Added FIR filter to libfilter

src/libfilter/Makefile.am

@@ -3,4 +3,6 @@ AM_CPPFLAGS = -Wall -Wextra -g $(all_includes)
 noinst_LIBRARIES = libfilter.a
 
 libfilter_a_SOURCES = \
-	iir_filter.c
+	iir_filter.c \
+	fir_filter.c
+

src/libfilter/fir_filter.c

@@ -0,0 +1,197 @@
+/* FIR filter
+ *
+ * (C) 2017 by Andreas Eversberg <jolly@eversberg.eu>
+ * All Rights Reserved
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+#include <stdlib.h>
+#include <math.h>
+#include "../libsample/sample.h"
+#include "fir_filter.h"
+
+//#define DEBUG_TAPS
+
+static void kernel(double *taps, int M, double cutoff, int invert)
+{
+	int i;
+	double sum;
+
+	for (i = 0; i <= M; i++) {
+		/* gen sinc */
+		if (i == M / 2)
+			taps[i] = 2.0 * M_PI * cutoff;
+		else
+			taps[i] = sin(2.0 * M_PI * cutoff * (double)(i - M / 2))
+				/ (double)(i - M / 2);
+		/* blackman window */
+		taps[i] *= 0.42 - 0.50 * cos(2 * M_PI * (double)(i / M))
+				+ 0.08 * cos(4 * M_PI * (double)(i / M));
+	}
+
+	/* normalize */
+	sum = 0;
+	for (i = 0; i <= M; i++)
+		sum += taps[i];
+	for (i = 0; i <= M; i++)
+		taps[i] /= sum;
+
+	/* invert */
+	if (invert) {
+		for (i = 0; i <= M; i++)
+			taps[i] = -taps[i];
+		taps[M / 2] += 1.0;
+	}
+
+#ifdef DEBUG_TAPS
+	puts("start");
+	for (i = 0; i <= M; i++)
+		puts(debug_amplitude(taps[i]));
+#endif
+}
+
+static fir_filter_t *fir_init(double samplerate, double transition_bandwidth)
+{
+	fir_filter_t *fir;
+	int M;
+
+	/* alloc struct */
+	fir = calloc(1, sizeof(*fir));
+	if (!fir) {
+		fprintf(stderr, "No memory creating FIR filter!\n");
+		return NULL;
+	}
+
+	/* transition bandwidth */
+	M = ceil(1.0 / (transition_bandwidth / samplerate));
+	if ((M & 1))
+		M++;
+
+//	printf("cutoff=%.4f\n", cutoff / samplerate);
+//	printf("tb=%.4f\n", transition_bandwidth / samplerate);
+	fir->ntaps = M + 1;
+	fir->delay = M / 2;
+
+	/* alloc taps */
+	fir->taps = calloc(fir->ntaps, sizeof(*fir->taps));
+	if (!fir->taps) {
+		fprintf(stderr, "No memory creating FIR filter!\n");
+		fir_exit(fir);
+		return NULL;
+	}
+
+	/* alloc ring buffer */
+	fir->buffer = calloc(fir->ntaps, sizeof(*fir->buffer));
+	if (!fir->buffer) {
+		fprintf(stderr, "No memory creating FIR filter!\n");
+		fir_exit(fir);
+		return NULL;
+	}
+
+
+	return fir;
+}
+
+fir_filter_t *fir_lowpass_init(double samplerate, double cutoff, double transition_bandwidth)
+{
+	/* calculate kernel */
+	fir_filter_t *fir =  fir_init(samplerate, transition_bandwidth);
+	if (!fir)
+		return NULL;
+	kernel(fir->taps, fir->ntaps - 1, cutoff / samplerate, 0);
+	return fir;
+}
+
+fir_filter_t *fir_highpass_init(double samplerate, double cutoff, double transition_bandwidth)
+{
+	fir_filter_t *fir =  fir_init(samplerate, transition_bandwidth);
+	if (!fir)
+		return NULL;
+	kernel(fir->taps, fir->ntaps - 1, cutoff / samplerate, 1);
+	return fir;
+}
+
+fir_filter_t *fir_allpass_init(double samplerate, double transition_bandwidth)
+{
+	fir_filter_t *fir =  fir_init(samplerate, transition_bandwidth);
+	if (!fir)
+		return NULL;
+	fir->taps[(fir->ntaps - 1) / 2] = 1.0;
+	return fir;
+}
+
+fir_filter_t *fir_twopass_init(double samplerate, double cutoff_low, double cutoff_high, double transition_bandwidth)
+{
+	int i;
+	double sum;
+	fir_filter_t *fir =  fir_init(samplerate, transition_bandwidth);
+	if (!fir)
+		return NULL;
+	double lp_taps[fir->ntaps], hp_taps[fir->ntaps];
+	kernel(lp_taps, fir->ntaps - 1, cutoff_low / samplerate, 0);
+	kernel(hp_taps, fir->ntaps - 1, cutoff_high / samplerate, 1);
+	sum = 0;
+	printf("#warning does not work as expected\n");
+	abort();
+	for (i = 0; i < fir->ntaps; i++) {
+		fir->taps[i] = lp_taps[i] + hp_taps[i];
+		sum += fir->taps[i];
+	}
+	/* hp will die */
+//	for (i = 0; i < fir->ntaps; i++)
+	//	fir->taps[i] /= sum;
+	return fir;
+}
+
+void fir_exit(fir_filter_t *fir)
+{
+	if (!fir)
+		return;
+	free(fir->taps);
+	free(fir->buffer);
+	free(fir);
+}
+
+void fir_process(fir_filter_t *fir, sample_t *samples, int num)
+{
+	int i, j;
+	double y;
+
+	for (i = 0; i < num; i++) {
+		/* put sample in ring buffer */
+		fir->buffer[fir->buffer_pos] = samples[i];
+		if (++fir->buffer_pos == fir->ntaps)
+			fir->buffer_pos = 0;
+
+		/* convolve samples */
+		y = 0;
+		for (j = 0; j < fir->ntaps; j++) {
+			/* convolve sample from ring buffer, starting with oldes */
+			y += fir->buffer[fir->buffer_pos] * fir->taps[j];
+			if (++fir->buffer_pos == fir->ntaps)
+				fir->buffer_pos = 0;
+		}
+		samples[i] = y;
+	}
+}
+
+int fir_get_delay(fir_filter_t *fir)
+{
+	return fir->delay;
+}
+

src/libfilter/fir_filter.h

@@ -0,0 +1,21 @@
+#ifndef _FIR_FILTER_H
+#define _FIR_FILTER_H
+
+typedef struct fir_filter {
+	int	ntaps;
+	int	delay;
+	double	*taps;
+	double	*buffer;
+	int	buffer_pos;
+} fir_filter_t;
+
+fir_filter_t *fir_lowpass_init(double samplerate, double cutoff, double transition_bandwidth);
+fir_filter_t *fir_highpass_init(double samplerate, double cutoff, double transition_bandwidth);
+fir_filter_t *fir_allpass_init(double samplerate, double transition_bandwidth);
+fir_filter_t *fir_twopass_init(double samplerate, double cutoff_low, double cutoff_high, double transition_bandwidth);
+void fir_exit(fir_filter_t *fir);
+void fir_process(fir_filter_t *fir, sample_t *samples, int num);
+int fir_get_delay(fir_filter_t *fir);
+
+#endif /* _FIR_FILTER_H */
+

src/test/test_filter.c

@@ -4,6 +4,7 @@
 #include <string.h>
 #include "../libsample/sample.h"
 #include "../libfilter/iir_filter.h"
+#include "../libfilter/fir_filter.h"
 #include "../libdebug/debug.h"
 
 #define level2db(level)		(20 * log10(level))
@@ -40,6 +41,7 @@ int main(void)
 {
 	iir_filter_t filter_low;
 	iir_filter_t filter_high;
+	fir_filter_t	*fir_low/*, *fir_high*/;
 	sample_t samples[SAMPLERATE];
 	double level;
 	int iter = 2;
@@ -102,6 +104,46 @@ int main(void)
 			printf("\n");
 	}
 
+	double freq = 2000.0;
+	double tb = 400.0;
+	printf("testing low-pass FIR filter with %.0fHz transition bandwidth\n", tb);
+
+	fir_low = fir_lowpass_init(SAMPLERATE, freq, tb);
+	printf("Using %d taps\n", fir_low->ntaps);
+
+	for (i = 0; i < 4001; i += 100) {
+		gen_samples(samples, (double)i);
+		fir_process(fir_low, samples, SAMPLERATE);
+		level = get_level(samples);
+		printf("%s%s%4d Hz: %.1f dB", debug_amplitude(level), debug_db(level), i, level2db(level));
+		if (i == freq)
+			printf(" cutoff\n");
+		else
+			printf("\n");
+	}
+	fir_exit(fir_low);
+
+#if 0
+	double freq1 = 1000.0, freq2 = 2000.0;
+	tb = 100.0;
+	printf("testing two-pass FIR filter\n");
+
+	fir_high = fir_twopass_init(SAMPLERATE, freq1, freq2, tb);
+	printf("Using %d taps\n", fir_high->ntaps);
+
+	for (i = 0; i < 4001; i += 100) {
+		gen_samples(samples, (double)i);
+		fir_process(fir_high, samples, SAMPLERATE);
+		level = get_level(samples);
+		printf("%s%s%4d Hz: %.1f dB", debug_amplitude(level), debug_db(level), i, level2db(level));
+		if (i == freq1 || i == freq2)
+			printf(" cutoff\n");
+		else
+			printf("\n");
+	}
+	fir_exit(fir_high);
+#endif
+
 	return 0;
 }