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comparison spandsp-0.0.6pre17/src/spandsp/echo.h @ 4:26cd8f1ef0b1

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import spandsp-0.0.6pre17
author Peter Meerwald <pmeerw@cosy.sbg.ac.at>
date Fri, 25 Jun 2010 15:50:58 +0200
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3:c6c5a16ce2f2 4:26cd8f1ef0b1
1 /*
2 * SpanDSP - a series of DSP components for telephony
3 *
4 * echo.h - An echo cancellor, suitable for electrical and acoustic
5 * cancellation. This code does not currently comply with
6 * any relevant standards (e.g. G.164/5/7/8).
7 *
8 * Written by Steve Underwood <steveu@coppice.org>
9 *
10 * Copyright (C) 2001 Steve Underwood
11 *
12 * All rights reserved.
13 *
14 * This program is free software; you can redistribute it and/or modify
15 * it under the terms of the GNU Lesser General Public License version 2.1,
16 * as published by the Free Software Foundation.
17 *
18 * This program is distributed in the hope that it will be useful,
19 * but WITHOUT ANY WARRANTY; without even the implied warranty of
20 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
21 * GNU Lesser General Public License for more details.
22 *
23 * You should have received a copy of the GNU Lesser General Public
24 * License along with this program; if not, write to the Free Software
25 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
26 *
27 * $Id: echo.h,v 1.20 2009/09/22 13:11:04 steveu Exp $
28 */
29
30 /*! \file */
31
32 #if !defined(_SPANDSP_ECHO_H_)
33 #define _SPANDSP_ECHO_H_
34
35 /*! \page echo_can_page Line echo cancellation for voice
36
37 \section echo_can_page_sec_1 What does it do?
38 This module aims to provide G.168-2002 compliant echo cancellation, to remove
39 electrical echoes (e.g. from 2-4 wire hybrids) from voice calls.
40
41 \section echo_can_page_sec_2 How does it work?
42 The heart of the echo cancellor is FIR filter. This is adapted to match the echo
43 impulse response of the telephone line. It must be long enough to adequately cover
44 the duration of that impulse response. The signal transmitted to the telephone line
45 is passed through the FIR filter. Once the FIR is properly adapted, the resulting
46 output is an estimate of the echo signal received from the line. This is subtracted
47 from the received signal. The result is an estimate of the signal which originated
48 at the far end of the line, free from echos of our own transmitted signal.
49
50 The least mean squares (LMS) algorithm is attributed to Widrow and Hoff, and was
51 introduced in 1960. It is the commonest form of filter adaption used in things
52 like modem line equalisers and line echo cancellers. There it works very well.
53 However, it only works well for signals of constant amplitude. It works very poorly
54 for things like speech echo cancellation, where the signal level varies widely.
55 This is quite easy to fix. If the signal level is normalised - similar to applying
56 AGC - LMS can work as well for a signal of varying amplitude as it does for a modem
57 signal. This normalised least mean squares (NLMS) algorithm is the commonest one used
58 for speech echo cancellation. Many other algorithms exist - e.g. RLS (essentially
59 the same as Kalman filtering), FAP, etc. Some perform significantly better than NLMS.
60 However, factors such as computational complexity and patents favour the use of NLMS.
61
62 A simple refinement to NLMS can improve its performance with speech. NLMS tends
63 to adapt best to the strongest parts of a signal. If the signal is white noise,
64 the NLMS algorithm works very well. However, speech has more low frequency than
65 high frequency content. Pre-whitening (i.e. filtering the signal to flatten
66 its spectrum) the echo signal improves the adapt rate for speech, and ensures the
67 final residual signal is not heavily biased towards high frequencies. A very low
68 complexity filter is adequate for this, so pre-whitening adds little to the
69 compute requirements of the echo canceller.
70
71 An FIR filter adapted using pre-whitened NLMS performs well, provided certain
72 conditions are met:
73
74 - The transmitted signal has poor self-correlation.
75 - There is no signal being generated within the environment being cancelled.
76
77 The difficulty is that neither of these can be guaranteed.
78
79 If the adaption is performed while transmitting noise (or something fairly noise
80 like, such as voice) the adaption works very well. If the adaption is performed
81 while transmitting something highly correlative (typically narrow band energy
82 such as signalling tones or DTMF), the adaption can go seriously wrong. The reason
83 is there is only one solution for the adaption on a near random signal - the impulse
84 response of the line. For a repetitive signal, there are any number of solutions
85 which converge the adaption, and nothing guides the adaption to choose the generalised
86 one. Allowing an untrained canceller to converge on this kind of narrowband
87 energy probably a good thing, since at least it cancels the tones. Allowing a well
88 converged canceller to continue converging on such energy is just a way to ruin
89 its generalised adaption. A narrowband detector is needed, so adapation can be
90 suspended at appropriate times.
91
92 The adaption process is based on trying to eliminate the received signal. When
93 there is any signal from within the environment being cancelled it may upset the
94 adaption process. Similarly, if the signal we are transmitting is small, noise
95 may dominate and disturb the adaption process. If we can ensure that the
96 adaption is only performed when we are transmitting a significant signal level,
97 and the environment is not, things will be OK. Clearly, it is easy to tell when
98 we are sending a significant signal. Telling, if the environment is generating a
99 significant signal, and doing it with sufficient speed that the adaption will
100 not have diverged too much more we stop it, is a little harder.
101
102 The key problem in detecting when the environment is sourcing significant energy
103 is that we must do this very quickly. Given a reasonably long sample of the
104 received signal, there are a number of strategies which may be used to assess
105 whether that signal contains a strong far end component. However, by the time
106 that assessment is complete the far end signal will have already caused major
107 mis-convergence in the adaption process. An assessment algorithm is needed which
108 produces a fairly accurate result from a very short burst of far end energy.
109
110 \section echo_can_page_sec_3 How do I use it?
111 The echo cancellor processes both the transmit and receive streams sample by
112 sample. The processing function is not declared inline. Unfortunately,
113 cancellation requires many operations per sample, so the call overhead is only a
114 minor burden.
115 */
116
117 #include "fir.h"
118
119 /* Mask bits for the adaption mode */
120 enum
121 {
122 ECHO_CAN_USE_ADAPTION = 0x01,
123 ECHO_CAN_USE_NLP = 0x02,
124 ECHO_CAN_USE_CNG = 0x04,
125 ECHO_CAN_USE_CLIP = 0x08,
126 ECHO_CAN_USE_SUPPRESSOR = 0x10,
127 ECHO_CAN_USE_TX_HPF = 0x20,
128 ECHO_CAN_USE_RX_HPF = 0x40,
129 ECHO_CAN_DISABLE = 0x80
130 };
131
132 /*!
133 G.168 echo canceller descriptor. This defines the working state for a line
134 echo canceller.
135 */
136 typedef struct echo_can_state_s echo_can_state_t;
137
138 #if defined(__cplusplus)
139 extern "C"
140 {
141 #endif
142
143 /*! Create a voice echo canceller context.
144 \param len The length of the canceller, in samples.
145 \return The new canceller context, or NULL if the canceller could not be created.
146 */
147 SPAN_DECLARE(echo_can_state_t *) echo_can_init(int len, int adaption_mode);
148
149 /*! Release a voice echo canceller context.
150 \param ec The echo canceller context.
151 \return 0 for OK, else -1.
152 */
153 SPAN_DECLARE(int) echo_can_release(echo_can_state_t *ec);
154
155 /*! Free a voice echo canceller context.
156 \param ec The echo canceller context.
157 \return 0 for OK, else -1.
158 */
159 SPAN_DECLARE(int) echo_can_free(echo_can_state_t *ec);
160
161 /*! Flush (reinitialise) a voice echo canceller context.
162 \param ec The echo canceller context.
163 */
164 SPAN_DECLARE(void) echo_can_flush(echo_can_state_t *ec);
165
166 /*! Set the adaption mode of a voice echo canceller context.
167 \param ec The echo canceller context.
168 \param adaption_mode The mode.
169 */
170 SPAN_DECLARE(void) echo_can_adaption_mode(echo_can_state_t *ec, int adaption_mode);
171
172 /*! Process a sample through a voice echo canceller.
173 \param ec The echo canceller context.
174 \param tx The transmitted audio sample.
175 \param rx The received audio sample.
176 \return The clean (echo cancelled) received sample.
177 */
178 SPAN_DECLARE(int16_t) echo_can_update(echo_can_state_t *ec, int16_t tx, int16_t rx);
179
180 /*! Process to high pass filter the tx signal.
181 \param ec The echo canceller context.
182 \param tx The transmitted auio sample.
183 \return The HP filtered transmit sample, send this to your D/A.
184 */
185 SPAN_DECLARE(int16_t) echo_can_hpf_tx(echo_can_state_t *ec, int16_t tx);
186
187 SPAN_DECLARE(void) echo_can_snapshot(echo_can_state_t *ec);
188
189 #if defined(__cplusplus)
190 }
191 #endif
192
193 #endif
194 /*- End of file ------------------------------------------------------------*/

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