_EXAMINING AUDIO DSP ALGORITHS_
by Dennis Cronin
Listing One

/* flange_chorus. Does flanging/chorusing family of effects based on a single
    varying delay.
    
    dry_mix     mix of unaffected signal (-0.999 to 0.999)
    wet_mix     mix of affected signal (-0.999 - 0.999)
    feedback    amount of recirculation (-0.9 - 0.9)
    rate        rate of delay change in millisecs per sec
    sweep       sweep range in millisecs
    delay       fixed additional delay in millisecs
*/
void
flange_chorus(struct program *p)
{
    int fp,ep1,ep2;
    int step,depth,delay,min_sweep,max_sweep;
    double inval,outval,ifac = 65536.0;
    long scan = 0;
    bw data;
    wl sweep;
    /* fetch params */
    step = (int)(p->rate * 65.536);
    depth = (int)(p->depth * (double)SampleRate / 1000.0);
    delay = (int)(p->delay * (double)SampleRate / 1000.0);
    /* init/calc some stuff */
    max_sweep = BFSZ - 2 - delay;
    min_sweep = max_sweep - depth;
    if(min_sweep < 0) {
        printf("Can't do that much delay or depth at this sample rate.\n");
        exit(1);
    }
    sweep.w[1] = (min_sweep + max_sweep) / 2;
    sweep.w[0] = 0;
    /* init store and read ptrs to known value */
    fp = ep1 = ep2 = 0;
    /* disable interrupts, go to it */
    disable();
    while(1) {
        while((inp(SR) & 0x20) == 0);       /* wait for input ready */
        data.b[0] = inp(PDR);               /* read input from chip */
        data.b[1] = inp(PDR);
        /* interpolate from the 2 read values */
        outval =
         (Buf[ep1] * sweep.w[0] + Buf[ep2] * (ifac - sweep.w[0])) / ifac;
        /* store finished input plus feedback */
        Buf[fp] = (inval = (double)data.w) + outval * p->feedback;
        /* develop final output mix */
        outval = outval * p->wet_mix + inval * p->dry_mix;
        if(outval > 32767.0)
            data.w = 32767;
        else if(outval < -32768.0)
            data.w = -32768;
        else
            data.w = (int)outval;
        while((inp(SR) & 0x2) == 0);        /* wait for output ready */
        outp(PDR,data.b[0]);                /* write output to chip */
        outp(PDR,data.b[1]);
        /* update ptrs */
        fp = (fp + 1) & (BFSZ - 1);
        sweep.l += step;
        ep1 = (fp + sweep.w[1]) & (BFSZ - 1);
        ep2 = (ep1 - 1) & (BFSZ - 1);
        /* check for sweep reversal */      
        if(sweep.w[1] > max_sweep)          /* see if we hit top of sweep */
            step = -step;                   /* reverse */
        else if(sweep.w[1] < min_sweep)     /* or if we hit bottom of sweep */
            step = -step;                   /* reverse */
            
        check_human();                      /* check on human every so often */
    }
}