musicdsp.org source code archive - WSInf

}
double y;
y = lp->b * (x + lp->x1) - lp->a * lp->y1;
lp->x1 = x;
lp->y1 = y;
return y;
/* dsf blit datatype
*
*/
typedef struct
{
double phase; /* phase accumulator */
double aNQ; /* attenuation at nyquist */
double curcps; /* current frequency, updated once per cycle */
double curper; /* current period, updated once per cycle */
lowpass_t leaky; /* leaky integrator */
double N; /* # partials */
double a; /* dsf parameter which controls roll-off */
double aN; /* former to the N */
} blit_t;
/* initializes a blit structure
*
* The aNQ parameter is the desired attenuation
* at nyquist. A low value yields a duller
* sawtooth but gets rid of those annoying CLICKS
* when sweeping the frequency up real high. |aNQ|
* must be strictly less than 1.0! Find a setting
* which works for you.
*
* The cutoff parameter controls the leakiness of
* the integrator.
*/
void init_blit(blit_t *b, double aNQ, double cutoff)
{
b->phase = 0.0;
b->aNQ = aNQ;
b->curcps = 0.0;
b->curper = 0.0;
init_lowpass(&b->leaky, cutoff);
}
/* Returns a sawtooth computed from a leaky integration
* of a DSF bandlimited impulse train.
*
* cps (cycles per sample) is the fundamental
* frequency: 0 -> 0.5 == 0 -> nyquist
*/
double update_blit(blit_t *b, double cps)
{
double P2, beta, Nbeta, cosbeta, n, d, blit, saw;
if(b->phase >= 1.0 || b->curcps == 0.0)
{
/* New cycle, update frequency and everything
* that depends on it
*/
if(b->phase >= 1.0)
b->phase -= 1.0;
}
b->curcps = cps; /* this cycle\'s frequency */
b->curper = 1.0 / cps; /* this cycle\'s period */
P2 = b->curper / 2.0;
b->N = 1.0 + floor(P2); /* # of partials incl. dc */
/* find the roll-off parameter which gives
* the desired attenuation at nyquist
*/
b->a = pow(b->aNQ, 1.0 / P2);
b->aN = pow(b->a, b->N);
beta = twopi * b->phase;