On 2024-02-16 07:25 pm, Andreas Rheinhardt wrote:
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.
This is not true: It can not be streamcopied into formats expecting
ordinary PCM or Dolby-e/AC-3. Which is why exporting the data without
the unnecessary packetization is preferable.
With this decoder patch, the packet exported from the demuxer remains
s302m. That may contain either PCM or non-PCM payload[+ ADM metadata].
A streamcopied output into TS or raw data allows the user to parse out
the ADM on their own. I don't have the standard or samples for ADM so I
can't extract it with either a decoder or demuxer patch.
If they want the non-PCM coded stream then they select the pre-existing
method of non_pcm_mode copy in the decoder and a suitable pcm_* encoder
to extract the coded stream, else set the non pcm mode to decode to
obtain playable LPCM AVFrames.
With a demuxer patch, the packet extracted from the demuxer is either
PCM* or some non-PCM codec. The ADM portion if present is lost since
there is no parser for it. And if we add an option for passthrough
demuxing and leave the s302m decoder as it is, then we're back with the
status quo of a 2-step transcoding pipeline.
Regards,
Gyan
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