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/source/tuner/airspy/AirspySampleAdapter.java

@@ -0,0 +1,149 @@
+package source.tuner.airspy;
+
+/*******************************************************************************
+ *     SDR Trunk 
+ *     Copyright (C) 2015 Dennis Sheirer
+ * 
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ * 
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ * 
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <http://www.gnu.org/licenses/>
+ ******************************************************************************/
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import sample.adapter.ISampleAdapter;
+
+public class AirspySampleAdapter implements ISampleAdapter
+{
+	private final static Logger mLog = LoggerFactory.getLogger( AirspySampleAdapter.class );
+
+	private static final float SCALE_SIGNED_12_BIT_TO_FLOAT = 1.0f / 2048.0f;
+	
+	private boolean mSamplePacking = false;
+
+	/**
+	 * Adapter to translate byte buffers received from the airspy tuner into 
+	 * float buffers for processing.
+	 */
+	public AirspySampleAdapter()
+	{
+	}
+
+	/**
+	 * Sample packing places two 12-bit samples into 3 bytes when enabled or 
+	 * places two 12-bit samples into 4 bytes when disabled.
+	 * 
+	 * @param enabled 
+	 */
+	public void setSamplePacking( boolean enabled )
+	{
+		mSamplePacking = enabled;
+	}
+	
+	@Override
+	public float[] convert( byte[] samples )
+	{
+		if( mSamplePacking )
+		{
+			return convertPacked( samples );
+		}
+		else
+		{
+			return convertUnpacked( samples );
+		}
+	}
+
+	/**
+	 * Converts the byte array containing unsigned 12-bit short values into
+	 * signed float values in the range -1 to 1;
+	 * 
+	 * @param data - byte array of unsigned 16-bit values
+	 * @return converted float values
+	 */
+	private float[] convertUnpacked( byte[] data )
+	{
+		float[] samples = new float[ data.length / 2 ];
+
+		int pointer = 0;
+		
+		for( int x = 0; x < data.length; x += 2 )
+		{
+			samples[ pointer++ ] = scale( ( data[ x ] & 0xFF ) | 
+										  ( data[ x + 1 ] << 8 ) );
+		}
+		
+		return samples;
+	}
+	
+	/**
+	 * Converts every 3 bytes containing a pair of 12-bit unsigned values into
+	 * a pair of float values in the range -1 to 1;
+	 * 
+	 * @param data - byte array of unsigned 12-bit values
+	 * @return converted float values
+	 */
+	private float[] convertPacked( byte[] data1 )
+	{
+		byte[] data = new byte[ data1.length ];
+
+		//Convert big-endian to little-endian
+		for( int x = 0; x < data1.length; x += 4 )
+		{
+			data[ x ] = data1[ x + 3 ];
+			data[ x + 1 ] = data1[ x + 2 ];
+			data[ x + 2 ] = data1[ x + 1 ];
+			data[ x + 3 ] = data1[ x ];
+		}
+		
+		int count = (int)( (float)data.length / 1.5f );
+
+		/* Ensure we have an even number of samples */
+		if( count % 2 == 1 )
+		{
+			count--;
+		}
+
+		int bytes = (int)((float)count * 1.5f);
+
+		float[] samples = new float[ count ];
+		
+		int pointer = 0;
+		
+		int first;
+		int second;
+		
+		for( int x = 0; x < bytes; x += 3 )
+		{
+			first = ( ( data[ x ] << 4 ) & 0xFF0 ) | 
+					( ( data[ x + 1 ] >> 4 ) & 0xF );
+
+			samples[ pointer++ ] = scale( first );
+
+			second = ( ( data[ x + 1 ] << 8 ) & 0xF00 ) |
+					   ( data[ x + 2 ] & 0xFF );
+			
+			samples[ pointer++ ] = scale( second );
+		}
+		
+		return samples;
+	}
+
+	/**
+	 * Converts unsigned 12-bit values to signed 12-bit values and then scales
+	 * the signed value to a signed float value in range: -1.0 : +1.0
+	 */
+	public static float scale( int value )
+	{
+		return (float)(( value & 0xFFF ) - 2048 ) * SCALE_SIGNED_12_BIT_TO_FLOAT;
+	}
+}