Category : Music and Digitized Voice
Archive   : SOX7DOS.ZIP
Filename : LIBST.C

/* libst.c - portable sound tools library
*/

/*
** This routine converts from linear to ulaw.
**
** Craig Reese: IDA/Supercomputing Research Center
** Joe Campbell: Department of Defense
** 29 September 1989
**
** References:
** 1) CCITT Recommendation G.711 (very difficult to follow)
** 2) "A New Digital Technique for Implementation of Any
** Continuous PCM Companding Law," Villeret, Michel,
** et al. 1973 IEEE Int. Conf. on Communications, Vol 1,
** 1973, pg. 11.12-11.17
** 3) MIL-STD-188-113,"Interoperability and Performance Standards
** for Analog-to_Digital Conversion Techniques,"
** 17 February 1987
**
** Input: Signed 16 bit linear sample
** Output: 8 bit ulaw sample
*/

#define ZEROTRAP /* turn on the trap as per the MIL-STD */
#define BIAS 0x84 /* define the add-in bias for 16 bit samples */
#define CLIP 32635

unsigned char
st_linear_to_ulaw( sample )
int sample;
{
static int exp_lut[256] = {0,0,1,1,2,2,2,2,3,3,3,3,3,3,3,3,
4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,
5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7};
int sign, exponent, mantissa;
unsigned char ulawbyte;

/* Get the sample into sign-magnitude. */
sign = (sample >> 8) & 0x80; /* set aside the sign */
if ( sign != 0 ) sample = -sample; /* get magnitude */
if ( sample > CLIP ) sample = CLIP; /* clip the magnitude */

/* Convert from 16 bit linear to ulaw. */
sample = sample + BIAS;
exponent = exp_lut[( sample >> 7 ) & 0xFF];
mantissa = ( sample >> ( exponent + 3 ) ) & 0x0F;
ulawbyte = ~ ( sign | ( exponent << 4 ) | mantissa );
#ifdef ZEROTRAP
if ( ulawbyte == 0 ) ulawbyte = 0x02; /* optional CCITT trap */
#endif

return ulawbyte;
}

/*
** This routine converts from ulaw to 16 bit linear.
**
** Craig Reese: IDA/Supercomputing Research Center
** 29 September 1989
**
** References:
** 1) CCITT Recommendation G.711 (very difficult to follow)
** 2) MIL-STD-188-113,"Interoperability and Performance Standards
** for Analog-to_Digital Conversion Techniques,"
** 17 February 1987
**
** Input: 8 bit ulaw sample
** Output: signed 16 bit linear sample
*/

int
st_ulaw_to_linear_slow( ulawbyte )
unsigned char ulawbyte;
{
static int exp_lut[8] = { 0, 132, 396, 924, 1980, 4092, 8316, 16764 };
int sign, exponent, mantissa, sample;

ulawbyte = ~ ulawbyte;
sign = ( ulawbyte & 0x80 );
exponent = ( ulawbyte >> 4 ) & 0x07;
mantissa = ulawbyte & 0x0F;
sample = exp_lut[exponent] + ( mantissa << ( exponent + 3 ) );
if ( sign != 0 ) sample = -sample;

return sample;
}