[MPlayer-dev-eng] Surround plugin now low-pass filters surround channels
Stephen Davies
steve at daviesfam.org
Fri Dec 14 00:47:32 CET 2001
Hi,
In pl_surround.c, the Dolby Surround decoder plugin, I've added a 7kHz
low-pass filter on the surround channels.
So that plugin now implements all of Dolby's Dolby Surround decoder
design. (its NOT Pro Logic though).
I did the FIR filter in C, lightly optimized. It seems pretty efficient -
taking just a couple of percent of the total mplayer CPU by gprof. (Its
only a 32-tap FIR). But could be redone in mmx/3dnow versions. I believe
Anders will need a FIR for sample rate conversion and has some MMX code,
so we'll merge his then.
In the meantime, I reckon it sounds quite good. I'd be pleased to hear
reports. One good test is to take an AC3 VOB file or DVD and compare
-ac ac3 -channels 4
with -ac ac3 -channels 2 -aop list=surround
The first is the true Dolby Digital, the second downmixes the Dolby
Digital to Dolby Surround, and then decodes the Dolby Surround using
pl_surround.c
Thanks and respect are due to Jake Janovetz, for his LGPLed remez.[ch]
code for computing FIR coefficients. Who knows how it works, seems
like magic to me.
Steve
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