/* Get the tilt factor of a formant filter from its transfer function  */
static float tilt_factor(const float *lpcs, int n_lpcs)
/* Get the tilt factor of a formant filter from its transfer function  */
static float tilt_factor(const float *lpcs, int n_lpcs)
{
    float rh0, rh1;
    rh0 = 1.0     + ff_dot_productf(lpcs,  lpcs,    n_lpcs);
    rh1 = lpcs[0] + ff_dot_productf(lpcs, &lpcs[1], n_lpcs - 1);
    return rh1 / rh0;
}