echo.h

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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  * Based on a bit from here, a bit from there, eye of toad,
00013  * ear of bat, etc - plus, of course, my own 2 cents.
00014  *
00015  * All rights reserved.
00016  *
00017  * This program is free software; you can redistribute it and/or modify
00018  * it under the terms of the GNU General Public License version 2, as
00019  * published by the Free Software Foundation.
00020  *
00021  * This program is distributed in the hope that it will be useful,
00022  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00023  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
00024  * GNU General Public License for more details.
00025  *
00026  * You should have received a copy of the GNU General Public License
00027  * along with this program; if not, write to the Free Software
00028  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
00029  *
00030  * $Id: echo.h,v 1.10 2007/04/05 19:20:49 steveu Exp $
00031  */
00032 
00033 /*! \file */
00034 
00035 #if !defined(_SPANDSP_ECHO_H_)
00036 #define _SPANDSP_ECHO_H_
00037 
00038 /*! \page echo_can_page Line echo cancellation for voice
00039 
00040 \section echo_can_page_sec_1 What does it do?
00041 This module aims to provide G.168-2002 compliant echo cancellation, to remove
00042 electrical echoes (e.g. from 2-4 wire hybrids) from voice calls.
00043 
00044 \section echo_can_page_sec_2 How does it work?
00045 The heart of the echo cancellor is FIR filter. This is adapted to match the echo
00046 impulse response of the telephone line. It must be long enough to adequately cover
00047 the duration of that impulse response. The signal transmitted to the telephone line
00048 is passed through the FIR filter. Once the FIR is properly adapted, the resulting
00049 output is an estimate of the echo signal received from the line. This is subtracted
00050 from the received signal. The result is an estimate of the signal which originated
00051 at the far end of the line, free from echos of our own transmitted signal. 
00052 
00053 The least mean squares (LMS) algorithm is attributed to Widrow and Hoff, and was
00054 introduced in 1960. It is the commonest form of filter adaption used in things
00055 like modem line equalisers and line echo cancellers. There it works very well.
00056 However, it only works well for signals of constant amplitude. It works very poorly
00057 for things like speech echo cancellation, where the signal level varies widely.
00058 This is quite easy to fix. If the signal level is normalised - similar to applying
00059 AGC - LMS can work as well for a signal of varying amplitude as it does for a modem
00060 signal. This normalised least mean squares (NLMS) algorithm is the commonest one used
00061 for speech echo cancellation. Many other algorithms exist - e.g. RLS (essentially
00062 the same as Kalman filtering), FAP, etc. Some perform significantly better than NLMS.
00063 However, factors such as computational complexity and patents favour the use of NLMS.
00064 
00065 A simple refinement to NLMS can improve its performance with speech. NLMS tends
00066 to adapt best to the strongest parts of a signal. If the signal is white noise,
00067 the NLMS algorithm works very well. However, speech has more low frequency than
00068 high frequency content. Pre-whitening (i.e. filtering the signal to flatten
00069 its spectrum) the echo signal improves the adapt rate for speech, and ensures the
00070 final residual signal is not heavily biased towards high frequencies. A very low
00071 complexity filter is adequate for this, so pre-whitening adds little to the
00072 compute requirements of the echo canceller.
00073 
00074 An FIR filter adapted using pre-whitened NLMS performs well, provided certain
00075 conditions are met: 
00076 
00077     - The transmitted signal has poor self-correlation.
00078     - There is no signal being generated within the environment being cancelled.
00079 
00080 The difficulty is that neither of these can be guaranteed.
00081 
00082 If the adaption is performed while transmitting noise (or something fairly noise
00083 like, such as voice) the adaption works very well. If the adaption is performed
00084 while transmitting something highly correlative (typically narrow band energy
00085 such as signalling tones or DTMF), the adaption can go seriously wrong. The reason
00086 is there is only one solution for the adaption on a near random signal - the impulse
00087 response of the line. For a repetitive signal, there are any number of solutions
00088 which converge the adaption, and nothing guides the adaption to choose the generalised
00089 one. Allowing an untrained canceller to converge on this kind of narrowband
00090 energy probably a good thing, since at least it cancels the tones. Allowing a well
00091 converged canceller to continue converging on such energy is just a way to ruin
00092 its generalised adaption. A narrowband detector is needed, so adapation can be
00093 suspended at appropriate times.
00094 
00095 The adaption process is based on trying to eliminate the received signal. When
00096 there is any signal from within the environment being cancelled it may upset the
00097 adaption process. Similarly, if the signal we are transmitting is small, noise
00098 may dominate and disturb the adaption process. If we can ensure that the
00099 adaption is only performed when we are transmitting a significant signal level,
00100 and the environment is not, things will be OK. Clearly, it is easy to tell when
00101 we are sending a significant signal. Telling, if the environment is generating a
00102 significant signal, and doing it with sufficient speed that the adaption will
00103 not have diverged too much more we stop it, is a little harder. 
00104 
00105 The key problem in detecting when the environment is sourcing significant energy
00106 is that we must do this very quickly. Given a reasonably long sample of the
00107 received signal, there are a number of strategies which may be used to assess
00108 whether that signal contains a strong far end component. However, by the time
00109 that assessment is complete the far end signal will have already caused major
00110 mis-convergence in the adaption process. An assessment algorithm is needed which
00111 produces a fairly accurate result from a very short burst of far end energy. 
00112 
00113 \section echo_can_page_sec_3 How do I use it?
00114 The echo cancellor processes both the transmit and receive streams sample by
00115 sample. The processing function is not declared inline. Unfortunately,
00116 cancellation requires many operations per sample, so the call overhead is only a
00117 minor burden. 
00118 */
00119 
00120 #include "fir.h"
00121 
00122 #define NONUPDATE_DWELL_TIME    600     /* 600 samples, or 75ms */
00123 
00124 /* Mask bits for the adaption mode */
00125 #define ECHO_CAN_USE_NLP            0x01
00126 #define ECHO_CAN_USE_SUPPRESSOR     0x02
00127 #define ECHO_CAN_USE_CNG            0x04
00128 #define ECHO_CAN_USE_ADAPTION       0x08
00129 
00130 /*!
00131     G.168 echo canceller descriptor. This defines the working state for a line
00132     echo canceller.
00133 */
00134 typedef struct
00135 {
00136     int tx_power[4];
00137     int rx_power[3];
00138     int clean_rx_power;
00139 
00140     int rx_power_threshold;
00141     int nonupdate_dwell;
00142 
00143     fir16_state_t fir_state;
00144     /*! Echo FIR taps (16 bit version) */
00145     int16_t *fir_taps16[4];
00146     /*! Echo FIR taps (32 bit version) */
00147     int32_t *fir_taps32;
00148 
00149     int curr_pos;
00150         
00151     int taps;
00152     int tap_mask;
00153     int adaption_mode;
00154     
00155     int32_t supp_test1;
00156     int32_t supp_test2;
00157     int32_t supp1;
00158     int32_t supp2;
00159     int vad;
00160     int cng;
00161     /* Parameters for the Hoth noise generator */
00162     int cng_level;
00163     int cng_rndnum;
00164     int cng_filter;
00165     
00166     int16_t geigel_max;
00167     int geigel_lag;
00168     int dtd_onset;
00169     int tap_set;
00170     int tap_rotate_counter;
00171 
00172     int32_t latest_correction;  /* Indication of the magnitude of the latest
00173                                    adaption, or a code to indicate why adaption
00174                                    was skipped, for test purposes */
00175     int32_t last_acf[28];
00176     int narrowband_count;
00177     int narrowband_score;
00178 } echo_can_state_t;
00179 
00180 /*! Create a voice echo canceller context.
00181     \param len The length of the canceller, in samples.
00182     \return The new canceller context, or NULL if the canceller could not be created.
00183 */
00184 echo_can_state_t *echo_can_create(int len, int adaption_mode);
00185 
00186 /*! Free a voice echo canceller context.
00187     \param ec The echo canceller context.
00188 */
00189 void echo_can_free(echo_can_state_t *ec);
00190 
00191 /*! Flush (reinitialise) a voice echo canceller context.
00192     \param ec The echo canceller context.
00193 */
00194 void echo_can_flush(echo_can_state_t *ec);
00195 
00196 /*! Set the adaption mode of a voice echo canceller context.
00197     \param ec The echo canceller context.
00198     \param adapt The mode.
00199 */
00200 void echo_can_adaption_mode(echo_can_state_t *ec, int adaption_mode);
00201 
00202 /*! Process a sample through a voice echo canceller.
00203     \param ec The echo canceller context.
00204     \param tx The transmitted audio sample.
00205     \param rx The received audio sample.
00206     \return The clean (echo cancelled) received sample.
00207 */
00208 int16_t echo_can_update(echo_can_state_t *ec, int16_t tx, int16_t rx);
00209 
00210 #endif
00211 /*- End of file ------------------------------------------------------------*/

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