spandsp  0.0.6
g711.h
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1 /*
2  * SpanDSP - a series of DSP components for telephony
3  *
4  * g711.h - In line A-law and u-law conversion routines
5  *
6  * Written by Steve Underwood <steveu@coppice.org>
7  *
8  * Copyright (C) 2001 Steve Underwood
9  *
10  * All rights reserved.
11  *
12  * This program is free software; you can redistribute it and/or modify
13  * it under the terms of the GNU Lesser General Public License version 2.1,
14  * as published by the Free Software Foundation.
15  *
16  * This program is distributed in the hope that it will be useful,
17  * but WITHOUT ANY WARRANTY; without even the implied warranty of
18  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
19  * GNU Lesser General Public License for more details.
20  *
21  * You should have received a copy of the GNU Lesser General Public
22  * License along with this program; if not, write to the Free Software
23  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
24  */
25 
26 /*! \file */
27 
28 /*! \page g711_page A-law and mu-law handling
29 Lookup tables for A-law and u-law look attractive, until you consider the impact
30 on the CPU cache. If it causes a substantial area of your processor cache to get
31 hit too often, cache sloshing will severely slow things down. The main reason
32 these routines are slow in C, is the lack of direct access to the CPU's "find
33 the first 1" instruction. A little in-line assembler fixes that, and the
34 conversion routines can be faster than lookup tables, in most real world usage.
35 A "find the first 1" instruction is available on most modern CPUs, and is a
36 much underused feature.
37 
38 If an assembly language method of bit searching is not available, these routines
39 revert to a method that can be a little slow, so the cache thrashing might not
40 seem so bad :(
41 
42 Feel free to submit patches to add fast "find the first 1" support for your own
43 favourite processor.
44 
45 Look up tables are used for transcoding between A-law and u-law, since it is
46 difficult to achieve the precise transcoding procedure laid down in the G.711
47 specification by other means.
48 */
49 
50 #if !defined(_SPANDSP_G711_H_)
51 #define _SPANDSP_G711_H_
52 
53 /*! The A-law alternate mark inversion mask */
54 #define G711_ALAW_AMI_MASK 0x55
55 
56 /* The usual values to use on idle channels, to emulate silence */
57 /*! Idle value for A-law channels */
58 #define G711_ALAW_IDLE_OCTET (0x80 ^ G711_ALAW_AMI_MASK)
59 /*! Idle value for u-law channels */
60 #define G711_ULAW_IDLE_OCTET 0xFF
61 
62 enum
63 {
64  G711_ALAW = 0,
65  G711_ULAW
66 };
67 
68 /*!
69  G.711 state
70  */
71 typedef struct g711_state_s g711_state_t;
72 
73 #if defined(__cplusplus)
74 extern "C"
75 {
76 #endif
77 
78 /* N.B. It is tempting to use look-up tables for A-law and u-law conversion.
79  * However, you should consider the cache footprint.
80  *
81  * A 64K byte table for linear to x-law and a 512 byte table for x-law to
82  * linear sound like peanuts these days, and shouldn't an array lookup be
83  * real fast? No! When the cache sloshes as badly as this one will, a tight
84  * calculation may be better. The messiest part is normally finding the
85  * segment, but a little inline assembly can fix that on an i386, x86_64 and
86  * many other modern processors.
87  */
88 
89 /*
90  * Mu-law is basically as follows:
91  *
92  * Biased Linear Input Code Compressed Code
93  * ------------------------ ---------------
94  * 00000001wxyza 000wxyz
95  * 0000001wxyzab 001wxyz
96  * 000001wxyzabc 010wxyz
97  * 00001wxyzabcd 011wxyz
98  * 0001wxyzabcde 100wxyz
99  * 001wxyzabcdef 101wxyz
100  * 01wxyzabcdefg 110wxyz
101  * 1wxyzabcdefgh 111wxyz
102  *
103  * Each biased linear code has a leading 1 which identifies the segment
104  * number. The value of the segment number is equal to 7 minus the number
105  * of leading 0's. The quantization interval is directly available as the
106  * four bits wxyz. * The trailing bits (a - h) are ignored.
107  *
108  * Ordinarily the complement of the resulting code word is used for
109  * transmission, and so the code word is complemented before it is returned.
110  *
111  * For further information see John C. Bellamy's Digital Telephony, 1982,
112  * John Wiley & Sons, pps 98-111 and 472-476.
113  */
114 
115 /* Enable the trap as per the MIL-STD */
116 //#define G711_ULAW_ZEROTRAP
117 /*! Bias for u-law encoding from linear. */
118 #define G711_ULAW_BIAS 0x84
119 
120 /*! \brief Encode a linear sample to u-law
121  \param linear The sample to encode.
122  \return The u-law value.
123 */
124 static __inline__ uint8_t linear_to_ulaw(int linear)
125 {
126  uint8_t u_val;
127  int mask;
128  int seg;
129 
130  /* Get the sign and the magnitude of the value. */
131  if (linear >= 0)
132  {
133  linear = G711_ULAW_BIAS + linear;
134  mask = 0xFF;
135  }
136  else
137  {
138  linear = G711_ULAW_BIAS - linear;
139  mask = 0x7F;
140  }
141 
142  seg = top_bit(linear | 0xFF) - 7;
143  if (seg >= 8)
144  {
145  u_val = (uint8_t) (0x7F ^ mask);
146  }
147  else
148  {
149  /* Combine the sign, segment, quantization bits, and complement the code word. */
150  u_val = (uint8_t) (((seg << 4) | ((linear >> (seg + 3)) & 0xF)) ^ mask);
151  }
152 #if defined(G711_ULAW_ZEROTRAP)
153  /* Optional ITU trap */
154  if (u_val == 0)
155  u_val = 0x02;
156 #endif
157  return u_val;
158 }
159 /*- End of function --------------------------------------------------------*/
160 
161 /*! \brief Decode an u-law sample to a linear value.
162  \param ulaw The u-law sample to decode.
163  \return The linear value.
164 */
165 static __inline__ int16_t ulaw_to_linear(uint8_t ulaw)
166 {
167  int t;
168 
169  /* Complement to obtain normal u-law value. */
170  ulaw = ~ulaw;
171  /*
172  * Extract and bias the quantization bits. Then
173  * shift up by the segment number and subtract out the bias.
174  */
175  t = (((ulaw & 0x0F) << 3) + G711_ULAW_BIAS) << (((int) ulaw & 0x70) >> 4);
176  return (int16_t) ((ulaw & 0x80) ? (G711_ULAW_BIAS - t) : (t - G711_ULAW_BIAS));
177 }
178 /*- End of function --------------------------------------------------------*/
179 
180 /*
181  * A-law is basically as follows:
182  *
183  * Linear Input Code Compressed Code
184  * ----------------- ---------------
185  * 0000000wxyza 000wxyz
186  * 0000001wxyza 001wxyz
187  * 000001wxyzab 010wxyz
188  * 00001wxyzabc 011wxyz
189  * 0001wxyzabcd 100wxyz
190  * 001wxyzabcde 101wxyz
191  * 01wxyzabcdef 110wxyz
192  * 1wxyzabcdefg 111wxyz
193  *
194  * For further information see John C. Bellamy's Digital Telephony, 1982,
195  * John Wiley & Sons, pps 98-111 and 472-476.
196  */
197 
198 /*! \brief Encode a linear sample to A-law
199  \param linear The sample to encode.
200  \return The A-law value.
201 */
202 static __inline__ uint8_t linear_to_alaw(int linear)
203 {
204  uint8_t a_val;
205  int mask;
206  int seg;
207 
208  if (linear >= 0)
209  {
210  /* Sign (bit 7) bit = 1 */
211  mask = 0x80 | G711_ALAW_AMI_MASK;
212  }
213  else
214  {
215  /* Sign (bit 7) bit = 0 */
216  mask = G711_ALAW_AMI_MASK;
217  linear = -linear - 1;
218  }
219 
220  /* Convert the scaled magnitude to segment number. */
221  seg = top_bit(linear | 0xFF) - 7;
222  if (seg >= 8)
223  {
224  a_val = (uint8_t) (0x7F ^ mask);
225  }
226  else
227  {
228  /* Combine the sign, segment, and quantization bits. */
229  a_val = (uint8_t) (((seg << 4) | ((linear >> ((seg) ? (seg + 3) : 4)) & 0x0F)) ^ mask);
230  }
231  return a_val;
232 }
233 /*- End of function --------------------------------------------------------*/
234 
235 /*! \brief Decode an A-law sample to a linear value.
236  \param alaw The A-law sample to decode.
237  \return The linear value.
238 */
239 static __inline__ int16_t alaw_to_linear(uint8_t alaw)
240 {
241  int i;
242  int seg;
243 
244  alaw ^= G711_ALAW_AMI_MASK;
245  i = ((alaw & 0x0F) << 4);
246  seg = (((int) alaw & 0x70) >> 4);
247  if (seg)
248  i = (i + 0x108) << (seg - 1);
249  else
250  i += 8;
251  return (int16_t) ((alaw & 0x80) ? i : -i);
252 }
253 /*- End of function --------------------------------------------------------*/
254 
255 /*! \brief Transcode from A-law to u-law, using the procedure defined in G.711.
256  \param alaw The A-law sample to transcode.
257  \return The best matching u-law value.
258 */
259 SPAN_DECLARE(uint8_t) alaw_to_ulaw(uint8_t alaw);
260 
261 /*! \brief Transcode from u-law to A-law, using the procedure defined in G.711.
262  \param ulaw The u-law sample to transcode.
263  \return The best matching A-law value.
264 */
265 SPAN_DECLARE(uint8_t) ulaw_to_alaw(uint8_t ulaw);
266 
267 /*! \brief Decode from u-law or A-law to linear.
268  \param s The G.711 context.
269  \param amp The linear audio buffer.
270  \param g711_data The G.711 data.
271  \param g711_bytes The number of G.711 samples to decode.
272  \return The number of samples of linear audio produced.
273 */
274 SPAN_DECLARE(int) g711_decode(g711_state_t *s,
275  int16_t amp[],
276  const uint8_t g711_data[],
277  int g711_bytes);
278 
279 /*! \brief Encode from linear to u-law or A-law.
280  \param s The G.711 context.
281  \param g711_data The G.711 data.
282  \param amp The linear audio buffer.
283  \param len The number of samples to encode.
284  \return The number of G.711 samples produced.
285 */
286 SPAN_DECLARE(int) g711_encode(g711_state_t *s,
287  uint8_t g711_data[],
288  const int16_t amp[],
289  int len);
290 
291 /*! \brief Transcode between u-law and A-law.
292  \param s The G.711 context.
293  \param g711_out The resulting G.711 data.
294  \param g711_in The original G.711 data.
295  \param g711_bytes The number of G.711 samples to transcode.
296  \return The number of G.711 samples produced.
297 */
298 SPAN_DECLARE(int) g711_transcode(g711_state_t *s,
299  uint8_t g711_out[],
300  const uint8_t g711_in[],
301  int g711_bytes);
302 
303 /*! Initialise a G.711 encode or decode context.
304  \param s The G.711 context.
305  \param mode The G.711 mode.
306  \return A pointer to the G.711 context, or NULL for error. */
307 SPAN_DECLARE(g711_state_t *) g711_init(g711_state_t *s, int mode);
308 
309 /*! Release a G.711 encode or decode context.
310  \param s The G.711 context.
311  \return 0 for OK. */
312 SPAN_DECLARE(int) g711_release(g711_state_t *s);
313 
314 /*! Free a G.711 encode or decode context.
315  \param s The G.711 context.
316  \return 0 for OK. */
317 SPAN_DECLARE(int) g711_free(g711_state_t *s);
318 
319 #if defined(__cplusplus)
320 }
321 #endif
322 
323 #endif
324 /*- End of file ------------------------------------------------------------*/
int g711_encode(g711_state_t *s, uint8_t g711_data[], const int16_t amp[], int len)
Encode from linear to u-law or A-law.
Definition: g711.c:123
int g711_release(g711_state_t *s)
Definition: g711.c:183
g711_state_t * g711_init(g711_state_t *s, int mode)
Definition: g711.c:171
int g711_transcode(g711_state_t *s, uint8_t g711_out[], const uint8_t g711_in[], int g711_bytes)
Transcode between u-law and A-law.
Definition: g711.c:147
uint8_t ulaw_to_alaw(uint8_t ulaw)
Transcode from u-law to A-law, using the procedure defined in G.711.
Definition: g711.c:93
#define G711_ALAW_AMI_MASK
Definition: g711.h:54
Definition: private/g711.h:32
int g711_free(g711_state_t *s)
Definition: g711.c:189
int mode
Definition: private/g711.h:35
uint8_t alaw_to_ulaw(uint8_t alaw)
Transcode from A-law to u-law, using the procedure defined in G.711.
Definition: g711.c:87
#define G711_ULAW_BIAS
Definition: g711.h:118
int g711_decode(g711_state_t *s, int16_t amp[], const uint8_t g711_data[], int g711_bytes)
Decode from u-law or A-law to linear.
Definition: g711.c:99