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