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modem_echo.h
Go to the documentation of this file.
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/*
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* SpanDSP - a series of DSP components for telephony
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*
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* modem_echo.h - An echo cancellor, suitable for electrical echos in GSTN modems
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*
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* Written by Steve Underwood <steveu@coppice.org>
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*
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* Copyright (C) 2001, 2004 Steve Underwood
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*
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* Based on a bit from here, a bit from there, eye of toad,
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* ear of bat, etc - plus, of course, my own 2 cents.
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*
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* All rights reserved.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU Lesser General Public License version 2.1,
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* as published by the Free Software Foundation.
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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 Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/*! \file */
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#if !defined(_SPANDSP_MODEM_ECHO_H_)
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#define _SPANDSP_MODEM_ECHO_H_
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/*! \page modem_echo_can_page Line echo cancellation for modems
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\section modem_echo_can_page_sec_1 What does it do?
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This module aims to cancel electrical echoes (e.g. from 2-4 wire hybrids)
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in modem applications. It is not very suitable for speech applications, which
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require additional refinements for satisfactory performance. It is, however, more
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efficient and better suited to modem applications.
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\section modem_echo_can_page_sec_2 How does it work?
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The heart of the echo cancellor is an adaptive FIR filter. This is adapted to
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match the impulse response of the environment being cancelled. It must be long
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enough to adequately cover the duration of that impulse response. The signal
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being transmitted into the environment being cancelled is passed through the
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FIR filter. The resulting output is an estimate of the echo signal. This is
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then subtracted from the received signal, and the result should be an estimate
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of the signal which originates within the environment being cancelled (people
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talking in the room, or the signal from the far end of a telephone line) free
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from the echos of our own transmitted signal.
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The FIR filter is adapted using the least mean squares (LMS) algorithm. This
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algorithm is attributed to Widrow and Hoff, and was introduced in 1960. It is
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the commonest form of filter adaption used in things like modem line equalisers
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and line echo cancellers. It works very well if the signal level is constant,
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which is true for a modem signal. To ensure good performa certain conditions must
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be met:
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- The transmitted signal has weak self-correlation.
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- There is no signal being generated within the environment being cancelled.
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The difficulty is that neither of these can be guaranteed. If the adaption is
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performed while transmitting noise (or something fairly noise like, such as
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voice) the adaption works very well. If the adaption is performed while
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transmitting something highly correlative (e.g. tones, like DTMF), the adaption
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can go seriously wrong. The reason is there is only one solution for the
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adaption on a near random signal. For a repetitive signal, there are a number of
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solutions which converge the adaption, and nothing guides the adaption to choose
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the correct one.
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\section modem_echo_can_page_sec_3 How do I use it?
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The echo cancellor processes both the transmit and receive streams sample by
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sample. The processing function is not declared inline. Unfortunately,
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cancellation requires many operations per sample, so the call overhead is only a
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minor burden.
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*/
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#include "fir.h"
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/*!
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Modem line echo canceller descriptor. This defines the working state for a line
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echo canceller.
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*/
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typedef
struct
modem_echo_can_state_s
modem_echo_can_state_t
;
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#if defined(__cplusplus)
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extern
"C"
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{
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#endif
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/*! Create a modem echo canceller context.
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\param len The length of the canceller, in samples.
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eturn The new canceller context, or NULL if the canceller could not be created.
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*/
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SPAN_DECLARE(
modem_echo_can_state_t
*)
modem_echo_can_init
(
int
len);
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/*! Free a modem echo canceller context.
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\param ec The echo canceller context.
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*/
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SPAN_DECLARE(
void
)
modem_echo_can_free
(
modem_echo_can_state_t
*ec);
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/*! Flush (reinitialise) a modem echo canceller context.
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\param ec The echo canceller context.
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*/
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SPAN_DECLARE(
void
)
modem_echo_can_flush
(
modem_echo_can_state_t
*ec);
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/*! Set the adaption mode of a modem echo canceller context.
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\param ec The echo canceller context.
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\param adapt The mode.
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*/
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SPAN_DECLARE(
void
)
modem_echo_can_adaption_mode
(
modem_echo_can_state_t
*ec,
int
adapt);
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/*! Process a sample through a modem echo canceller.
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\param ec The echo canceller context.
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\param tx The transmitted audio sample.
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\param rx The received audio sample.
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eturn The clean (echo cancelled) received sample.
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*/
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SPAN_DECLARE(int16_t)
modem_echo_can_update
(
modem_echo_can_state_t
*ec, int16_t tx, int16_t rx);
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#if defined(__cplusplus)
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}
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#endif
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#endif
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/*- End of file ------------------------------------------------------------*/
src
spandsp
modem_echo.h
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