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  Automatic sample loop finder  (Read 1460 times)
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Offline krasse
« Posted 2011-03-10 09:36:28 »

Here is a function I use to find a suitable loop in a sample. It uses a moving window to find two sample points that have similar surrounding values.

You'll have to download gervill.jar to get the AudioFloatConverter class or provide a byte-to-float converter yourself.

You can also modify some of the constants in the code (larger -> better loops perhaps + slower): windowSize, endSteps, startSteps

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   static long[] calculateLoopStartAndLength(AudioFormat format,
         byte[] sampleData, long initialLoopStart, long initialLoopLength) {
      long[] result = new long[2];

      AudioFloatConverter converter = AudioFloatConverter
            .getConverter(format);
      float[] floatData = new float[sampleData.length
            / (format.getFrameSize())];
      converter.toFloatArray(sampleData, floatData);

      int windowSize = 21;
      int endSteps = 2000;
      int startSteps = 2000;
      int bestEndIndex = (int) (initialLoopStart + initialLoopLength);
      int bestStartIndex = (int) initialLoopStart;
      double smallestValue = Double.POSITIVE_INFINITY;
      for (int i = -endSteps / 2; i < endSteps / 2; i++) {
         int endIndex = (int) (initialLoopStart + initialLoopLength + i);

         if (endIndex - windowSize / 2 < 0
               || endIndex + windowSize / 2 >= floatData.length) {
            continue;
         }
         for (int j = -startSteps / 2; j < startSteps / 2; j++) {
            int startIndex = (int) (initialLoopStart + j);
            // Calculate the weighted difference sum
            if (startIndex - windowSize / 2 < 0
                  || startIndex + windowSize / 2 >= floatData.length) {
               continue;
            }

            double sum = 0.0;
            for (int k = -windowSize / 2; k <= windowSize / 2; k++) {
               double endValue = floatData[endIndex + k];
               double weight = 1.0;
               if (k != 0) {
                  weight = Math.abs(1.0 / (double) (2.0 * k));
               }
               double startValue = floatData[startIndex + k];
               sum += weight * Math.abs(endValue - startValue);
            }
            if (sum < smallestValue) {
               smallestValue = sum;
               bestEndIndex = endIndex;
               bestStartIndex = startIndex;
            }
         }
      }
      result[0] = bestStartIndex;
      result[1] = bestEndIndex - bestStartIndex;
      return result;
   }

Offline CommanderKeith
« Reply #1 - Posted 2011-03-10 13:36:29 »

So it's used for finding a good start and end place to make a repeating loop?

Sounds pretty useful, thanks for the code.

If you get time, it'd be cool to see a simple use-case.

Offline krasse
« Reply #2 - Posted 2011-03-10 14:13:35 »

So it's used for finding a good start and end place to make a repeating loop?

Yes that is correct.

It tries to find similar surroundings for two places in a sample. The similarity at the two loop points is valued the most with the weight variable that decreases with distance.
There are a lot of possibilities for exploring other weights and also trying to get away from that nasty cubic-like complexity.

Some kind of use-case might pop up some day Smiley

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