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

[Gyan Doshi <ffmpeg@gyani.pro>]

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.
b) it would lead to a mapping of 1 actual to N virtual streams. How 
would stream_ids be assigned?
c) this codec specifies for multiple payloads but I haven't seen an 
actual sample


>>>> +
>>>> +    ret = init_get_bits8(&gb, buf, buf_size);
>>>> +    if (ret < 0)
>>>> +        return ret;
>>>> +
>>>> +    aes_frm_size = (s->bits + 4) * 2 / 8;
>>>> +    if (buf_size < aes_frm_size * 2)  // not enough to contain
>>>> data_type & length_code
>>>> +        return AVERROR_INVALIDDATA;
>>>> +
>>>> +    state = get_bits64(&gb, aes_frm_size * 8);
>>>> +
>>>> +    while (!IS_NONPCMSYNC(s->bits,state) && (get_bits_left(&gb) >= 8))
>>>> +        state = (state << 8) | get_bits(&gb, 8);
>>> Reading byte-aligned data with a GetBit context is very suboptimal.
>> What is the performance difference vs. uint8 pointers?
> Typically worse by a constant factor.

But in this scenario (of a few dozen bytes per packet if dolby_e), how 
significant is it?

Regards,
Gyan
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