ref: 8748bd3cde86dda00a454c8bf526f6c83976e6d4
dir: /src/band.h/
/* libSoX Bandpass effect file. July 5, 1991
* Copyright 1991 Lance Norskog And Sundry Contributors
*
* This source code is freely redistributable and may be used for
* any purpose. This copyright notice must be maintained.
* Lance Norskog And Sundry Contributors are not responsible for
* the consequences of using this software.
*
* Algorithm: 2nd order recursive filter.
* Formula stolen from MUSIC56K, a toolkit of 56000 assembler stuff.
* Quote:
* This is a 2nd order recursive band pass filter of the form.
* y(n)= a * x(n) - b * y(n-1) - c * y(n-2)
* where :
* x(n) = "IN"
* "OUT" = y(n)
* c = EXP(-2*pi*cBW/S_RATE)
* b = -4*c/(1+c)*COS(2*pi*cCF/S_RATE)
* if cSCL=2 (i.e. noise input)
* a = SQT(((1+c)*(1+c)-b*b)*(1-c)/(1+c))
* else
* a = SQT(1-b*b/(4*c))*(1-c)
* endif
* note : cCF is the center frequency in Hertz
* cBW is the band width in Hertz
* cSCL is a scale factor, use 1 for pitched sounds
* use 2 for noise.
*
*
* July 1, 1999 - Jan Paul Schmidt <jps@fundament.org>
*
* This looks like the resonator band pass in SPKit. It's a
* second order all-pole (IIR) band-pass filter described
* at the pages 186 - 189 in
* Dodge, Charles & Jerse, Thomas A. 1985:
* Computer Music -- Synthesis, Composition and Performance.
* New York: Schirmer Books.
* Reference from the SPKit manual.
*/
p->a2 = exp(-2 * M_PI * bw_Hz / effp->in_signal.rate);
p->a1 = -4 * p->a2 / (1 + p->a2) * cos(2 * M_PI * p->fc / effp->in_signal.rate);
p->b0 = sqrt(1 - p->a1 * p->a1 / (4 * p->a2)) * (1 - p->a2);
if (p->filter_type == filter_BPF_SPK_N) {
mult = sqrt(((1+p->a2) * (1+p->a2) - p->a1*p->a1) * (1-p->a2) / (1+p->a2)) / p->b0;
p->b0 *= mult;
}