[MPlayer-dev-eng] Surround plugin now low-pass filters surround channels

[Stephen Davies]

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