[FFmpeg-devel] [FFmpeg-cvslog] avcodec/dca: add new decoder based on libdcadec

[Michael Niedermayer]

On Sun, Jan 31, 2016 at 04:04:41PM -0300, James Almer wrote:
> On 1/31/2016 3:57 PM, James Almer wrote:
> > On 1/31/2016 3:51 PM, Michael Niedermayer wrote:
> >> On Sun, Jan 31, 2016 at 05:14:14PM +0100, foo86 wrote:
> >>> ffmpeg | branch: master | foo86 <foobaz86 at gmail.com> | Sat Jan 16 11:54:38 2016 +0300| [ae5b2c52501d5009fe712334428138a9b758849b] | committer: Hendrik Leppkes
> >>>
> >>> avcodec/dca: add new decoder based on libdcadec
> >>>
> >>>> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=ae5b2c52501d5009fe712334428138a9b758849b
> >>> ---
> >>>
> >>
> >>
> >> this breaks request_channel_layout
> >>
> >> example:
> >> ./ffmpeg -i dts/lotr_5.1_768.dts test.wav
> >>
> >> Input #0, dts, from '/home/michael/videos/dts/lotr_5.1_768.dts':
> >>   Duration: 00:02:05.18, start: 0.000000, bitrate: 768 kb/s
> >>     Stream #0:0: Audio: dts (DTS-ES), 48000 Hz, 6.1, fltp, 768 kb/s
> >>
> >>
> >> vs.
> >> ./ffmpeg -request_channel_layout 3 -i dts/lotr_5.1_768.dts test-req3.wav
> >>
> >> Input #0, dts, from '/home/michael/videos/dts/lotr_5.1_768.dts':
> >>   Duration: 00:02:05.18, start: 0.000000, bitrate: 768 kb/s
> >>     Stream #0:0: Audio: dts (DTS), 48000 Hz, 5.1(side), fltp, 768 kb/s
> >>
> >> vs.
> >> ./ffmpeg-ref -request_channel_layout 3 -i dts/lotr_5.1_768.dts test-ref-req3.wav
> >>
> >> Input #0, dts, from '/home/michael/videos/dts/lotr_5.1_768.dts':
> >>   Duration: 00:02:05.18, start: 0.000000, bitrate: 768 kb/s
> >>     Stream #0:0: Audio: dts (DTS-ES), 48000 Hz, stereo, fltp, 768 kb/s
> >>
> >>
> >> previously it resulted in 2 channels now it produces 5.1 when
> >> requesting stereo
> > 
> > libdcadec (and thus this new decoder) doesn't force stereo when downmix
> > coefficients are not embedded in the stream, leaving the work to the
> > resampler instead. Try the libdcadec wrapper and it will do the same.
> > 
> > It's an intended change in behavior compared to the old dca decoder, so
> > i guess it could documented.
> > 
> 
> To expand a bit based on what foo86 mentioned in a patch for the wrapper,
> with DTS-ES, DTS-HDMA and other extensions, if you request a stereo downmix
> and coefficients are not available it will not bother applying all the
> extensions to output 7 or 8 channels and instead output the core 5.1 stream
> as is, to simplify the work of the resampler.
> Trying to force a downmix using default coefficients may result in clipping.
> 
> He can explain it much better than me, though.

the output is float, that should not clip
also how is downmixing after the decoder better, it too would apply
some coefficients (either risking an increase in peak volume or
a average loss of volume or inconsistet attenuation

[...]
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