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/*
* Kaiser Window digital filter
*
* Copyright © 2001, Keith Packard
* All Rights Reserved. See the file COPYING in this directory
* for licensing information.
*/
real i0(real x)
{
real ds, d, s;
ds = 1;
d = 0;
s = 0;
do
{
d = d + 2;
ds = ds * x**2 / d**2;
s = s + ds;
# printf ("ds %g s %g dist %g\n", ds, s,
# ds - 0.2e-8 * s);
}
while (ds - 0.2e-8 * s > 0);
return s;
}
real highpass (real n, real m, real wc)
{
real alpha = m/2;
real dist;
dist = n - alpha;
if (dist == 0)
return (pi/2 - wc) / pi;
return -sin(dist * (pi/2-wc)) / (pi * dist);
}
real lowpass (real n, real m, real wc)
{
real alpha = m/2;
real dist;
dist = n - alpha;
if (dist == 0)
return wc / pi;
return sin (wc * dist) / (pi * dist);
}
real kaiser (real n, real m, real beta)
{
real alpha = m / 2;
return i0 (beta * sqrt (1 - ((n - alpha) / alpha)**2)) / i0(beta);
}
void write_high (string filename,
real m,
real wc,
real beta,
real step)
{
real n;
file f;
f = File::open (filename, "w");
for (n = 0; n <= m; n += step)
File::fprintf (f, "%g,\n", highpass (n, m, wc) * kaiser (n, m, beta));
File::close (f);
}
void write_low (string filename,
real m,
real wc,
real beta,
real step)
{
real n;
file f;
f = File::open (filename, "w");
for (n = 0; n <= m; n += step)
File::fprintf (f, "%g,\n", lowpass (n, m, wc) * kaiser (n, m, beta));
File::close (f);
}
real Beta (real A)
{
if (A > 50)
return 0.1102 * (A - 8.7);
else if (A >= 21)
return 0.5842 * (A - 21) ** 0.4 + 0.07886 * (A - 21);
else
return 0.0;
}
int M (real A, real deltaw)
{
return ceil ((A - 8) / (2.285 * deltaw));
}
real filter (real wpass, real wstop, real error, *int mp)
{
real deltaw = wstop - wpass;
real A = -20 * log10 (error);
real beta;
beta = Beta (A);
*mp = M (A, deltaw);
printf ("Beta %g M %g\n", beta, *mp);
return beta;
}
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