Adds de-emphasis filter to P25 decoded IMBE audio output.

src/dsp/filter/iir/DeemphasisFilter.java

@@ -0,0 +1,60 @@
+package dsp.filter.iir;
+
+import sample.real.RealBuffer;
+
+public class DeemphasisFilter
+{
+	private static final float MAX_SAMPLE_VALUE = 0.95f;
+	private float mAlpha;
+	private float mGain;
+	private float mPrevious = 0.0f;
+	
+	public DeemphasisFilter( float sampleRate, float cutoff, float gain )
+	{
+		mAlpha = (float)Math.exp( -2.0 * Math.PI * cutoff * ( 1.0 / sampleRate ) );
+		mGain = gain;
+	}
+	
+	public float filter( float sample )
+	{
+		mPrevious = sample + ( mAlpha * mPrevious );
+		
+		return declip( mPrevious * mGain );
+	}
+	
+	private float declip( float value )
+	{
+		if( value > MAX_SAMPLE_VALUE )
+		{
+			return MAX_SAMPLE_VALUE;
+		}
+		else if( value < -MAX_SAMPLE_VALUE )
+		{
+			return -MAX_SAMPLE_VALUE;
+		}
+		
+		else return value;
+	}
+	
+	public float[] filter( float[] samples )
+	{
+		for( int x = 0; x < samples.length; x++ )
+		{
+			samples[ x ] = filter( samples[ x ] );
+		}
+		
+		return samples;
+	}
+	
+	public RealBuffer filter( RealBuffer buffer )
+	{
+		float[] samples = buffer.getSamples();
+		
+		for( int x = 0; x < samples.length; x++ )
+		{
+			samples[ x ] = filter( samples[ x ] );
+		}
+
+		return buffer;
+	}
+}