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POJava algorithm for normalizing audio
 

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    <p>I'm trying to normalize an audio file of speech.</p> <p>Specifically, where an audio file contains peaks in volume, I'm trying to level it out, so the quiet sections are louder, and the peaks are quieter.</p> <p>I know very little about audio manipulation, beyond what I've learnt from working on this task. Also, my math is embarrassingly weak.</p> <p>I've done some research, and the Xuggle site provides a sample which shows reducing the volume using the following code: (<a href="http://build.xuggle.com/view/Stable/job/xuggler_jdk5_stable/ws/workingcopy/src/com/xuggle/mediatool/demos/ModifyAudioAndVideo.java">full version here</a>)</p> <pre><code>@Override public void onAudioSamples(IAudioSamplesEvent event) { // get the raw audio byes and adjust it's value ShortBuffer buffer = event.getAudioSamples().getByteBuffer().asShortBuffer(); for (int i = 0; i &lt; buffer.limit(); ++i) buffer.put(i, (short)(buffer.get(i) * mVolume)); super.onAudioSamples(event); } </code></pre> <p>Here, they modify the bytes in <code>getAudioSamples()</code> by a constant of <code>mVolume</code>.</p> <p>Building on this approach, I've attempted a normalisation modifies the bytes in <code>getAudioSamples()</code> to a normalised value, considering the max/min in the file. (See below for details). I have a simple filter to leave "silence" alone (ie., anything below a value).</p> <p>I'm finding that the output file is <strong><em>very</em></strong> noisy (ie., the quality is seriously degraded). I assume that the error is either in my normalisation algorithim, or the way I manipulate the bytes. However, I'm unsure of where to go next.</p> <p>Here's an abridged version of what I'm currently doing.</p> <h3>Step 1: Find peaks in file:</h3> <p>Reads the full audio file, and finds this highest and lowest values of <code>buffer.get()</code> for all AudioSamples</p> <pre><code> @Override public void onAudioSamples(IAudioSamplesEvent event) { IAudioSamples audioSamples = event.getAudioSamples(); ShortBuffer buffer = audioSamples.getByteBuffer().asShortBuffer(); short min = Short.MAX_VALUE; short max = Short.MIN_VALUE; for (int i = 0; i &lt; buffer.limit(); ++i) { short value = buffer.get(i); min = (short) Math.min(min, value); max = (short) Math.max(max, value); } // assign of min/max ommitted for brevity. super.onAudioSamples(event); } </code></pre> <h3>Step 2: Normalize all values:</h3> <p>In a loop similar to step1, replace the buffer with normalized values, calling:</p> <pre><code> buffer.put(i, normalize(buffer.get(i)); public short normalize(short value) { if (isBackgroundNoise(value)) return value; short rawMin = // min from step1 short rawMax = // max from step1 short targetRangeMin = 1000; short targetRangeMax = 8000; int abs = Math.abs(value); double a = (abs - rawMin) * (targetRangeMax - targetRangeMin); double b = (rawMax - rawMin); double result = targetRangeMin + ( a/b ); // Copy the sign of value to result. result = Math.copySign(result,value); return (short) result; } </code></pre> <h2>Questions:</h2> <ul> <li>Is this a valid approach for attempting to normalize an audio file?</li> <li>Is my math in <code>normalize()</code> valid?</li> <li>Why would this cause the file to become noisy, where a similar approach in the demo code doesn't?</li> </ul>
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    1. CO"I'm trying to level it out, so the quiet sections are louder, and the peaks are quieter." - That's called [dynamic range compression](http://en.wikipedia.org/wiki/Dynamic_range_compression), which is not the same as normalization.
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