Add example program that generates resampling coefficients

examples/Makefile.in

@@ -13,7 +13,7 @@ LDADD = -lsndio @ldadd@
 # variables defined on configure script command line (if any)
 @vars@
 
-PROG = play rec fd vol cap
+PROG = play rec fd vol cap gen-fir
 
 all:		${PROG}
 
@@ -32,6 +32,9 @@ vol:		vol.o tools.o
 cap:		cap.o tools.o
 		${CC} ${LDFLAGS} ${LIB} -o cap cap.o tools.o ${LDADD}
 
+gen-fir:	gen-fir.c
+		${CC} ${LDFLAGS} ${LIB} -o gen-fir gen-fir.c -lm
+
 .c.o:
 		${CC} ${CFLAGS} ${INCLUDE} ${DEFS} -c $<
 

examples/gen-fir.c

@@ -0,0 +1,119 @@
+/*
+ * Copyright (c) 2021 Alexandre Ratchov <alex@caoua.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * This program generates coefficients for the resampling low-pass
+ * filter used in sndiod and aucat.
+ *
+ * The coefficients correspond to the filter impulse response sampled
+ * with 64 points per time unit; they are represented as 24-bit
+ * fixed-point numbers.
+ *
+ * The filter is an ideal low-pass (sinc function), multiplied by a 8
+ * time units long Blackman window. The filter cut-off frequency is
+ * set at 0.75 of the Nyquist frequency (for instance, at 48kHz, the
+ * cut-off will be at 18kHz, beyond audible frequencies).
+ *
+ * References:
+ *	https://en.wikipedia.org/wiki/Sinc_filter
+ *	https://en.wikipedia.org/wiki/Window_function
+ *	https://ccrma.stanford.edu/~jos/resample/
+ */
+
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+int order = 8, factor = 64, bits = 24;
+
+/*
+ * convert double to fixed-point integer
+ */
+int
+mkfp(double x)
+{
+	long long r, unit;
+
+	unit = 1LL << (bits - 1);
+	r = x * unit + ((x >= 0) ? 0.5 : -0.5);
+	if (r < -unit || r >= unit) {
+		fprintf(stderr, "%g: sample out of range\n", x);
+		exit(1);
+	}
+	return r;
+}
+
+/*
+ * Blackman window function
+ */
+double
+win_blackman(double a, double x)
+{
+	if (x < 0)
+		x = -x;
+	return (x >= 0.5) ? 0 :
+		0.5 * (1 - a) +
+		0.5 * cos(2 * M_PI * x) +
+		0.5 * a * cos(4 * M_PI * x);
+}
+
+/*
+ * Ideal low-pass filter response
+ */
+double
+lowpass_sinc(double cutoff, double t)
+{
+	double arg;
+
+	if (t == 0)
+		return cutoff;
+	arg = M_PI * t;
+	return sin(cutoff * arg) / arg;
+}
+
+/*
+ * Convert table index to time
+ */
+double
+timeof(int i)
+{
+	return (double)(i - order * factor / 2) / factor;
+}
+
+int
+main(void)
+{
+	double t;
+	double h;
+	double cutoff = 3. / 4.;
+	double param = 0.16;
+	int i;
+
+	printf("{");
+	for (i = 0; i <= order * factor; i++) {
+		t = timeof(i);
+		h = lowpass_sinc(cutoff, t) * win_blackman(param, t / order);
+		if (i != 0)
+			printf(",");
+		printf("%s", (i % 8 == 0) ? "\n\t" : " ");
+		printf("%7d", mkfp(h));
+	}
+	printf("\n};\n");
+
+	return 0;
+}