wavetab.c

DSP算法（ANSI_C） Wavetable Music Generator 4-20-94 PME

#include <stdlib.h>
#include <math.h>
#include "rtdspc.h"
#include "gwave.h"        /* gwave[301] array */

/* Wavetable Music Generator 4-20-94 PME */

    int key;

void main()
{
    int t,told,ci,k;
    float ampold,rate,env,wave_size,dec,phase,frac,delta,sample;
    register long int i,endi;
    register float sig_out;

    static float trel[5] = {    0.02 ,    0.14,     0.6,  1.0, 0.0 };
    static float amps[5] = { 25000.0 , 15000.0,  6000.0, 10.0, 0.0 };
    static float rates[10];
    static int tbreaks[10];

    wave_size = 300.0;      /* dimension of original wave */
    endi = 22050;           /* 2 second notes */

    for(key = -6 ; key < 48 ; key++) {

/* decimation ratio for key semitones down */
        dec = pow(2.0,0.0833333333*(float)key);

/* calculate the rates required to get the desired amps */
        i = 0;
        told = 0;
        ampold = 1.0;       /* always starts at unity */
        while(amps[i] > 1.0) {
            t = trel[i]*endi;
            rates[i] = exp(log(amps[i]/ampold)/(t-told));
            ampold = amps[i];
            tbreaks[i] = told = t;
            i++;
        }

        phase = 0.0;
        rate = rates[0];
        env = 1.0;
        ci = 0;
        for(i = 0 ; i < endi ; i++) {
/* calculate envelope amplitude */
            if(i == tbreaks[ci]) rate = rates[++ci];
            env = rate*env;
/* determine interpolated sample value from table */
            k = (int)phase;
            frac = phase - (float)k;
            sample = gwave[k];
            delta = gwave[k+1] - sample; /* possible wave_size+1 access */
            sample += frac*delta;
/* calculate output and send to DAC */
            sig_out = env*sample;
            sendout(sig_out);
/* calculate next phase value */
            phase += dec;
            if(phase >= wave_size) phase -= wave_size;
        }
    }
    flush();
}