[FFmpeg-user] ffmpeg's Dolby Pro Logic feature is leaking center channel into surround channel

[Carles Vila]

Hi
There are many variables in your setup, and I believe you have some
confusion regarding how film audio works. I work in audio postproduction
for film and TV, I hope I am able to help.
I truly believe the root of your problem is not in ffmpeg, but in the setup
of your system. In that respect I would recommend exposing your issue to a
film-sound forum (https://www.gearslutz.com/board/post-production-forum/)
Where many professionals are familiar with your situation.

Your situation is very specific, so we need specific details. What Cinema
server are you using? what sound processor(s) are you using?
For example doremi DCP-2000 + Dolby CP650. Your limiting factor as you
describe seems to be the kind of MPEG files the server is able to play
back. It would be useful in this regard to know if it's limited to stereo
(2 channel) output.

It is also crucial to know what kind of video files and audio mixes you are
receiving as source material. From what I read I think you are receiving
stereo mixes, which is OK for film festival, but you must accept compromise.

That being said, there's something that does not make sense to me: You say
if you play back plain stereo through the cinema sound processor you only
hear a whisper. Whereas if you playback a "Dolby prologic" encoded sound
you made from converting an ac3, which in turn you created by faking a 5.1
mix, the loudness is ok.
"dolby prologic" is a particularly encoded  stereo file but it is nothing
more than two channel audio. There's nothing that makes it louder per se
because the levels are the same as can be a plain stereo file.
Futhermore, the dolby prologic encoder expects to receive at least 4
discrete, different, audio channels (Left, Right, Center, Surround). If you
fake the C by summing L+R and put nothing in the surrounds, it is normal
that the decoder does weird, impredictable stuff like leaking to the
surrounds, because that's not what it was designed for.
My advice would be to avoid experimentation and convoluted steps and play
back the stereo you receive as is. Find out the problem why it sounds so
quiet.

Best regards,
Carles






On 27 November 2015 at 23:49, TBUFFestival <tbuffestival at gmail.com> wrote:

> From: Projection Journeyman
>
> The full scope of my situation can be found here:
>
>
> http://superuser.com/questions/1006255/ffmpegs-dolby-pro-logic-feature-is-leaking-center-channel-into-surround-channel?noredirect=1
>
> Including all things we have tried and why we need to do this, but just to
> quickly summarize:
>
> We are attempting to take a Dolby Digital .ac3 file we created from a
> stereo file (left = left, right = right, center = mono of left+right, LFE
> and surround sound are blank) and convert it to Dolby Pro Logic using
> ffmpeg's aresample:
>
> ffmpeg -i test-dolby-digital.ac3 -acodec libmp3lame -b:a 320k -af
> "aresample=matrix_encoding=dplii" test-dolby-pro-logic.mp3
>
> The center channel of the matrixed .mp3 comes out pretty good, but it is
> leaking into the surround channel, even though we did not put anything at
> all in the surround channel of the .ac3 file.  Can this be stopped?  We
> don't see this in professionally rendered DPL soundtracks.  We did notice
> there exists in ffmpeg aresample something called "surround_level" or
> "slev" that can be set to zero but aren't sure if it applies here and even
> if it does, how the syntax works and where it should be within the above
> statement.  We also have our doubts it will even stop the leaking -- maybe
> it is a byproduct of attempting to backconvert a stereo file into digital
> and back to analog but matrixed?  But any idea anyone can come up with to
> overcome this, even if it involves some software other than aresample or
> ffmpeg for that matter would be greatly appreciated.
>
> Projection Journeyman
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