[MPlayer-dev-eng] PATCH 4/5: substract surround from front in pl_surround

[Stephen Davies]

Hi Tobias,

I don't think this manipulation is OK - remember that, by nature of the
matrix decoding process, the decoded surround output includes L-R
crosstalk too.  If you subtract this surround off L and R you will have a
big effect on the front sound.

This would be OK for a sound strongly to the rear (and, in fact, a Pro
Logic decoder would do something like this for that case).  

The math is as follows:

  The input tracks are
    Lt = L + (C-3dB) + (S-3dB)
    Rt = R + (C-3dB) - (S-3dB)
  Basic decoded outputs are:
    Lout = Lt
         = L + (C-3dB) + (S-3dB)
    Rout = Rt
         = R + (C-3dB) - (S-3dB)
    Cout = Lt+Rt
         = C + L + R
    Sout = Lt-Rt
         = S + (L - R)

  Your manipulation is (using Left as an example):
    Lout = L + (C-3dB) + (S-3dB) - (S + (L - R))
         = L + (C-3dB) + (S-3dB) - S - L + R
         = (C-3dB) - (S-10dB) + R

The output doesn't bear much relation to the input "L".

Regards,
Steve