www.pudn.com > slow.zip > coder.C

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	coder.c 
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/* Modem for MIPS   AJF	  March 1998 
   V.29 encoding / decoding routines */ 
 
#include  
 
#include  
 
#include "modem.h" 
#include "coder.h" 
 
/* tables for DPSK encoding */ 
static uchar fwd_phinctab[8] = { 1, 0, 2, 3, 6, 7, 5, 4 }; 
static uchar rev_phinctab[8] = { 1, 0, 2, 3, 7, 6, 4, 5 }; 
 
static c_complex constellation[8] = 
  { { +3.0, 0.0 }, { +1.0, +1.0 }, { 0.0, +3.0 }, { -1.0, +1.0 },	/*  0 -	 3 */ 
    { -3.0, 0.0 }, { -1.0, -1.0 }, { 0.0, -3.0 }, { +1.0, -1.0 },	/*  4 -	 7 */ 
  }; 
 
static int locate(complex); 
 
 
complex traininggen::get(int bc) 
  { /* V.29 training sequence */ 
    complex z; 
    if (bc < SEG_2) 
      { /* segment 1 : silence */ 
	z = 0.0; 
      } 
    else 
      { int k; 
	if (bc < SEG_3) 
	  { /* segment 2 : ABAB... */ 
	    k = (bc & 1) ? 7 : 4; 
	  } 
	else 
	  { /* segment 3 : CDCD... */ 
	    int b6 = (reg >> 1) & 1; 
	    int b7 = reg & 1; 
	    k = b7 ? 3 : 0; 
	    reg >>= 1; 
	    if (b6 ^ b7) reg |= 0x40; 
	  } 
	z = constellation[k]; 
      } 
    return z; 
  } 
 
complex encoder::encode(int bits) 
  { state += fwd_phinctab[bits & 7]; 
    state &= 7; 
    return constellation[state]; 
  } 
 
int decoder::decode(complex z) 
  { int nst = locate(z); 
    int bits = rev_phinctab[(nst-state) & 7]; 
    state = nst; 
    return bits; 
  } 
 
static int locate(complex z) 
  { int k; 
    float m = fabsf(z.re) - fabsf(z.im); 
    if (m > +1.5 || m < -1.5) 
      { int b1 = (z.im > z.re); 
	int b2 = (z.im < -z.re); 
	k = (b2 << 2) | ((b1 ^ b2) << 1); 
      } 
    else 
      { int b1 = (z.re < 0.0); 
	int b2 = (z.im < 0.0); 
	k = (b2 << 2) | ((b1 ^ b2) << 1) | 1; 
      } 
    return k; 
  } 
 
complex decoder::getez() 
  { return constellation[state];    /* return exact (quantized) value of last decoded symbol */ 
  }