[FFmpeg-devel] FFmpeg TC input needed

[Gyan Doshi]

Ping x2.

On 2024-04-02 10:55 am, Gyan Doshi wrote:
> Ping.
>
> As the TC rules matter has been concluded, this should go ahead.
>
> Regards,
> Gyan
>
>
> On 2024-02-17 05:15 pm, Gyan Doshi wrote:
>> 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
>>
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