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

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

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