ulaw.txt

4.8kcelp语音压缩编码程序

Path: super!cfreese
From: [e-mail] (Craig F. Reese)
Newsgroups: comp.dsp
Subject: SPARCStation audio conversion (ulaw <-> linear)
Message-ID: <[e-mail]>
Date: 29 Sep 89 23:23:49 GMT
Sender: [e-mail]
Reply-To: [e-mail] (Craig F. Reese)
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Organization: Supercomputing Research Center, Bowie, Md.
Lines: 120

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: [e-mail]
Institute for Defense Analyses/
Supercomputing Research Center
17100 Science Dr.
Bowie, MD  20715-4300