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

[Anton Khirnov <anton@khirnov.net>]

Quoting Gyan Doshi (2024-01-26 05:23:50)
> 
> 
> On 2024-01-25 06:47 pm, Andreas Rheinhardt wrote:
> > Gyan Doshi:
> >>
> >> On 2024-01-25 10:29 am, Andreas Rheinhardt wrote:
> >>> Gyan Doshi:
> >>>> Set up framework for non-PCM decoding in-place and
> >>>> add support for Dolby-E decoding.
> >>>>
> >>>> Useful for direct transcoding of non-PCM audio in live inputs.
> >>>> ---
> >>>>    configure          |   1 +
> >>>>    doc/decoders.texi  |  40 +++
> >>>>    libavcodec/s302m.c | 609 +++++++++++++++++++++++++++++++++++++--------
> >>>>    3 files changed, 543 insertions(+), 107 deletions(-)
> >>>>
> >>>> diff --git a/configure b/configure
> >>>> index c8ae0a061d..8db3fa3f4b 100755
> >>>> --- a/configure
> >>>> +++ b/configure
> >>>> @@ -2979,6 +2979,7 @@ rv20_decoder_select="h263_decoder"
> >>>>    rv20_encoder_select="h263_encoder"
> >>>>    rv30_decoder_select="golomb h264pred h264qpel mpegvideodec rv34dsp"
> >>>>    rv40_decoder_select="golomb h264pred h264qpel mpegvideodec rv34dsp"
> >>>> +s302m_decoder_select="dolby_e_decoder"
> >>>>    screenpresso_decoder_deps="zlib"
> >>>>    shorten_decoder_select="bswapdsp"
> >>>>    sipr_decoder_select="lsp"
> >>>> diff --git a/doc/decoders.texi b/doc/decoders.texi
> >>>> index 293c82c2ba..9f85c876bf 100644
> >>>> --- a/doc/decoders.texi
> >>>> +++ b/doc/decoders.texi
> >>>> @@ -347,6 +347,46 @@ configuration. You need to explicitly configure
> >>>> the build with
> >>>>    An FFmpeg native decoder for Opus exists, so users can decode Opus
> >>>>    without this library.
> >>>>    +@section s302m
> >>>> +
> >>>> +SMPTE ST 302 decoder.
> >>>> +
> >>>> +SMPTE ST 302 is a method for storing AES3 data format within an MPEG
> >>>> Transport
> >>>> +Stream. AES3 streams can contain LPCM streams of 2, 4, 6 or 8
> >>>> channels with a
> >>>> +bit depth of 16, 20 or 24-bits at a sample rate of 48 kHz.
> >>>> +They can also contain non-PCM codec streams such as AC-3 or Dolby-E.
> >>>> +
> >>> This sounds like we should add bitstream filters to extract the proper
> >>> underlying streams instead.
> >>> (I see only two problems with this approach: The BSF API needs to set
> >>> the CodecID of the output during init, but at this point no packet has
> >>> reached the BSF to determine it. And changing codec IDs mid-stream is
> >>> also not supported.)
> >> In theory, this decoder shouldn't exist, as it is just a carrier,
> >> whether of LPCM or non-PCM.
> >> FFmpeg architecture also imposes a fundamental limitation in that one
> >> s302m stream may
> >> carry multiple payload streams and we support only one decoding context
> >> per input stream
> > Then why does the demuxer not separate the data into multiple streams?
> 
> I didn't add demuxing support for this codec in MPEGTS, but I can venture
> 
> 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.

-- 
Anton Khirnov
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