitstream filter (possibly inserted automatically by the demuxer).

s302m is a hybrid creature and does not slot cleanly into any role. So 
there is no theoretically proper place for this component - any choice 
is a least-out-of-place accommodation.

But it is much more out of place inside a demuxer. Analyzing packet 
payload and then manipulating that entire payload is much closer to a 
decoding role than data chunk extraction for packetization. And the 
stream extracted from the container is meant to be SMPTE ST 302 not PCM* 
or Dolby-E/AC-3..etc, which will both misrepresent what the container 
carries
and possibly discard S-ADM metadata, if present, in the packet. With 
passthrough demuxing, a stream can be mapped for both decoding and 
streamcopying.

A bsf in principle would work but in practice, can't as Andreas 
clarified that bsfs can't set or alter codec_id after init. And 
resetting the codec id requires packet inspection.

Nested decoders are used without issue in components like imm5 or ftr 
(upto 64 nested decoders!) among others. There's no breaking of new 
ground here.

If you feel strongly about this, please refer this to the TC.

Regards,
Gyan