spandsp 0.0.6
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00001 /* 00002 * SpanDSP - a series of DSP components for telephony 00003 * 00004 * echo.h - An echo cancellor, suitable for electrical and acoustic 00005 * cancellation. This code does not currently comply with 00006 * any relevant standards (e.g. G.164/5/7/8). 00007 * 00008 * Written by Steve Underwood <steveu@coppice.org> 00009 * 00010 * Copyright (C) 2001 Steve Underwood 00011 * 00012 * All rights reserved. 00013 * 00014 * This program is free software; you can redistribute it and/or modify 00015 * it under the terms of the GNU Lesser General Public License version 2.1, 00016 * as published by the Free Software Foundation. 00017 * 00018 * This program is distributed in the hope that it will be useful, 00019 * but WITHOUT ANY WARRANTY; without even the implied warranty of 00020 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 00021 * GNU Lesser General Public License for more details. 00022 * 00023 * You should have received a copy of the GNU Lesser General Public 00024 * License along with this program; if not, write to the Free Software 00025 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. 00026 * 00027 * $Id: echo.h,v 1.20 2009/09/22 13:11:04 steveu Exp $ 00028 */ 00029 00030 /*! \file */ 00031 00032 #if !defined(_SPANDSP_ECHO_H_) 00033 #define _SPANDSP_ECHO_H_ 00034 00035 /*! \page echo_can_page Line echo cancellation for voice 00036 00037 \section echo_can_page_sec_1 What does it do? 00038 This module aims to provide G.168-2002 compliant echo cancellation, to remove 00039 electrical echoes (e.g. from 2-4 wire hybrids) from voice calls. 00040 00041 \section echo_can_page_sec_2 How does it work? 00042 The heart of the echo cancellor is FIR filter. This is adapted to match the echo 00043 impulse response of the telephone line. It must be long enough to adequately cover 00044 the duration of that impulse response. The signal transmitted to the telephone line 00045 is passed through the FIR filter. Once the FIR is properly adapted, the resulting 00046 output is an estimate of the echo signal received from the line. This is subtracted 00047 from the received signal. The result is an estimate of the signal which originated 00048 at the far end of the line, free from echos of our own transmitted signal. 00049 00050 The least mean squares (LMS) algorithm is attributed to Widrow and Hoff, and was 00051 introduced in 1960. It is the commonest form of filter adaption used in things 00052 like modem line equalisers and line echo cancellers. There it works very well. 00053 However, it only works well for signals of constant amplitude. It works very poorly 00054 for things like speech echo cancellation, where the signal level varies widely. 00055 This is quite easy to fix. If the signal level is normalised - similar to applying 00056 AGC - LMS can work as well for a signal of varying amplitude as it does for a modem 00057 signal. This normalised least mean squares (NLMS) algorithm is the commonest one used 00058 for speech echo cancellation. Many other algorithms exist - e.g. RLS (essentially 00059 the same as Kalman filtering), FAP, etc. Some perform significantly better than NLMS. 00060 However, factors such as computational complexity and patents favour the use of NLMS. 00061 00062 A simple refinement to NLMS can improve its performance with speech. NLMS tends 00063 to adapt best to the strongest parts of a signal. If the signal is white noise, 00064 the NLMS algorithm works very well. However, speech has more low frequency than 00065 high frequency content. Pre-whitening (i.e. filtering the signal to flatten 00066 its spectrum) the echo signal improves the adapt rate for speech, and ensures the 00067 final residual signal is not heavily biased towards high frequencies. A very low 00068 complexity filter is adequate for this, so pre-whitening adds little to the 00069 compute requirements of the echo canceller. 00070 00071 An FIR filter adapted using pre-whitened NLMS performs well, provided certain 00072 conditions are met: 00073 00074 - The transmitted signal has poor self-correlation. 00075 - There is no signal being generated within the environment being cancelled. 00076 00077 The difficulty is that neither of these can be guaranteed. 00078 00079 If the adaption is performed while transmitting noise (or something fairly noise 00080 like, such as voice) the adaption works very well. If the adaption is performed 00081 while transmitting something highly correlative (typically narrow band energy 00082 such as signalling tones or DTMF), the adaption can go seriously wrong. The reason 00083 is there is only one solution for the adaption on a near random signal - the impulse 00084 response of the line. For a repetitive signal, there are any number of solutions 00085 which converge the adaption, and nothing guides the adaption to choose the generalised 00086 one. Allowing an untrained canceller to converge on this kind of narrowband 00087 energy probably a good thing, since at least it cancels the tones. Allowing a well 00088 converged canceller to continue converging on such energy is just a way to ruin 00089 its generalised adaption. A narrowband detector is needed, so adapation can be 00090 suspended at appropriate times. 00091 00092 The adaption process is based on trying to eliminate the received signal. When 00093 there is any signal from within the environment being cancelled it may upset the 00094 adaption process. Similarly, if the signal we are transmitting is small, noise 00095 may dominate and disturb the adaption process. If we can ensure that the 00096 adaption is only performed when we are transmitting a significant signal level, 00097 and the environment is not, things will be OK. Clearly, it is easy to tell when 00098 we are sending a significant signal. Telling, if the environment is generating a 00099 significant signal, and doing it with sufficient speed that the adaption will 00100 not have diverged too much more we stop it, is a little harder. 00101 00102 The key problem in detecting when the environment is sourcing significant energy 00103 is that we must do this very quickly. Given a reasonably long sample of the 00104 received signal, there are a number of strategies which may be used to assess 00105 whether that signal contains a strong far end component. However, by the time 00106 that assessment is complete the far end signal will have already caused major 00107 mis-convergence in the adaption process. An assessment algorithm is needed which 00108 produces a fairly accurate result from a very short burst of far end energy. 00109 00110 \section echo_can_page_sec_3 How do I use it? 00111 The echo cancellor processes both the transmit and receive streams sample by 00112 sample. The processing function is not declared inline. Unfortunately, 00113 cancellation requires many operations per sample, so the call overhead is only a 00114 minor burden. 00115 */ 00116 00117 #include "fir.h" 00118 00119 /* Mask bits for the adaption mode */ 00120 enum 00121 { 00122 ECHO_CAN_USE_ADAPTION = 0x01, 00123 ECHO_CAN_USE_NLP = 0x02, 00124 ECHO_CAN_USE_CNG = 0x04, 00125 ECHO_CAN_USE_CLIP = 0x08, 00126 ECHO_CAN_USE_SUPPRESSOR = 0x10, 00127 ECHO_CAN_USE_TX_HPF = 0x20, 00128 ECHO_CAN_USE_RX_HPF = 0x40, 00129 ECHO_CAN_DISABLE = 0x80 00130 }; 00131 00132 /*! 00133 G.168 echo canceller descriptor. This defines the working state for a line 00134 echo canceller. 00135 */ 00136 typedef struct echo_can_state_s echo_can_state_t; 00137 00138 #if defined(__cplusplus) 00139 extern "C" 00140 { 00141 #endif 00142 00143 /*! Create a voice echo canceller context. 00144 \param len The length of the canceller, in samples. 00145 \return The new canceller context, or NULL if the canceller could not be created. 00146 */ 00147 SPAN_DECLARE(echo_can_state_t *) echo_can_init(int len, int adaption_mode); 00148 00149 /*! Release a voice echo canceller context. 00150 \param ec The echo canceller context. 00151 \return 0 for OK, else -1. 00152 */ 00153 SPAN_DECLARE(int) echo_can_release(echo_can_state_t *ec); 00154 00155 /*! Free a voice echo canceller context. 00156 \param ec The echo canceller context. 00157 \return 0 for OK, else -1. 00158 */ 00159 SPAN_DECLARE(int) echo_can_free(echo_can_state_t *ec); 00160 00161 /*! Flush (reinitialise) a voice echo canceller context. 00162 \param ec The echo canceller context. 00163 */ 00164 SPAN_DECLARE(void) echo_can_flush(echo_can_state_t *ec); 00165 00166 /*! Set the adaption mode of a voice echo canceller context. 00167 \param ec The echo canceller context. 00168 \param adaption_mode The mode. 00169 */ 00170 SPAN_DECLARE(void) echo_can_adaption_mode(echo_can_state_t *ec, int adaption_mode); 00171 00172 /*! Process a sample through a voice echo canceller. 00173 \param ec The echo canceller context. 00174 \param tx The transmitted audio sample. 00175 \param rx The received audio sample. 00176 \return The clean (echo cancelled) received sample. 00177 */ 00178 SPAN_DECLARE(int16_t) echo_can_update(echo_can_state_t *ec, int16_t tx, int16_t rx); 00179 00180 /*! Process to high pass filter the tx signal. 00181 \param ec The echo canceller context. 00182 \param tx The transmitted auio sample. 00183 \return The HP filtered transmit sample, send this to your D/A. 00184 */ 00185 SPAN_DECLARE(int16_t) echo_can_hpf_tx(echo_can_state_t *ec, int16_t tx); 00186 00187 SPAN_DECLARE(void) echo_can_snapshot(echo_can_state_t *ec); 00188 00189 #if defined(__cplusplus) 00190 } 00191 #endif 00192 00193 #endif 00194 /*- End of file ------------------------------------------------------------*/