decoder.java

基于java开发的mp3播放器

/*
 * 1/12/99		Initial version.	[e-mail]
/*-----------------------------------------------------------------------
 *  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 2 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, write to the Free Software
 *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 *----------------------------------------------------------------------
 */



package javazoom.jl.decoder;

import java.io.IOException;


/**
 * The <code>Decoder</code> class encapsulates the details of
 * decoding an MPEG audio frame. 
 * 
 * @author	MDM	
 * @version 0.0.7 12/12/99
 * @since	0.0.5
 */

public class Decoder implements DecoderErrors
{
	static private final Params DEFAULT_PARAMS = new Params();
	
	/**
	 * The Bistream from which the MPEG audio frames are read.
	 */
	//private Bitstream				stream;
	
	/**
	 * The Obuffer instance that will receive the decoded
	 * PCM samples.
	 */
	private Obuffer			output;
		
	/**
	 * Synthesis filter for the left channel.
	 */
	private SynthesisFilter			filter1;
	
	/**
	 * Sythesis filter for the right channel.
	 */
	private SynthesisFilter			filter2;	
			
	/**
	 * The decoder used to decode layer III frames.
	 */
	private LayerIIIDecoder			l3decoder;
	
	
	private Params					params;
	
	private boolean					initialized;
		
	
	/**
	 * Creates a new <code>Decoder</code> instance with default 
	 * parameters.
	 */
	
	public Decoder()
	{
		this(null);
	}

	/**
	 * Creates a new <code>Decoder</code> instance with default 
	 * parameters.
	 * 
	 * @param params	The <code>Params</code> instance that describes
	 *					the customizable aspects of the decoder.  
	 */
	public Decoder(Params params0)
	{
		if (params0==null)
			params0 = DEFAULT_PARAMS;
	
		params = params0;
	}
	
	static public Params getDefaultParams()
	{
		return (Params)DEFAULT_PARAMS.clone();
	}
	
	/**
	 * Decodes one frame from an MPEG audio bitstream.
	 * 
	 * @param header		The header describing the frame to decode.
	 * @param bitstream		The bistream that provides the bits for te body of the frame. 
	 * 
	 * @return A SampleBuffer containing the decoded samples.
	 */
	public Obuffer decodeFrame(Header header, Bitstream stream)
		throws DecoderException
	{
		if (!initialized)
		{
			initialize(header);
		}
		
		int layer = header.layer();
		
		output.clear_buffer();
		
		FrameDecoder decoder = retrieveDecoder(header, stream, layer);
		
		decoder.decodeFrame();
		output.write_buffer(1);
		
		return output;	
	}
	
	/**
	 * Changes the output buffer. This will take effect the next time
	 * decodeFrame() is called. 
	 */
	public void setOutputBuffer(Obuffer out)
	{
		output = out;
	}
	
	protected DecoderException newDecoderException(int errorcode)
	{
		return new DecoderException(errorcode, null);
	}
	
	protected DecoderException newDecoderException(int errorcode, Throwable throwable)
	{
		return new DecoderException(errorcode, throwable);
	}
	
	protected FrameDecoder retrieveDecoder(Header header, Bitstream stream, int layer)
		throws DecoderException
	{
		FrameDecoder decoder = null;
		
		// REVIEW: allow channel output selection type
		// (LEFT, RIGHT, BOTH, DOWNMIX)
		if (layer == 3) 
		{
			if (l3decoder==null)
			{
				l3decoder = new LayerIIIDecoder(stream, 
					header, filter1, filter2, 
					output, OutputChannels.BOTH_CHANNELS);
			}						
			
			decoder = l3decoder;
		}
						
		if (decoder==null)
		{
			throw newDecoderException(UNSUPPORTED_LAYER, null);
		}
		
		return decoder;
	}
	
	private void initialize(Header header)
		throws DecoderException
	{
		
		// REVIEW: allow customizable scale factor
		float scalefactor = 32767.0f;
		
		int mode = header.mode();
		int layer = header.layer();
		int channels = mode==Header.SINGLE_CHANNEL ? 1 : 2;

		// REVIEW: remove this and expose Equalizer object instance.
		float[] eq = new float[32];
		for (int i=0; i<32; i++)
		{
			eq[i] = 1.0f;
		}
			
		// set up output buffer if not set up by client.
		if (output==null)
			output = new SampleBuffer(header.frequency(), channels);
		
		filter1 = new SynthesisFilter(0, scalefactor, eq);
   		
		// REVIEW: allow mono output for stereo
		if (channels==2) 
			filter2 = new SynthesisFilter(1, scalefactor, eq);

		initialized = true;
	}
	
	/**
	 * The <code>Params</code> class presents the customizable
	 * aspects of the decoder. 
	 * <p>
	 * Instances of this class are not thread safe. 
	 */
	public static class Params implements Cloneable
	{
		private OutputChannels	outputChannels = OutputChannels.BOTH;
		
		private Equalizer		equalizer = new Equalizer(Equalizer.PASS_THRU);
		
		public Params()
		{			
		}
		
		public Object clone()
		{
			try
			{
				return super.clone();
			}
			catch (CloneNotSupportedException ex)
			{				
				throw new InternalError(this+": "+ex);
			}
		}
				
		public void setOutputChannels(OutputChannels out)
		{
			if (out==null)
				throw new NullPointerException("out");
			
			outputChannels = out;
		}
		
		public OutputChannels getOutputChannels()
		{
			return outputChannels;
		}
		
		/**
		 * Retrieves the equalizer settings that the decoder's equalizer
		 * will be initialized from.
		 * <p>
		 * The <code>Equalizer</code> instance returned 
		 * cannot be changed in real time to affect the 
		 * decoder output as it is used only to initialize the decoders
		 * EQ settings. To affect the decoder's output in realtime,
		 * use the Equalizer returned from the getEqualizer() method on
		 * the decoder. 
		 * 
		 * @return	The <code>Equalizer</code> used to initialize the
		 *			EQ settings of the decoder. 
		 */
		public Equalizer getInitialEqualizerSettings()
		{
			return equalizer;	
		}
				
	};
}