/************************************************************************ * *  sac.c, part of tmndecode (H.263 decoder) *  Copyright (C) 1996  Telenor R&D, Norway *        Karl Olav Lillevold <Karl.Lillevold@nta.no> * *  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. * *  Karl Olav Lillevold               <Karl.Lillevold@nta.no> *  Telenor Research and Development *  P.O.Box 83                        tel.:   +47 63 84 84 00 *  N-2007 Kjeller, Norway            fax.:   +47 63 81 00 76 * *  Robert Danielsen                  e-mail: Robert.Danielsen@nta.no *  Telenor Research and Development  www:    http://www.nta.no/brukere/DVC/ *  P.O.Box 83                        tel.:   +47 63 84 84 00 *  N-2007 Kjeller, Norway            fax.:   +47 63 81 00 76 *   ************************************************************************//********************************************************************* *        SAC Decoder Module *        Algorithm as Specified in H26P Annex -E *              (c) 1995 BT Labs * *	Author:	Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk> * *********************************************************************/#include <stdio.h>#include <string.h>#include "config.h"#include "tmndec.h"#include "global.h"#define   q1    16384#define   q2    32768#define   q3    49152#define   top   65535/* local prototypes */void bit_out_psc_layer();/********************************************************************* *        SAC Decoder Algorithm as Specified in H26P Annex -E * *        Name:        decode_a_symbol * *	Description:	Decodes an Aritmetically Encoded Symbol * *	Input:        array holding cumulative freq. data *        also uses static data for decoding endpoints *        and code_value variable * *	Returns:	Index to relevant symbol model * *	Side Effects:	Modifies low, high, length, cum and code_value * *	Author:        Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk> * *********************************************************************/ static long low, high, code_value, bit, length, sacindex, cum, zerorun=0;int decode_a_symbol(int cumul_freq[ ])//sac解碼器中，對一個符號的解碼是通過使用一個基于句法的模型并調用下面的C程序而完成的{  length = high - low + 1;  cum = (-1 + (code_value - low + 1) * cumul_freq[0]) / length;  for (sacindex = 1; cumul_freq[sacindex] > cum; sacindex++);  high = low - 1 + (length * cumul_freq[sacindex-1]) / cumul_freq[0];  low += (length * cumul_freq[sacindex]) / cumul_freq[0];  for ( ; ; ) {      if (high < q2) ;    else if (low >= q2) {      code_value -= q2;       low -= q2;       high -= q2;    }    else if (low >= q1 && high < q3) {      code_value -= q1;       low -= q1;       high -= q1;    }    else {      break;    }     low *= 2;     high = 2*high + 1;    bit_out_psc_layer();     code_value = 2*code_value + bit;  }  return (sacindex-1);} /********************************************************************* * *        Name:        decoder_reset * *	Description:	Fills Decoder FIFO after a fixed word length *        string has been detected. * *	Input:        None * *	Returns:	Nothing * *	Side Effects:	Fills Arithmetic Decoder FIFO * *	Author:        Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk> * *********************************************************************/void decoder_reset( ){  int i;  zerorun = 0;        /* clear consecutive zero's counter */  code_value = 0;  low = 0;  high = top;  for (i = 1;   i <= 16;   i++) {    bit_out_psc_layer();     code_value = 2 * code_value + bit;  }  if (trace)    printf("Arithmetic Decoder Reset \n");}/********************************************************************* * *        Name:        bit_out_psc_layer * *	Description:	Gets a bit from the Encoded Stream, Checks for *        and removes any PSC emulation prevention bits *        inserted at the decoder, provides 'zeros' to the *        Arithmetic Decoder FIFO to allow it to finish  *        data prior to the next PSC. (Garbage bits) * *	Input:        None * *	Returns:	Nothing * *	Side Effects:	Gets a bit from the Input Data Stream * *	Author:        Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk> * *********************************************************************/void bit_out_psc_layer(){  if (showbits(17)!=1) { /* check for startcode in Arithmetic Decoder FIFO */    bit = getbits(1);    if(zerorun > 13) {	/* if number of consecutive zeros = 14 */	       if (!bit) {        if (trace)          printf("PSC/GBSC, Header Data, or Encoded Stream Error \n");        zerorun = 1;              }      else { /* if there is a 'stuffing bit present */        if (trace)          printf("Removing Startcode Emulation Prevention bit \n");        bit = getbits(1);        /* overwrite the last bit */	        zerorun = !bit;        /* zerorun=1 if bit is a '0' */      }    }    else { /* if consecutive zero's not exceeded 14 */      if (!bit)        zerorun++;      else        zerorun = 0;    }  } /* end of if !(showbits(17)) */  else {    bit = 0;    if (trace)      printf("Startcode Found:Finishing Arithmetic Decoding using 'Garbage bits'\n");  }   /*	   printf("lastbit = %ld bit = %ld zerorun = %ld \n", lastbit, bit, zerorun);    lastbit = bit;   */        /* latent diagnostics */        }