[FFmpeg-devel] [PATCH] rematrix DPLII fix for channel downmixing. Surround now adds to front rather than taking away.

[Peter Robertson]

On 15/03/14 16:51, Hendrik Leppkes wrote:
> On Sat, Mar 15, 2014 at 4:46 PM, Peter Robertson
> <Peter at corrosivetruths.org> wrote:
>> This fixes a problem where trying to convert gameplay sound to Dolby Pro
>> Logic II would result in a left channel that would be missing the sounds
>> that were in center and back left at the same time. I also applied it to
>> Dolby Pro Logic as it should work the same way. Sorry if I've done
>> anything wrong, first patch. Fixes #3455
>>
> The encoding matrix for DPL/DPLII is defined by Dolby, and only sounds
> good if you use a DPL compatible decoder to reconstruct the original
> signal.
> Its not necessarily supposed to be played as stereo through stereo speakers.
>
> All Information I could find about DPL encoding suggests that the
> implementation is correct as it is today. Left channel subtracts both
> surrounds, and right channel adds them.
>
> - Hendrik
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The idea is that the sound can be played stereo or through a decoder.
After patching the way it works and putting a surround file through it;
a given game footage waveform looks very close to a 'SurCode for Dolby®
Pro Logic II Encoder & Decoder' produced version I'm using as reference
whereas the way it works now, having a sound in the back left speaker
will cancel out sound in the left front / center which the SurCode
doesn't do. I discovered this when enoding game footage that has a
foot-step sound coming from center and surrounds which produces a stereo
track which is too quiet on the left, and a surround track which has no
footstep sound on the left either. Version with the patch works
perfectly in stereo or decoded into surround, version without patch also
works perfectly in stereo and surround; until you feed it a file with
repeated sounds in surrounds and front (like I say, I noticed with
videogame footage) at which point you get a bias to the right speaker
(which like I say doesn't exist in a certified encoder).

What can I do?