Re: [hatari-devel] YM2149 + C10 lowpass filter mapping

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On 11/12/2011 03:41, David Savinkoff wrote:
Hi,

*** Here are 2 patches to provide YM2149 + C10 lowpass filtering for
the STf. Filtering is acivated in ST mode for 44100 Hz and 48000 Hz.
The STe, TT and Falcon do not have C10 so they sound brighter.
You can test the sound difference on the ST by selecting 50066 Hz,
or change audio.c to filter only 44100 or 48000 Hz.



Hello

from your comments,

#define B_z (yms32)( 0.2667*(1<<15))
#define A_z (yms32)(-0.4667*(1<<15))
y0 = (B_z*(x0 + x1) - A_z*y0) >> 15;
x1 = x0;

But then the code in LowPassFilter becomes :

       y0 = ((x0 + x1) + (y0<<1)) >> 2;
       x1 = x0;

Did you simplify it by approximating A_z ~= -2 * B_z
Doesn't it alter filtering too much ?


Nicolas



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