/*  *    dither.c * *	Copyright (C) Aaron Holtzman - May 1999 * *  This file is part of ac3dec, a free Dolby AC-3 stream decoder. *	 *  ac3dec 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, or (at your option) *  any later version. *    *  ac3dec 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 GNU Make; see the file COPYING.  If not, write to *  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.  * */#include <stdlib.h>#include <stdio.h>#include "ac3.h"#include "dither.h"static uint_32 lfsr_state = 1;/*  * Generate eight bits of pseudo-entropy using a 16 bit linear * feedback shift register (LFSR). The primitive polynomial used * is 1 + x^4 + x^14 + x^16. * * The distribution is uniform, over the range [-0.707,0.707] * */uint_16 dither_gen(void){	int i;	uint_32 state;	//explicitly bring the state into a local var as gcc > 3.0?	//doesn't know how to optimize out the stores	state = lfsr_state;	//Generate eight pseudo random bits	for(i=0;i<8;i++)	{		state <<= 1;			if(state & 0x10000)			state ^= 0xa011;	}	lfsr_state = state;	return (((((sint_32)state<<8)>>8) * (sint_32) (0.707106 * 256.0))>>16);}