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

Carles Vila cvilad at gmail.com
Mon Nov 30 13:04:56 CET 2015

On 30 November 2015 at 12:00, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:

> On Monday 30 November 2015 11:09:55 am Carles Vila wrote:
> [...]
> I believe you completely misunderstood the report, do you
> realize that FFmpeg contains a "dolby prologic encoder"?
> I am particularly interested in this issue, and I would
> recommend not to move the discussion about a possible issue
> in FFmpeg to another mailing list or forum.
> Please do not top-post here, Carl Eugen
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The fact that ffmpeg contains a dolby prologic encoder was new to me I must
say before reading this thread. I have used commercial encoders/decoders,
software and hardware, uncountable times.
In the overall context of the OP's problem (playing back stereo contents in
a cinema) I was trying to help him with relevant advice, which happens to
be unrelated to ffmpeg.
FFmpeg's dolby prologic encoder is interesting to me also. But if you
believe it has issues, I think it's better to test it in a more controlled
environment. It is normal to get L and R leaked signal in the surrounds
even if no surround signal was present at the encoder. Of course this is a
"feature/problem" of the decoder, not the encoder. But it is certainly not
OK to feed the encoder with C=L+R, and furthermore expect a certain result
without knowing the theory behind the dolby system. A dolby PLII codec
expects to receive a proper 5.1, encodes a stereo-compatible two-channel
audio, which the decoder can decode back to an approximation of the
original 5.1. There is leakage, there is stereo-steering, which are
inherent side-effects of matrixing. BTW how did you implement the PLII

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