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Old 18th August 2003, 12:28   #16  |  Link
Valex
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Join Date: Sep 2002
Posts: 144
@bleo

Very good! I'll update AC3Filter's downmix matrix in the next version.

But however I think that real DPLII downmixer is little more complicated thing that just a passive matrix downmixer so I consider all this as just a simple hack. .

I think that downmix coefficients depend on relative audio level in input channels. So it is needed to know coefficients as function of input levels:
Lt = L + 0.7 C + K1 * BL + K2 * BR
Rt = R + 0.7 C + K3 * BL + K4 * BR
where K1..K4 are functions of input levels.

To know this functions we need better to analyze encoder but not decoder.

Imagine that we have 2 signals at input: L with level lev_L and frequency f1 and BR with level lev_BR and frequency f2. After downmixing we have 2 signals Lt and Rt. Lt should contain f1 and f2 frequencies. Rt should contain only f2. We supposed that L level at output does not depends on anything (but we may be wrong), so f1 level in L should not depend on other channels level, but f2 level in Lt and Rt is nothing else that K2 and K4 coefficients! When changing f1 and f2 levels at input we can get K2 and K4 as functions K2(lev_L, lev_BR) and K4(lev_L, lev_BR) and then devise the total function!

I do not think that this functions are too complicated. It should be very simple. Somewhere at Dolby site I've read that DPLII encoder is rather simple thing but decoder is very complicated (it is why better to analyze the encoder).

@all
Anybody can make such a complicated experiment?
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