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authorMathias Agopian <mathias@google.com>2007-08-23 03:16:02 -0700
committerGlenn Kasten <gkasten@google.com>2012-10-17 11:24:23 -0700
commit4b8a3d8a89814dc3fb365f18d01733e26eb495a1 (patch)
treeb2332f82504febf88d9fa672f6833cf37a78d83d /tools/resampler_tools
parenta923dd4e6c14f5cba2fc5f6f152619ec6bc70c5d (diff)
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This is a tool to compute the the reconstruction filter coefficients for a sinc audio resampler.
Change-Id: I99be2505139b8e0e7647200e1647509d4f7e6067 Signed-off-by: Glenn Kasten <gkasten@google.com>
Diffstat (limited to 'tools/resampler_tools')
-rw-r--r--tools/resampler_tools/Makefile17
-rw-r--r--tools/resampler_tools/fir.cpp109
2 files changed, 126 insertions, 0 deletions
diff --git a/tools/resampler_tools/Makefile b/tools/resampler_tools/Makefile
new file mode 100644
index 0000000..28bd993
--- /dev/null
+++ b/tools/resampler_tools/Makefile
@@ -0,0 +1,17 @@
+# Copyright 2005 Google Inc. All Rights Reserved.
+#
+# Makefile for resampler_tools
+#
+
+
+LOCAL_PATH:= $(call my-dir)
+include $(CLEAR_VARS)
+
+LOCAL_SRC_FILES := \
+ fir.cpp
+
+LOCAL_TARGET := fir
+
+include $(BUILD_HOST_EXECUTABLE)
+
+
diff --git a/tools/resampler_tools/fir.cpp b/tools/resampler_tools/fir.cpp
new file mode 100644
index 0000000..919a97b
--- /dev/null
+++ b/tools/resampler_tools/fir.cpp
@@ -0,0 +1,109 @@
+/*
+ * Copyright (C) 2007 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <math.h>
+#include <stdio.h>
+
+static double sinc(double x) {
+ if (fabs(x) == 0.0f) return 1.0f;
+ return sin(x) / x;
+}
+
+static double sqr(double x) {
+ return x*x;
+}
+
+static double I0(double x) {
+ // from the Numerical Recipes in C p. 237
+ double ax,ans,y;
+ ax=fabs(x);
+ if (ax < 3.75) {
+ y=x/3.75;
+ y*=y;
+ ans=1.0+y*(3.5156229+y*(3.0899424+y*(1.2067492
+ +y*(0.2659732+y*(0.360768e-1+y*0.45813e-2)))));
+ } else {
+ y=3.75/ax;
+ ans=(exp(ax)/sqrt(ax))*(0.39894228+y*(0.1328592e-1
+ +y*(0.225319e-2+y*(-0.157565e-2+y*(0.916281e-2
+ +y*(-0.2057706e-1+y*(0.2635537e-1+y*(-0.1647633e-1
+ +y*0.392377e-2))))))));
+ }
+ return ans;
+}
+
+static double kaiser(int k, int N, double alpha) {
+ if (k < 0 || k > N)
+ return 0;
+ return I0(M_PI*alpha * sqrt(1.0 - sqr((2.0*k)/N - 1.0))) / I0(M_PI*alpha);
+}
+
+int main(int argc, char** argv)
+{
+ // nc is the number of bits to store the coefficients
+ int nc = 16;
+
+ // ni is the minimum number of bits needed for interpolation
+ // (not used for generating the coefficients)
+ const int ni = nc / 2;
+
+ // nzc is the number of zero-crossing on one half of the filter
+ int nzc = 12;
+
+ // alpha parameter of the kaiser window
+ // Larger numbers reduce ripples in the rejection band but increase
+ // the width of the transition band. In reality there doesn't seem to be
+ // a good reason to choose a big number because of the limited range
+ // of our coefficients (16 bits).
+ double alpha = 3.0;
+
+ // cut off frequency ratio Fc/Fs
+ double Fcr = 20000.0 / 44100.0;
+
+ // 2^nz is the number coefficients per zero-crossing
+ // (int theory this should be 1<<(nc/2))
+ const int nz = 4;
+
+ // total number of coefficients
+ const int N = (1 << 4) * nzc;
+
+ // generate the right half of the filter
+
+ printf("const int32_t RESAMPLE_FIR_SIZE = %d;\n", N);
+ printf("const int32_t RESAMPLE_FIR_NUM_COEF = %d;\n", nzc);
+ printf("const int32_t RESAMPLE_FIR_COEF_BITS = %d;\n", nc);
+ printf("const int32_t RESAMPLE_FIR_LERP_FRAC_BITS = %d;\n", ni);
+ printf("const int32_t RESAMPLE_FIR_LERP_INT_BITS = %d;\n", nz);
+ printf("\n");
+ printf("static int16_t resampleFIR[%d] = {", N);
+ for (int i=0 ; i<N ; i++)
+ {
+ double x = (2.0 * M_PI * i * Fcr) / (1 << 4);
+ double y = kaiser(i+N, 2*N, alpha) * sinc(x);
+
+ int yi = floor(y * (1<<(nc-1)) + 0.5);
+ if (yi >= (1<<(nc-1))) yi = (1<<(nc-1))-1;
+
+ //printf("%6d\n", yi);
+
+ if ((i % (1 << 4)) == 0) printf("\n ");
+ printf("0x%04x, ", yi & 0xFFFF);
+ }
+ printf("\n};\n");
+
+ return 0;
+ }
+ \ No newline at end of file