University of Manchester
			School of Computer Science
			Comp30242 Mobile Computing
			Laboratory Assignment
		Appendix C: Conversion of A-law to Uniform


Attached are two routines in 'C':  'bbalawexp' and 'bbalawexptable'.
They do exactly the same thing, i.e. convert a single A-law (G711) sample
to a 16-bit uniformly quantised sample which is ready to be output directly
to a sound card. 
If you are a 'C' programmer, just use either routine.
If you are not, you may find the table version easier to convert to 
your chosen language.
(There is a little test program attached to convince you they are identical.)
This Appendix can be found in text form on 
www.cs.man.ac.uk/~barry/mydocs/myComp60242
The file-name is: MC07-Lab-App-C.txt
That's all!


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

/* Convert one sample of ALAW to one uniform sample. */
short  bbalawexp(short alaw)
{
    short    unif, ix, mant, iexp;

    ix = alaw ^ (0x0055);	/*Invert even bits */
    ix &= (0x007F);		      /* remove sign */
    iexp = ix >> 4;		      /* remove exponent */
    mant = ix & (0x000F);	      /* get mantissa */
    if (iexp > 0)  
        mant = mant + 16;  /* prefix '1', if exp > 0 */

    mant = (mant << 4) + (0x0008);	/* mantissa left justified and round */
    if (iexp > 1) 
        mant = mant << (iexp - 1);  /*left shift*/

    if (alaw > 127) 
        unif = mant ;
    else unif = -mant;

    return(unif);
}

/*============================================================================*/
short  bbalawexptable(short alaw)
{
   short unif;
   /* Convert one sample of ALAW to one uniform sample. */
   short alawtable[256] = {
   -5504, -5248,   -6016,  -5760,  -4480,  -4224,  -4992, -4736,   -7552,  -7296,
   -8064,  -7808,  -6528,  -6272,  -7040,  -6784,  -2752, -2624,   -3008,  -2880,
   -2240,  -2112,  -2496,  -2368,  -3776,  -3648,  -4032,  -3904,  -3264,  -3136,
   -3520,  -3392,  -22016, -20992, -24064, -23040, -17920, -16896, -19968, -18944,
   -30208, -29184, -32256, -31232, -26112, -25088, -28160, -27136, -11008, -10496,
   -12032, -11520, -8960,  -8448,  -9984,  -9472,  -15104, -14592, -16128, -15616,
   -13056, -12544, -14080, -13568, -344,   -328,   -376,   -360,   -280,  -264,
   -312,   -296,   -472,   -456,   -504,   -488,   -408,   -392,   -440,  -424,
   -88,    -72,    -120,   -104,   -24,     -8,    -56,    -40,    -216 , -200,
   -248,   -232,   -152,   -136,   -184,    -168,  -1376,  -1312,  -1504, -1440,
   -1120,  -1056,  -1248,  -1184,  -1888,   -1824,  -2016, -1952,  -1632, -1568,
   -1760,  -1696,  -688,   -656,   -752,    -720,   -560,  -528,   -624,  -592,
   -944,   -912,   -1008,  -976,   -816,    -784,   -880,  -848,   5504,  5248,
   6016,   5760,   4480,   4224,   4992,    4736,   7552,  7296,   8064,  7808,
   6528,   6272,   7040,   6784,   2752,    2624,   3008,  2880,   2240,  2112,
   2496,   2368,   3776,   3648,   4032,    3904,   3264,  3136,   3520,  3392,
   22016,  20992,  24064,  23040,  17920,   16896,  19968, 18944,  30208, 29184,
   32256,  31232,  26112,  25088,  28160,   27136,  11008, 10496,  12032, 11520,
   8960,   8448,   9984,   9472,   15104,   14592,  16128, 15616,  13056, 12544,
   14080,  13568,  344,    328,    376,     360,    280,   264,    312,   296,
   472,    456,    504,    488,    408,     392,    440,   424,    88,    72,
   120,    104,    24,     8,      56,      40,     216,   200,    248,   232,
   152,    136,    184,    168,    1376,    1312,   1504,  1440,   1120,  1056,
   1248,   1184,   1888,   1824,   2016,    1952,   1632,  1568,   1760,  1696,
   688,    656,    752,    720,    560,     528,    624,   592,    944,    912,
   1008,   976,    816,    784,    880,     848,
   };

   unif = alawtable[alaw];
   return(unif);

}

int  main (int argc, char* argv[])
{
    short alaw, uniffunc, uniftbl;

    for(alaw = 0; alaw < 256; alaw ++)
    {
      uniffunc =  bbalawexp(alaw);
      uniftbl = bbalawexptable(alaw);
      if (uniffunc != uniftbl)
          printf("error at alaw  = %d! \n", alaw);
    }
    return (0);
}