From: cpirazzi@bonk.esd.sgi.com (Chris Pirazzi)
Subject: Re: ulaw format wanted
Keywords: mulaw sound file format
Organization: Silicon Graphics, Inc., Mountain View, CA
Date: Fri, 24 Jul 1992 01:43:57 GMT

In article <23286@alice.att.com> jsh@alice.att.com (Juergen Schroeter) writes:
>Hi,
>
>I'd like to get hold of the exact file format of the SGI utilities'
>(e.g., sfconvert, sfplay) mulaw option.  Any soul out there who could
>give me a pointer/share it with me?
>
>Juergen Schroeter, jsh@alice.att.com
>(908) 582-7059
>AT&T Bell Labs, Murray Hill, NJ 07974-0636

Hi,

Here are the formulas used:

/*
 * This routine converts from ulaw to 16 bit linear
 * 29 September 1989
 *
 * Craig Reese: IDA/Supercomputing Research Center
 *
 * 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
 */

short 
ulaw2linear(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);
}

/*
 * This routine converts from linear to ulaw
 * 29 September 1989
 *
 * Craig Reese: IDA/Supercomputing Research Center
 * Joe Campbell: Department of Defense
 *
 * 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 
linear2ulaw(short 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 the result **/
    return(ulawbyte);
}


	- Chris Pirazzi