[FFmpeg-trac] #4175(swresample:new): support phase shift for Dolby Pro Logic II / Dolby matrix downmix

[FFmpeg]

#4175: support phase shift for Dolby Pro Logic II / Dolby matrix downmix
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             Reporter:  ranutso      |                    Owner:
                 Type:  enhancement  |                   Status:  new
             Priority:  normal       |                Component:
              Version:  git-master   |  swresample
             Keywords:  dplii        |               Resolution:
             Blocking:               |               Blocked By:
Analyzed by developer:  0            |  Reproduced by developer:  0
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Comment (by heleppkes):

 Its possible that the missing phase shift causes the difference you are
 observing, however blindly adjusting the formula to something that "feels"
 good in one particular circumstance is not the way to go here.

 Any reference and independent sources I can find suggest that the current
 code is correct, albeit missing the phase shift. It also matches your qaac
 matrix (sans phase shift).

 So we know its incomplete and therefor results in "wrong" output, no
 reason to actually "break" it more (by diversing the formula from the
 references), imho. The mixer in swresample (and avresample for that
 matter) is only a simple matrix mixer, until someone teaches it how to
 apply the phase shift, any modifications to the DPL/DPLII mixing are
 rather pointless.

 Fact of the matter is, no change to the matrix coefficients will give you
 actual proper DPLII encoded audio.

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Ticket URL: <https://trac.ffmpeg.org/ticket/4175#comment:9>
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