From: cfreese@super.org (Craig F. Reese)
Subject: Re: Format of .AU Sound files               
Organization: Supercomputing Research Center (Bowie, MD)
Date: Thu, 31 Dec 1992 18:56:50 GMT

In article <memo.834974@cix.compulink.co.uk> rspence@cix.compulink.co.uk writes:
>christos@kuhub.cc.ukans.edu () writes:
>
>> Hello there,
>
>> I am using the Public Domain program Sound Tools on the Sun station
>> to record speech. The speech samples are saved in a mu-law 8bits/sample
>> encoding rule with a standard ".au" header precceding the data. What I
>> would like to know is what the format of the speech samples is? In other
    .
    .
    .
>
>So much for background. If I remember correctly, the most significant bit is
>a sign bit, indicating positive (0) or negative (1) value. The remaining 7
>bits are the encoded amplitude of the sample. Manipulation will require you
>to decode the sample correctly, and I don't have the details of mu-law
>encoding to hand. But, I know where it is described. Try and get hold of the
>CCITT's G.7xx series recommendations which is one large volume. Try a
>technical library of a university. Alternatively, you could ask very nicely
>and I will try to mail the relevant pages when I'm over in the US in January.
>

Here's some code I wrote many moons ago.  It should help.

Craig

=============================================================

TWIMC,

Below are two routines I wrote for converting between ulaw and linear. 
I use them with the SparcStation and they seem to work fine.
I am pretty sure (99.9%) that they implement the standard as specified
in the references. 

Note that the standard deals with converting between 12 bit linear
and 8 bit ulaw.  These routines assume 16 bit linear.  Thus, some
bit shifting may be necessary.

craig

------------------------------------------------------------------
/**
 ** Signal conversion routines for use with the Sun4/60 audio chip
 **/

/*
 * 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(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 the result **/
    return(ulawbyte);
    }

/*
 * 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
 */

int
ulaw2linear(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);
    }

-----------------
Craig F. Reese                           Email: cfreese@super.org
Institute for Defense Analyses/
Supercomputing Research Center
17100 Science Dr.
Bowie, MD  20715-4300