kaiser.c
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上传日期:2007-06-06
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- /* ***** BEGIN LICENSE BLOCK *****
- * Version: RCSL 1.0/RPSL 1.0
- *
- * Portions Copyright (c) 1995-2002 RealNetworks, Inc. All Rights Reserved.
- *
- * The contents of this file, and the files included with this file, are
- * subject to the current version of the RealNetworks Public Source License
- * Version 1.0 (the "RPSL") available at
- * http://www.helixcommunity.org/content/rpsl unless you have licensed
- * the file under the RealNetworks Community Source License Version 1.0
- * (the "RCSL") available at http://www.helixcommunity.org/content/rcsl,
- * in which case the RCSL will apply. You may also obtain the license terms
- * directly from RealNetworks. You may not use this file except in
- * compliance with the RPSL or, if you have a valid RCSL with RealNetworks
- * applicable to this file, the RCSL. Please see the applicable RPSL or
- * RCSL for the rights, obligations and limitations governing use of the
- * contents of the file.
- *
- * This file is part of the Helix DNA Technology. RealNetworks is the
- * developer of the Original Code and owns the copyrights in the portions
- * it created.
- *
- * This file, and the files included with this file, is distributed and made
- * available on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
- * EXPRESS OR IMPLIED, AND REALNETWORKS HEREBY DISCLAIMS ALL SUCH WARRANTIES,
- * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY, FITNESS
- * FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
- *
- * Technology Compatibility Kit Test Suite(s) Location:
- * http://www.helixcommunity.org/content/tck
- *
- * Contributor(s):
- *
- * ***** END LICENSE BLOCK ***** */
- #include "hlxclib/math.h"
- #include "kaiser.h"
- #include "allresamplers.h"
- #ifndef M_PI
- #define M_PI 3.14159265358979323846
- #endif
- #define IZEROEPS (1E-21) /* max error in Izero */
- /*
- * KaiserEstim() estimates the window length and beta needed to
- * meet the given filter specifications.
- *
- * fpass, fstop are normalized freq (1.0 == Nyquist).
- * atten is stopband attenuation in dB.
- */
- void
- KaiserEstim(float fpass, float fstop, float atten, int *length, float *beta)
- {
- double d, b;
- /* estimate the required beta */
- if (atten < 21.0f) {
- b = 0.0;
- } else if (atten <= 50.0f) {
- b = 0.5842 * pow((atten-21.0), 0.4) + 0.07886 * (atten-21.0);
- } else { /* atten > 50 */
- b = 0.1102 * (atten-8.7);
- }
- *beta = (float)b;
- /* estimate the required length */
- d = (atten - 7.95) / (M_PI * 2.285);
- *length = 1 + (int)(d / (fstop - fpass));
- }
- /*
- * inf
- * Io(x) = 1 + sum(((x/2)^r / r!)^2)
- * r=1
- */
- double
- Izero(double x)
- {
- double halfx, term, sum, r;
- double temp;
- halfx = 0.5 * x;
- term = sum = r = 1.0;
- do {
- temp = halfx / r;
- term *= temp * temp;
- sum += term;
- r += 1.0;
- } while (term >= (sum * IZEROEPS));
- return sum;
- }
- /*
- * KaiserLowpass() creates a Kaiser-windowed lowpass filter.
- *
- * length is length of filter wing.
- * cutoff is normalized cutoff freq (-6dB down).
- * beta is the Kaiser window parameter.
- * gain is the desired dc gain.
- *
- * The Kaiser window is given by:
- * w[n] = Io(beta * sqrt(1 - (n/M)^2)) / Io(beta) (0 <= n <= M)
- *
- * Note: sampling of the "analog" lowpass is offset by 0.5 sample,
- * so that all phases are an even length.
- */
- void
- KaiserLowpass(int length, float cutoff, float beta, float gain, double *filter)
- {
- double ibeta, ilength, x, w;
- int i;
- ibeta = 1.0 / Izero(beta);
- ilength = 1.0 / (length - 0.5); /* last window value should be ibeta */
- for (i = 0; i < length; i++) {
- x = i + 0.5;
- /* Kaiser window */
- w = x * ilength;
- w = 1.0 - w * w;
- w = MAX(w, 0.0);
- w = Izero(beta * sqrt(w)) * ibeta;
- /* windowed ideal lowpass filter */
- filter[i] = w * gain * sin(cutoff * M_PI * x) / (M_PI * x);
- }
- }