[FFmpeg-devel] FFmpeg TC input needed

[Gyan Doshi]

Issue:

Patch: avcodec/s302m: enable non-PCM decoding
URL: 
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20240127103854.9971-1-ffmpeg@gyani.pro/

The issue needing resolution is whether the patch should be added to the 
existing s302m decoder or should that decoder
be removed and all old and new patched features inlined into the mpeg-ts 
demuxer.

Summary:

SMPTE ST 302 specifies for carriage of LPCM media in MPEG-TS. SMPTE ST 
337 enables this for non-PCM codecs. The payload
has a custom layout so it can't be directly processed hence lavc/s302m 
decodes the packet data to yield LPCM media. But
it can only deal at present with LPCM payloads, meaning that non-PCM 
payloads need to be exported to a raw bytestream
format and then decoded in a 2nd step, which prohibits direct 
transcoding of live/streaming inputs. This patch corrects
the identification process for non-PCM payloads, reformats the payload 
and then carries out in-place decoding by calling
a nested decoder similar to the ftr or imm5 decoders in lavc.

In the v1 patch review, Andreas, in response to the proposed doc entry 
describing the feature capability of multiple
or differing payloads in a s302m stream, suggested[1] that s302m should 
be a bitstream filter instead, but I did not
see that as an actionable suggestion as he immediately listed the bsf 
limitations preventing the possibility. I also
had not seen an actual sample of s302m with multiple embedded streams. 
Kieran also observed[2] that he had not seen
such a stream in the wild. So the added features of this patch, wherever 
they are ultimately located, shall not yield
more than one media stream. Anton suggested[3] that the decoder should 
instead be a demuxer. I saw no other objections
to the architecture of the patch.

I posted the v2 patch, incorporating some changes suggested by Andreas, 
4 days later. This had gone uncommented for
over two weeks when I posted a notice stating an intention to push. 
Anton posted[4] a new objection that "If it
dynamically generates nested decoders, then it's not a proper codec in 
our model". This new objection is not connected
to multiple streams but only to a codec 'model' that I don't see 
described anywhere and which contradicts the
implementations of multiple decoders with a nested decoder, including 
the ftr and imm5 decoders, which are most similar
in design to the patched s302m decoder. Anton later on mentioned[5] that 
nested decoders are "a constant source of
issues". However, I didn't find anything on trac reporting an issue with 
the nested decoders of ftr and imm5 nor
anything on ffmpeg-devel-ml or ffmpeg-user-ml. Nothing in their commit 
history either points to architectural bugs.
These decoders have been around for 6 years among themselves. The 
testing of the patched s302m decoder over the past
month by myself, an OTT provider and others shows no issues either. 
Finally, Anton speculates[6] that the burden of
fixes will likely fall upon him. In none of his objections, till the 
time of writing, did I see specific concerns with
the patch.

There are some limitations in shifting this decoder wholesale to inside 
the MPEG-TS demuxer. A s302m stream may contain
some non-media payload accompanying non-PCM media i.e. S-ADM metadata. 
At present, I have neither the samples nor the
specification needed in order to locate and extract or parse this 
metadata. Formatting the payload data inside the
demuxer will lead to irrevocable loss of such metadata if present. 
However, a decoder patch allows simultaneuous output
of both a decoded stream alongside a copied stream. The end-user can 
then do with the raw data whatever they wish.

Ultimately, s302m is specified an an elementary stream inside a MPEG-TS 
container. Its internal handling is better left
to a dedicated module like a decoder. A bitstream filter might be a 
better fit if s302m streams with multiple media
payloads ever start appearing - none have, so far - but for single media 
payloads, a decoder remains the best place.

Regards,
Gyan

[1]: https://ffmpeg.org/pipermail/ffmpeg-devel/2024-January/320119.html
[2]: https://ffmpeg.org/pipermail/ffmpeg-devel/2024-January/320321.html
[3]: https://ffmpeg.org/pipermail/ffmpeg-devel/2024-January/320258.html
[4]: https://ffmpeg.org/pipermail/ffmpeg-devel/2024-February/321514.html
[5]: https://ffmpeg.org/pipermail/ffmpeg-devel/2024-February/321523.html
[6]: https://ffmpeg.org/pipermail/ffmpeg-devel/2024-February/321539.html