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

FFmpeg trac at avcodec.org
Thu Mar 26 21:06:44 CET 2015

#4175: support phase shift for Dolby Pro Logic II / Dolby matrix downmix
             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

Comment (by Peter):

 I wouldn't recommend using the patch, I didn't realise about the phase
 shifting when I wrote it (It will stop the waveform cancellation, but also
 get steered to the wrong channel).

 To quote some documentation from Dolby:

 "The 90-Degree Phase Shift filter provides a means for an encoding
 engineer to create a multichannel Dolby Digital bitstream that can be
 downmixed to a Dolby Surround compatible Lt/Rt output. Without this
 filter, point-source elements panned from Surround to Center in the
 multichannel mix would seem to pan from Surround to Left and then to
 Center when downmixed to Lt/Rt and reproduced using a Dolby Surround Pro
 Logic decoder.

 This filter should generally be used whenever encoding a multichannel
 signal unless it is known that the 5.1-channel source does not contain
 point-source element pans. For example, if the source was recorded using
 five discrete microphones placed in the corners of an auditorium, there is
 no panning between channels and the filter could be safely disabled. If in
 doubt, use a DP562 to downmix the 5.1-channel program to Lt/Rt, Dolby
 Surround Pro Logic decode the Lt/Rt signals, and then set the filter to
 the setting that sounds best."

 This is pretty much what happened in #3455. Basically, as heleppkes says,
 it will not give you proper encoded audio even if the coefficients are
 corrected (I do think they're off too).

Ticket URL: <https://trac.ffmpeg.org/ticket/4175#comment:11>
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