convolve
*out_ptr++ = double2short(convolve(coef, in_buf, i) +
convolve(coef + i, state_ptr++, init_order--));
double2short(convolve(coef, state_ptr++, order + 1));
*out_ptr++ = double2short(convolve(coef, in_buf, i + 1) +
convolve(coef + i + 1, state + i, order - i));
*out_ptr++ = double2short(convolve(coef, in_buf + i - order,
extern double convolve(double *, double *, int);