/*******************************************************************************
 *     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/>
 ******************************************************************************/
package spectrum.converter;


/**
 * Converts complex DFT output to scaled dB values with a maximum amplitude of
 * 0 dB and all values scaled to the minimum dB value which is:
 * 
 * Max: 0dB
 * Min: 20 * log10( 1.0 / ( 2.0 ^ ( bit depth - 1) ) = scaling factor
 * 
 * Notes:
 * 
 * The magnitude of each DFT output bin must be scaled by the number of DFT 
 * points to normalize the output to a range of ( 0 to 1.0 ).  DFT bin magnitude
 * can be calculated as:
 * 
 * 1) sqrt( bin[ x ] * bin[ x ] + bin[ x + 1 ] * bin[ x + 1] ) / DFTSize or,
 * 2) ( bin[ x ] * bin[ x ] + bin[ x + 1 ] * bin[ x + 1] ) / ( DFTSize * DFTSize )
 * 
 * Convert each scaled DFT bin magnitude to decibels using this formula:
 * 
 * 20 * log10( scaledBinValue )
 * 
 * Max Value: 20 * log10( 1.0 ) = 0 dB
 * 
 * When plotting, these dB bin values should be scaled according to the maximum
 * dynamic range supported by the source.  Maximum dynamic range for bit depth
 * is calculated as:
 * 
 * 20 * log10( 1 / ( 2 ^ ( bit depth - 1 ) )
 * 
 * A source that provides 12-bit samples can produce values in the range of 
 * ( 0 to 4096 ) or -2048 to 2047.  The smallest observable value would then
 * be 1 / 2048.
 * 
 * So, the smallest decibel value of a source producing 12 bit samples is:
 * 
 * 20 * log10( 1 / 2048 ) = -66.23 dB
 * 
 * and the dynamic range is:  (-66.23 to 0.0 dB)
 * 
 */
public class ComplexDecibelConverter extends DFTResultsConverter
{
	/**
	 * Converts the output of the JTransforms FloatFFT_1D.complexForward()
	 * calculation into the power spectrum in decibels, normalized to the 
	 * sample bit depth.
	 */
	public ComplexDecibelConverter()
	{
	}
	
	@Override
    public void receive( float[] results )
    {
		int halfResults = results.length / 2;
		
//		float dftBinSizeScalor = 1.0f / (float)Math.pow( halfResults, 2.0 );
		float dftBinSizeScalor = 1.0f / (float)halfResults;
		
		float[] processed = new float[ halfResults ];

		int middle = processed.length / 2;
		
		for( int x = 0; x < results.length; x += 2 )
		{
			//Calculate the magnitude squared (power) value from each bin's real 
			//and imaginary value and scale it to the DFT bin size squared.
			//Convert the scaled value to decibels.
			float decibels = 10.0f * (float)Math.log10( 
				( ( results[ x ] * results[ x ] ) + 
				  ( results[ x + 1 ] * results[ x + 1 ] ) ) * dftBinSizeScalor ); 

			// We have to swap the upper and lower halves of the JTransforms
			// DFT results for correct display
			
			int index = x / 2;
			
			if( index >= middle )
			{
				processed[ index - middle ] = decibels;
			}
			else
			{
				processed[ index + middle ] = decibels;
			}
		}
		
		dispatch( processed );
    }
}