cfreese@super.ORG (Craig F. Reese) (09/30/89)
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 20715-4300pokey@well.UUCP (Jef Poskanzer) (10/23/89)
In the referenced message, cfreese@super.UUCP (Craig F. Reese) wrote: }Below are two routines I wrote for converting between ulaw and linear. }I use them with the SparcStation and they seem to work fine. Thanks, those will be very useful. Since I haven't seen anyone else post it, appended is the obvious table-lookup version of Craig's ulaw to linear converter. I did some timings on a SPARCstation. In nanoseconds: counting subroutine call not counting subroutine call old new old new 1217 557 913 253 Also, the table in interesting in its own right, to get a feel for just what ulaw does. Corrections welcome, otherwise enjoy. --- Jef Jef Poskanzer pokey@well.sf.ca.us {ucbvax, apple, hplabs}!well!pokey Specifications subject to change without notice. /* ** This routine converts from ulaw to 16 bit linear, but faster. ** ** Jef Poskanzer ** 23 October 1989 ** ** Input: 8 bit ulaw sample ** Output: signed 16 bit linear sample */ int ulaw2linear(ulawbyte) unsigned char ulawbyte; { static int ulaw_table[] = { -32124, -31100, -30076, -29052, -28028, -27004, -25980, -24956, -23932, -22908, -21884, -20860, -19836, -18812, -17788, -16764, -15996, -15484, -14972, -14460, -13948, -13436, -12924, -12412, -11900, -11388, -10876, -10364, -9852, -9340, -8828, -8316, -7932, -7676, -7420, -7164, -6908, -6652, -6396, -6140, -5884, -5628, -5372, -5116, -4860, -4604, -4348, -4092, -3900, -3772, -3644, -3516, -3388, -3260, -3132, -3004, -2876, -2748, -2620, -2492, -2364, -2236, -2108, -1980, -1884, -1820, -1756, -1692, -1628, -1564, -1500, -1436, -1372, -1308, -1244, -1180, -1116, -1052, -988, -924, -876, -844, -812, -780, -748, -716, -684, -652, -620, -588, -556, -524, -492, -460, -428, -396, -372, -356, -340, -324, -308, -292, -276, -260, -244, -228, -212, -196, -180, -164, -148, -132, -120, -112, -104, -96, -88, -80, -72, -64, -56, -48, -40, -32, -24, -16, -8, 0, 32124, 31100, 30076, 29052, 28028, 27004, 25980, 24956, 23932, 22908, 21884, 20860, 19836, 18812, 17788, 16764, 15996, 15484, 14972, 14460, 13948, 13436, 12924, 12412, 11900, 11388, 10876, 10364, 9852, 9340, 8828, 8316, 7932, 7676, 7420, 7164, 6908, 6652, 6396, 6140, 5884, 5628, 5372, 5116, 4860, 4604, 4348, 4092, 3900, 3772, 3644, 3516, 3388, 3260, 3132, 3004, 2876, 2748, 2620, 2492, 2364, 2236, 2108, 1980, 1884, 1820, 1756, 1692, 1628, 1564, 1500, 1436, 1372, 1308, 1244, 1180, 1116, 1052, 988, 924, 876, 844, 812, 780, 748, 716, 684, 652, 620, 588, 556, 524, 492, 460, 428, 396, 372, 356, 340, 324, 308, 292, 276, 260, 244, 228, 212, 196, 180, 164, 148, 132, 120, 112, 104, 96, 88, 80, 72, 64, 56, 48, 40, 32, 24, 16, 8, 0 }; return ulaw_table[ulawbyte]; }