FIR filter implementation for surround sound lowpass

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@3484 b3059339-0415-0410-9bf9-f77b7e298cf2

libao2/firfilter.c

@@ -0,0 +1,59 @@
+
+static double desired_7kHz_lowpass[] = {1.0, 0.0};
+static double weights_7kHz_lowpass[] = {0.1, 0.1};
+
+double *calc_coefficients_7kHz_lowpass(int rate)
+{
+  double *result = (double *)malloc(32*sizeof(double));
+  double bands[4];
+
+  bands[0] = 0.0;  bands[1] = 6800.0/rate;
+  bands[2] = 8500.0/rate;  bands[3] = 0.5;
+
+  remez(result, 32, 2, bands,
+	desired_7kHz_lowpass, weights_7kHz_lowpass, BANDPASS);
+
+  return result;
+}
+
+#if 0
+
+int16_t firfilter(int16_t *buf, int pos, int len, int count, double *coefficients)
+{
+  double result = 0.0;
+
+  // Back 32 samples, maybe wrapping in buffer.
+  pos = (pos+len-count)%len;
+  // And do the multiply-accumulate
+  while (count--) {
+    result += buf[pos++] * *coefficients++;  pos %= len;
+  }
+  return result;
+}
+
+#endif
+
+int16_t firfilter(int16_t *buf, int pos, int len, int count, double *coefficients)
+{
+  double result = 0.0;
+  int count1, count2;
+  int16_t *ptr;
+
+  if (pos >= count) {
+    pos -= count;
+    count1 = count; count2 = 0;
+  }
+  else {
+    count2 = pos;
+    count1 = count - pos;
+    pos = len - count1;
+  }
+  //fprintf(stderr, "pos=%d, count1=%d, count2=%d\n", pos, count1, count2);
+
+  // high part of window
+  ptr = &buf[pos];
+  while (count1--)  result += *ptr++ * *coefficients++;
+  // wrapped part of window
+  while (count2--)  result += *buf++ * *coefficients++;
+  return result;
+}