On 2024-01-29 02:57 pm, Nicolas Gaullier wrote:
On 2024-01-28 04:24 pm, Anton Khirnov wrote:
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.
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.
Regards,
Gyan
AFAIK, DolbyE may be found on satellite feeds, for carriage between broadcast facilities, and thus it makes them
accessible so they may be grabbed by "end users". Otherwise, it is "broadcast professional end
users", which are users too. AFAIK, its most common form is 20Bits and you simply "cannot" have a single
stream in a 20Bit carrier; but indeed, most of the time only the first stream ("program") is used and the
second is a downmix; but not always. For example, you can have a first program which is standard 5.1 and a second
program with Audio Description.
In the samples I have, including yours, the outermost layer is AES3
which contains one inner stream Dolby-E, which in turn contains 8
channels constituting multiple programs. Those are programs downstream
of the dolby_e decoder. That's not what we are talking about. The
discussion here is about multiple payload streams within the AES3 layer
- streams stored in subframe mode e.g. a Dolby-E and AC-3 or LPCM in
alternate subframes/frames. I've no such samples of that sort.
Regards,
Gyan
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