eid.c

Reference Implementation of G.711 standard and other voice codecs

                       with the EID hardware and Rec.G.192. Code not
                       changed <[e-mail]>
 ============================================================================
*/
void BER_insertion_stl92(lseg, xbuff,ybuff, EPbuff)
    long lseg;
    short *xbuff; 
    short *ybuff; 
    short *EPbuff;
{
   long i;
 
  /* FIRST STEP: Copy SYNC-word -> output buffer */
    ybuff[0] = xbuff[0];

  /* THEN: Process data bits */
    /*    for (i=1; i<lseg+1; i++) This was in the STL92, but this is wrong!
     */
    for (i=1; i<lseg; i++)
    {   
      if (xbuff[i]==(short)0x007F)
        {
          /* input bit = '0' ('hard bit' = 7F) */
          ybuff[i] = (short)(0x00FF) &   EPbuff[i-1];
        }
      else
        {  
          /* input bit = '1' ('hard bit' = 81) */
          ybuff[i] = (short)(0x00FF) & (~EPbuff[i-1] + 1);
        }
    }
}
/* ..................... End of BER_insertion_stl92() ..................... */

 
 
/*                                                                            
  ============================================================================ 

        void BER_insertion_stl96 (long lseg, short *xbuff, short *ybuff, 
        ~~~~~~~~~~~~~~~~~~~~~~~~  short *error_pattern);

        Description:
        ~~~~~~~~~~~~

        Disturbing an input bitstream according stored error
        patterns. The input (undisturbed) and output (disturbed)
        buffers have samples conforming to the bitstream
        representation description in Annex B of G.192. This bitstream
        has a synchronism (sync) header at frame boundaries, which
        consists of a synchronism (SYNC) word followed by a 16-bit,
        2-complement word with the number of softbits in the frame
        (i.e., the frame length). The error-pattern does not have the
        sync header.

        This function is aliased to BER_insertion by default in
        eid.h. If the pre-STL96 version is desired, the module needs
        to be compiled with the symbol STL92 defined.

        ---------------------------------------------------------------

                         NOTES on Data Representation:
                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

        Each individual bit is stored in one location of a  short-array,
        where in location 0 the SYNC-word is  stored (SYNC-word =
        0x6B21, 0x6B22, ... 0x6B2F).  To allow the processing of
        'soft-bits' (see separate  ITU-T-paper) in future applications,
        the following  definitions have been made: 

        a) input signal: 'hard' bit '0' must be represented as 0x007F 
                         'hard' bit '1' must be represented as 0x0081 

        b) error pattern: 
        * probability for undisturbed transmission: 
            0x0001 ... 0x007F       (0x0001: lowest probability that the 
            or   1 ... 127 dec.      transmission has been indeed undisturbed)

        * probability for disturbed transmission: 
            0x00FF ... 0x0081       (0x00FF: lowest probability that the
            or 255 ... 129 dec.      transmission has been really disturbed)


        c) output signal computation: 

        For input '1' (0x0081): 
          * if error pattern 0x00FF ... 0x0081 (255 ... 129), then
                   output = 0x0001 ... 0x007F  (  1 ... 127);
          * if error pattern 0x0001 ... 0x007F (  1 ... 127), then 
                   output = 0x00FF ... 0x0081  (255 ... 129).

        For input '0' (0x007F): 
          * if error pattern 0x00FF ... 0x0081 (255 ... 129), then 
                   output = 0x00FF ... 0x0081  (255 ... 129);
          * if error pattern 0x0001 ... 0x007F (  1 ... 127), then
                   output = 0x0001 ... 0x007F  (  1 ... 127).
        ---------------------------------------------------------------


        Parameters:  
        ~~~~~~~~~~~

        lseg: ..... length of current frame (WITH sync header). This 
                    has been MODIFIED from the definition in the
                    pre-STL96 version of this function. Therefore lseg
                    is the number of softbits in the input bitstream
                    plus two (to account for the sync word and for the
                    length word).

        xbuff: .... buffer with input G.192 bitstream (length = "lseg")

        ybuff: .... buffer with output G.192 bitstream (length = "lseg")

        EPbuff: ... buffer with error pattern (without sync header)
                    (length = "lseg-2") 

        Return value:
        ~~~~~~~~~~~~~
        None.

        Author:
        ~~~~~~~
        Simao Ferraz de Campos Neto
        Comsat Laboratories                  Tel:    [phone]
        22300 Comsat Drive                   Fax:    [phone]
        Clarksburg MD 20871 - USA            E-mail: [e-mail]

        History:
        ~~~~~~~~
        07.Mar.96 v1.0 Created based on the STL92 version of this function, 
                       which did not comply with the bitstream data
                       representation in Annex B of G.192.
                       <[e-mail]>
 ============================================================================
*/
void BER_insertion_stl96(lseg, xbuff,ybuff, EPbuff)
    long lseg;
    short *xbuff; 
    short *ybuff; 
    short *EPbuff;
{
  register long i;
 
  /* FIRST STEP: Copy sync header to the output buffer */
  ybuff[0] = xbuff[0];
  ybuff[1] = xbuff[1];

  /* THEN: Process data bits */
  for (i=2; i<lseg; i++)
  {   
    if (xbuff[i]==(short)0x007F)
    {
      /* input bit = '0' ('hard bit' = 7F) */
      ybuff[i] = (short)(0x00FF) &   EPbuff[i-2];
    }
    else
    {  
      /* input bit = '1' ('hard bit' = 81) */
      ybuff[i] = (short)(0x00FF) & (~EPbuff[i-2] + 1);
    }
  }
}
/* ..................... End of BER_insertion_stl96() ..................... */

 
 
/*
  ============================================================================ 

        double FER_generator_random (SCD_EID *EID);

        Description:
        ~~~~~~~~~~~~

        Implementation of the `Frame Erasure' function:
        - computes the 'frame erasure pattern' 


        Parameters:   
        ~~~~~~~~~~~ 

        EID: ..... pointer to EID struct.

        Return value:
        ~~~~~~~~~~~~~
        Returns (double)1 if the current frame has been erased, and
        (double)0 otherwise. 

        Author: <[e-mail]>
        ~~~~~~~
	Modified by G. Schroeder 

        History:
        ~~~~~~~~
        28.Feb.92 v1.0 Release of 1st version <[e-mail]>

 ============================================================================ 
*/
double FER_generator_random(EID)
SCD_EID *EID; 
{
   long n;                        /* value of random generator */
   double RAN,fer;
 

   /* Get next random number */
   RAN = EID_random(&(EID->seed));
 
   /* See if another channel state has to be entered */
   for (n=0; n<EID->nstates; n++)
   {
     if (RAN < EID->matrix[EID->current_state][n])
     {
       EID->current_state = n; /* go to the selected state */
       n = EID->nstates;       /* -> aborts loop */
     }
   }
 
/*
   * ......... Compute bit error in current state .........
   */

   /* Get next random number */
   RAN = EID_random(&(EID->seed));
 
   /* if random number below the threshold (bit error rate in current state), 
      erase current frame totally */
   if (RAN < EID->ber[EID->current_state])
   {
     fer = 1.0;
   }
   else
   {
     fer = 0.0;
   }

  /* Return the frame erasure flag */
    return(fer);
}
/* ....................... End of FER_generator_random() ................. */


/*
  ============================================================================ 

        double FER_module_stl92 (SCD_EID *EID, long lseg, short *xbuff, 
        ~~~~~~~~~~~~~~~~~~~~~~~  short *ybuff);

        Description:
        ~~~~~~~~~~~~

        Implementation of the `Frame Erasure' function:
        - computes the 'frame erasure pattern' 
        - erases all bits in one frame according the current 
          state of the pattern generator. 

        The input buffer contains 'lseg' words, containing the data bits,
        preceeded by a SYNC-word.  If the frame should be erased (depending 
        on the frame  erasure pattern), all bits are set to 0x0000.
        Since a '0' is represented by 0x7F and a '1' is  represented 
        by 0x81, this corresponds to a total uncertainty about the true bit
        values. 

        In addition the lower 4 bits of the SYNC-word in the sync
        header are set to '0'. This makes it easier for the
        succeeding software to detect an erased frame.


        Parameters:   
        ~~~~~~~~~~~ 

        EID: ..... pointer to EID struct.
        lseg: .... length of current frame (including SYNC-word).
        xbuff: ... buffer with input bit stream xbuff[0](= SYNC-word) is
                   not processed.
        ybuff: ... buffer with output bit stream 

        Return value:
        ~~~~~~~~~~~~~
        Returns (double)1 if the current frame has been erased, and
        (double)0 otherwise. 

        Author: <[e-mail]>
        ~~~~~~~

        History:
        ~~~~~~~~
        28.Feb.92 v1.0 Release of 1st version <[e-mail]>
        06.Jun.95 v1.1 Changed self-documentation to align the EID module 
                       with the EID hardware and Rec.G.192. Code not
                       changed <[e-mail]>

 ============================================================================ 
*/
double FER_module_stl92(EID, lseg,xbuff,ybuff)
SCD_EID *EID; 
long  lseg; 
short *xbuff; 
short *ybuff; 
{
   long i,n;                        /* value of random generator */
   double RAN,fer;
 

        /* Get next random number */
        RAN = EID_random(&(EID->seed));
 
        /* See if another channel state has to be entered */
        for (n=0; n<EID->nstates; n++)
        {
            if (RAN < EID->matrix[EID->current_state][n])
            {
                EID->current_state = n; /* go to the selected state */
                n = EID->nstates;       /* -> aborts loop */
            }
        }
 
/*
   * ......... Compute bit error in current state .........
   */

        /* Get next random number */
        RAN = EID_random(&(EID->seed));
 
        /* if random number below the threshold (bit error rate in current state), 
           erase current frame totally */
        if (RAN < EID->ber[EID->current_state])
        {
            ybuff[0] = xbuff[0] & 0x0000FFF0; /* modify SYNC-word */
            /* set all bits in current frame to 0 (which means 'not good - 
               not bad') */
            for (i=1; i<lseg; i++) ybuff[i] = (short)0;    
            fer = 1.0;
        }
        else
        {
        /* otherwise copy current frame into output buffer */
            ybuff[0] = xbuff[0];        /* store SYNC-word */
            for (i=1; i<lseg; i++)      /* store data */
                ybuff[i] = (short)(0x00FF) & (xbuff[i]);
            fer = 0.0;
        }

  /* Return the frame erasure flag */
    return(fer);
}
/* ....................... End of FER_module_stl92() ....................... */
 

/*
  ============================================================================ 

        double FER_module_stl96 (SCD_EID *EID, long lseg, short *xbuff, 
        ~~~~~~~~~~~~~~~~~~~~~~~  short *ybuff);

        Description:
        ~~~~~~~~~~~~

        Implementation of the `Frame Erasure' function:
        - computes the 'frame erasure pattern' 
        - erases all bits in one frame according the current 
          state of the pattern generator. 

        The input (undisturbed) and output (disturbed)
        buffers have samples conforming to the bitstream
        representation description in Annex B of G.192. This bitstream
        has a synchronism (sync) header at frame boundaries, which
        consists of a synchronism (SYNC) word followed by a 16-bit,
        2-complement word with the number of softbits in the frame
        (i.e., the frame length). Should the frame be erased (depending 
        on the frame  erasure pattern), all softbits are set to 0x0000.
        Since a '0' is represented by 0x7F and a '1' is  represented 
        by 0x81, this corresponds to a total uncertainty about the true bit
        values. 

        In addition the lower 4 bits of the SYNC-word in the sync
        header are set to '0'. This makes it easier for the succeeding
        software to detect an erased frame. The frame length word is
        copied unmodified to the output buffer.

        This function is aliased to FER_module by default in
        eid.h. If the pre-STL96 version is desired, the module needs
        to be compiled with the symbol STL92 defined.

        Parameters:   
        ~~~~~~~~~~~ 

        EID: ..... pointer to EID struct.
        lseg: .... length of current frame (including SYNC header).