[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>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker
More information about the FFmpeg-trac
mailing list