/* Mu-law code from Linux kernel
* Copyright (c) 1999 by Jaroslav Kysela
* Uros Bizjak
* Copyright Jeroen Vreeken (jeroen@vreeken.net), 2017
*
* Based on reference implementation by Sun Microsystems, Inc.
*
* This library is free software; you can redistribute it and/or modify
* it under the terms of the GNU Library General Public License as
* published by the Free Software Foundation; either version 2 of
* the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Library General Public License for more details.
*
* You should have received a copy of the GNU Library General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*
*/
#include "ulaw.h"
#define BIAS (0x84) /* Bias for linear code. */
#define SIGN_BIT (0x80) /* Sign bit for a u-law byte. */
#define QUANT_MASK (0xf) /* Quantization field mask. */
#define NSEGS (8) /* Number of u-law segments. */
#define SEG_SHIFT (4) /* Left shift for segment number. */
#define SEG_MASK (0x70) /* Segment field mask. */
static inline int val_seg(int val)
{
int r = 0;
val >>= 7;
if (val & 0xf0) {
val >>= 4;
r += 4;
}
if (val & 0x0c) {
val >>= 2;
r += 2;
}
if (val & 0x02)
r += 1;
return r;
}
/*
* linear2ulaw() - Convert a linear PCM value to u-law
*
* In order to simplify the encoding process, the original linear magnitude
* is biased by adding 33 which shifts the encoding range from (0 - 8158) to
* (33 - 8191). The result can be seen in the following encoding table:
*
* Biased Linear Input Code Compressed Code
* ------------------------ ---------------
* 00000001wxyza 000wxyz
* 0000001wxyzab 001wxyz
* 000001wxyzabc 010wxyz
* 00001wxyzabcd 011wxyz
* 0001wxyzabcde 100wxyz
* 001wxyzabcdef 101wxyz
* 01wxyzabcdefg 110wxyz
* 1wxyzabcdefgh 111wxyz
*
* Each biased linear code has a leading 1 which identifies the segment
* number. The value of the segment number is equal to 7 minus the number
* of leading 0's. The quantization interval is directly available as the
* four bits wxyz. * The trailing bits (a - h) are ignored.
*
* Ordinarily the complement of the resulting code word is used for
* transmission, and so the code word is complemented before it is returned.
*
* For further information see John C. Bellamy's Digital Telephony, 1982,
* John Wiley & Sons, pps 98-111 and 472-476.
*/
static uint8_t linear2ulaw(int16_t pcm_val) /* 2's complement (16-bit range) */
{
int mask;
int seg;
unsigned char uval;
/* Get the sign and the magnitude of the value. */
if (pcm_val < 0) {
pcm_val = BIAS - pcm_val;
mask = 0x7F;
} else {
pcm_val += BIAS;
mask = 0xFF;
}
if (pcm_val > 0x7FFF)
pcm_val = 0x7FFF;
/* Convert the scaled magnitude to segment number. */
seg = val_seg(pcm_val);
/*
* Combine the sign, segment, quantization bits;
* and complement the code word.
*/
uval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0xF);
return uval ^ mask;
}
/*
* ulaw2linear() - Convert a u-law value to 16-bit linear PCM
*
* First, a biased linear code is derived from the code word. An unbiased
* output can then be obtained by subtracting 33 from the biased code.
*
* Note that this function expects to be passed the complement of the
* original code word. This is in keeping with ISDN conventions.
*/
static int16_t ulaw2linear(uint8_t u_val)
{
int t;
/* Complement to obtain normal u-law value. */
u_val = ~u_val;
/*
* Extract and bias the quantization bits. Then
* shift up by the segment number and subtract out the bias.
*/
t = ((u_val & QUANT_MASK) << 3) + BIAS;
t <<= ((unsigned)u_val & SEG_MASK) >> SEG_SHIFT;
return ((u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS));
}
void ulaw_decode(int16_t *samples, uint8_t *ulaw, int nr)
{
int i;
for (i = 0; i < nr; i++) {
samples[i] = ulaw2linear(ulaw[i]);
}
}
void ulaw_encode(uint8_t *ulaw, int16_t *samples, int nr)
{
int i;
for (i = 0; i < nr; i++) {
ulaw[i] = linear2ulaw(samples[i]);
}
}