snd_flt.cpp

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/*
 * This program is  free software; you can redistribute it  and modify it
 * under the terms of the GNU  General Public License as published by the
 * Free Software Foundation; either version 2  of the license or (at your
 * option) any later version.
 *
 * Authors: Olivier Lapicque <olivierl@jps.net>
*/

#include "stdafx.h"
#include "sndfile.h"

// AWE32: cutoff = reg[0-255] * 31.25 + 100 -> [100Hz-8060Hz]
// EMU10K1 docs: cutoff = reg[0-127]*62+100
#define FILTER_PRECISION        8192

#ifndef NO_FILTER

#ifdef WIN32
#define _ASM_MATH
#endif

#ifdef _ASM_MATH

// pow(a,b) returns a^^b -> 2^^(b.log2(a))
static float pow(float a, float b)
{
        long tmpint;
        float result;
        _asm {
        fld b                           // Load b
        fld a                           // Load a
        fyl2x                           // ST(0) = b.log2(a)
        fist tmpint                     // Store integer exponent
        fisub tmpint            // ST(0) = -1 <= (b*log2(a)) <= 1
        f2xm1                           // ST(0) = 2^(x)-1
        fild tmpint                     // load integer exponent
        fld1                            // Load 1
        fscale                          // ST(0) = 2^ST(1)
        fstp ST(1)                      // Remove the integer from the stack
        fmul ST(1), ST(0)       // multiply with fractional part
        faddp ST(1), ST(0)      // add integer_part
        fstp result                     // Store the result
        }
        return result;
}


#else

#include <math.h>

#endif // _ASM_MATH


DWORD CSoundFile::CutOffToFrequency(UINT nCutOff, int flt_modifier) const
//-----------------------------------------------------------------------
{
        float Fc;

        if (m_dwSongFlags & SONG_EXFILTERRANGE)
                Fc = 110.0f * pow(2.0f, 0.25f + ((float)(nCutOff*(flt_modifier+256)))/(21.0f*512.0f));
        else
                Fc = 110.0f * pow(2.0f, 0.25f + ((float)(nCutOff*(flt_modifier+256)))/(24.0f*512.0f));
        LONG freq = (LONG)Fc;
        if (freq < 120) return 120;
        if (freq > 10000) return 10000;
        if (freq*2 > (LONG)gdwMixingFreq) freq = gdwMixingFreq>>1;
        return (DWORD)freq;
}


// Simple 2-poles resonant filter
void CSoundFile::SetupChannelFilter(MODCHANNEL *pChn, BOOL bReset, int flt_modifier) const
//----------------------------------------------------------------------------------------
{
        float fc = (float)CutOffToFrequency(pChn->nCutOff, flt_modifier);
        float fs = (float)gdwMixingFreq;
        float fg, fb0, fb1;

        fc *= (float)(2.0*3.14159265358/fs);
        float dmpfac = pow(10.0f, -((24.0f / 128.0f)*(float)pChn->nResonance) / 20.0f);
        float d = (1.0f-2.0f*dmpfac)* fc;
        if (d>2.0) d = 2.0;
        d = (2.0f*dmpfac - d)/fc;
        float e = pow(1.0f/fc,2.0);

        fg=1/(1+d+e);
        fb0=(d+e+e)/(1+d+e);
        fb1=-e/(1+d+e);

        pChn->nFilter_A0 = (int)(fg * FILTER_PRECISION);
        pChn->nFilter_B0 = (int)(fb0 * FILTER_PRECISION);
        pChn->nFilter_B1 = (int)(fb1 * FILTER_PRECISION);
        
        if (bReset)
        {
                pChn->nFilter_Y1 = pChn->nFilter_Y2 = 0;
                pChn->nFilter_Y3 = pChn->nFilter_Y4 = 0;
        }
        pChn->dwFlags |= CHN_FILTER;
}

#endif // NO_FILTER

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