Improved BPM detection routine to better spot harmonics for the strongest beat pattern.

include/BPMDetect.h

@@ -67,7 +67,7 @@ namespace soundtouch
 #define MIN_BPM 29
 
 /// Maximum allowed BPM rate. Used to restrict accepted result below a reasonable limit.
-#define MAX_BPM 230
+#define MAX_BPM 200
 
 
 /// Class for calculating BPM rate for audio data.

source/SoundTouch/PeakFinder.cpp

@@ -55,15 +55,46 @@ PeakFinder::PeakFinder()
 }
 
 
+// Finds real 'top' of a peak hump from neighnourhood of the given 'peakpos'.
+int PeakFinder::findTop(const float *data, int peakpos) const
+{
+    int i;
+    int start, end;
+    float refvalue;
+
+    refvalue = data[peakpos];
+
+    // seek within ±10 points
+    start = peakpos - 10;
+    if (start < minPos) start = minPos;
+    end = peakpos + 10;
+    if (end > maxPos) end = maxPos;
+
+    for (i = start; i <= end; i ++)
+    {
+        if (data[i] > refvalue)
+        {
+            peakpos = i;
+            refvalue = data[i];
+        }
+    }
+
+    // failure if max value is at edges of seek range => it's not peak, it's at slope.
+    if ((peakpos == start) || (peakpos == end)) return 0;
+
+    return peakpos;
+}
+
+
 // Finds 'ground level' of a peak hump by starting from 'peakpos' and proceeding
 // to direction defined by 'direction' until next 'hump' after minimum value will 
 // begin
 int PeakFinder::findGround(const float *data, int peakpos, int direction) const
 {
-    float refvalue;
     int lowpos;
     int pos;
     int climb_count;
+    float refvalue;
     float delta;
 
     climb_count = 0;
@@ -210,30 +241,41 @@ double PeakFinder::detectPeak(const float *data, int aminPos, int amaxPos)
     // Now check if the highest peak were in fact harmonic of the true base beat peak 
     // - sometimes the highest peak can be Nth harmonic of the true base peak yet 
     // just a slightly higher than the true base
-    for (i = 2; i < 10; i ++)
+
+    int hp = (int)(highPeak + 0.5);
+
+    for (i = 3; i < 10; i ++)
     {
-        double peaktmp, tmp;
+        double peaktmp, harmonic;
         int i1,i2;
 
-        peakpos = (int)(highPeak / (double)i + 0.5f);
+        harmonic = (double)i * 0.5;
+        peakpos = (int)(highPeak / harmonic + 0.5f);
         if (peakpos < minPos) break;
+        peakpos = findTop(data, peakpos);   // seek true local maximum index
+        if (peakpos == 0) continue;         // no local max here
 
-        // calculate mass-center of possible base peak
+        // calculate mass-center of possible harmonic peak
         peaktmp = getPeakCenter(data, peakpos);
 
+        // accept harmonic peak if 
+        // (a) it is found
+        // (b) is within ±4% of the expected harmonic interval
+        // (c) has at least half x-corr value of the max. peak
+
+        double diff = harmonic * peaktmp / highPeak;
+        if ((diff < 0.96) || (diff > 1.04)) continue;   // peak too afar from expected
+
         // now compare to highest detected peak
         i1 = (int)(highPeak + 0.5);
         i2 = (int)(peaktmp + 0.5);
-        tmp = 2 * (data[i2] - data[i1]) / (data[i2] + data[i1]);
-        if (fabs(tmp) < 0.1)
+        if (data[i2] >= 0.5*data[i1])
         {
-            // The highest peak is harmonic of almost as high base peak,
-            // thus use the base peak instead
+            // The harmonic is at least half as high primary peak,
+            // thus use the harmonic peak instead
             peak = peaktmp;
         }
     }
 
     return peak;
 }
-
-

source/SoundTouch/PeakFinder.h

@@ -63,6 +63,10 @@ protected:
                             int direction       /// Direction where to proceed from the peak: 1 = right, -1 = left.
                             ) const;
 
+    // Finds real 'top' of a peak hump from neighnourhood of the given 'peakpos'.
+    int findTop(const float *data, int peakpos) const;
+
+
     /// Finds the 'ground' level, i.e. smallest level between two neighbouring peaks, to right- 
     /// or left-hand side of the given peak position.
     int   findGround(const float *data,     /// Data vector.