File: energy.c

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/*
 *  TwoLAME: an optimized MPEG Audio Layer Two encoder
 *
 *  Copyright (C) 2001-2004 Michael Cheng
 *  Copyright (C) 2004-2006 The TwoLAME Project
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2.1 of the License, or (at your option) any later version.
 *
 *  This library 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
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 *  $Id: energy.c 158 2007-04-12 13:51:11Z nhumfrey $
 *
 */


#include <stdio.h>
#include <stdlib.h>

#include "twolame.h"
#include "common.h"
#include "bitbuffer.h"
#include "energy.h"



// Returns the desired number of ancillary bits to be 
// reserved at the end of the frame
int get_required_energy_bits( twolame_options *glopts )
{
	if (glopts->mode==TWOLAME_MONO) {
		// only 2 bytes + zero at n-3 needed for energy level for mono channel
		return 24;

	} else {
		// 5 bytes for the stereo energy info
		return 40;
	}

}


// Calculates the energy levels of current frame and
// inserts it into the end of the frame
void do_energy_levels(twolame_options *glopts, bit_stream *bs)
{
	/* Reference:
	   Using the BWF Energy Levels in AudioScience Bitstreams
	   http://www.audioscience.com/internet/download/notes/note0001_MPEG_energy.pdf
	  
	   Specification of the Broadcast Wave Format: Supplement 1 - MPEG audio 
	   http://www.ebu.ch/CMSimages/en/tec_doc_t3285_s1_tcm6-10545.pdf
	   http://www.sr.se/utveckling/tu/bwf/
	
	   The absolute peak of the PCM file for the left and right
	   channel in this frame are written into the last 5 bytes of the frame.
	
	   The last 5 bytes *must* be reserved for this to work correctly (otherwise
	   you'll be overwriting mpeg audio data)
	*/

	short int *leftpcm = glopts->buffer[0];
	short int *rightpcm = glopts->buffer[1];   
	
	int i, leftMax, rightMax;
	unsigned char rhibyte, rlobyte, lhibyte, llobyte;

	// Get the position (in butes) of the end of the mpeg audio frame
	int frameEnd = buffer_sstell(bs)/8;
	

	// find the maximum in the left and right channels
	leftMax = rightMax = -1;
	for (i=0; i<TWOLAME_SAMPLES_PER_FRAME; i++) { 
		if (abs(leftpcm[i])>leftMax)
			leftMax = abs(leftpcm[i]);
		if (abs(rightpcm[i])>rightMax)
			rightMax = abs(rightpcm[i]);
	}
	
	
	
	// fix any overflows
	if (leftMax > 32767)
		leftMax = 32767;
	
	if (rightMax > 32767)
		rightMax = 32767;
	

	
	// convert max value to hi/lo bytes and write into buffer
	lhibyte = leftMax/256;
	llobyte = leftMax - 256*lhibyte;

	// Write the left channel into the last two bytes of the frame
	bs->buf[frameEnd-1] = llobyte;
	bs->buf[frameEnd-2] = lhibyte;
    bs->buf[frameEnd-3] = 0;
	
	// Only write the right channel energy info
	// if we're in stereo mode.
	if (glopts->mode!=TWOLAME_MONO) {
		
		rhibyte = rightMax/256;
		rlobyte = rightMax - 256*rhibyte;
		
		bs->buf[frameEnd-4] = rlobyte;
		bs->buf[frameEnd-5] = rhibyte;
	}
	

}


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