Adds airspy USB tuner support and updates the spectral display

component.

-Airspy supported with 10 and 2.5 MSps sample rates.  Additional
decimation may be added later.

-Spectral display updated to include smoothing and filtering options.

Note: the decimated output of a tuner channel from the airspy is
48,076.9 samples per second, versus the normal 48 kHz decimated rate.
This is 1.0015 times the normal rate and doesn't appear to impact the
decoders or audio processing.  Therefore, no fractional resampling has
been added.

src/spectrum/converter/RealDecibelConverter.java

@@ -1,7 +1,31 @@
 package spectrum.converter;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 public class RealDecibelConverter extends DFTResultsConverter
 {
+	private static final Logger mLog = LoggerFactory.getLogger( RealDecibelConverter.class );
+
+	private float mDynamicRangeReference;
+
+	public RealDecibelConverter()
+	{
+		setSampleSize( 24.0 );
+	}
+	
+	/**
+	 * Specifies the bit depth to establish the maximum dynamic range.  All
+	 * FFT bin values will be scaled according to this value.
+	 */
+	@Override
+	public void setSampleSize( double size )
+	{
+		assert( 2.0 <= size && size <= 32.0 );
+		
+		mDynamicRangeReference = (float)Math.pow( 2.0d, size );
+	}
+
 	/**
 	 * Converts the output of the JTransforms FloatFFT_1D.realForward()
 	 * calculation into a normalized power spectrum in decibels, per description
@@ -10,29 +34,22 @@ public class RealDecibelConverter extends DFTResultsConverter
 	@Override
     public void receive( float[] results )
     {
-		float dc = Math.abs( results[ 0 ] );
+		float[] processed = new float[ results.length / 4 ];
 
-		/* Ensure we have power in the DC power bin (bin 0), otherwise don't
-		 * dispatch the results */
-		if( dc != 0 )
+		int index = 0;
+		
+		for( int x = 0; x < processed.length; x ++ )
 		{
-			/* Note: we're only processing 1/4 of the output samples */
-			float[] processed = new float[ results.length / 4 + 1 ];
+			index = x * 2;
 			
-			for( int x = 0; x < processed.length - 2; x ++ )
-			{
-				int index = ( x + 1 ) * 2;
+			float normalizedMagnitude = 10.0f * (float)Math.log10( 
+				( ( results[ index ] * results[ index ] ) + 
+				  ( results[ index + 1 ] * results[ index + 1 ] ) ) / 
+				  		mDynamicRangeReference );
 
-				double magnitude = ( ( results[ index ] * results[ index ] ) +
-						   ( results[ index + 1 ] * results[ index + 1 ] ) );
-				
-				processed[ x ] = 10.0f * 
-						(float)( Math.log10( magnitude / dc ) );
-			}
-
-			processed[ processed.length - 1 ] = processed[ 0 ];
-
-			dispatch( processed );
+			processed[ x ] = normalizedMagnitude;
 		}
+
+		dispatch( processed );
     }
 }