[Speex-dev] cheb_poly_eva using Clenshaw's recurrence formula

[Matthieu Poullet]

Hi,

After reading the paper entitled "The Computation of Line Spectral
Frequencies Using Chebyshev Polynomials", P. Kabal and R.
Ramachandran, IEEE Trans. on ASSP, Vol. 34, No. 6, December 1986, I
rewrite the function cheb_poly_eva in lsp.c using the Clenshaw's
recurrence formula, as described, for example, in Numerical Recipes in
C, Second Edition (5.5 and 5.8) :

static float cheb_poly_eva(spx_word32_t *coef, float x, int m, char *stack)
{
 int k;
 float b0, b1, tmp;
 int m2=m>>1;

 /* Initial conditions */
 b0=0; /* b_(m+1) */
 b1=0; /* b_(m+2) */

 x*=2;

 /* Calculate the b_(k) */
 for(k=m2;k>0;k--)
 {
    tmp=b0;                           /* tmp holds the previous value of b0 */
    b0=x*b0-b1+coef[m2-k];    /* b0 holds its new value based on b0 and b1 */
    b1=tmp;                           /* b1 holds the previous value of b0 */
 }

return(-b1+x/2*b0+coef[m2]);
}

I don't really know if it's a little improvement or not, so I'd like
to have feedback from the Speex experts here.

Thanks a lot for releasing Speex, it helps me a lot to understand
speech coding !

Matthieu Poullet