--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/spandsp-0.0.6pre17/src/spandsp/echo.h	Fri Jun 25 15:50:58 2010 +0200
@@ -0,0 +1,194 @@
+/*
+ * SpanDSP - a series of DSP components for telephony
+ *
+ * echo.h - An echo cancellor, suitable for electrical and acoustic
+ *	        cancellation. This code does not currently comply with
+ *	        any relevant standards (e.g. G.164/5/7/8).
+ *
+ * Written by Steve Underwood <steveu@coppice.org>
+ *
+ * Copyright (C) 2001 Steve Underwood
+ *
+ * All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 2.1,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ *
+ * $Id: echo.h,v 1.20 2009/09/22 13:11:04 steveu Exp $
+ */
+
+/*! \file */
+
+#if !defined(_SPANDSP_ECHO_H_)
+#define _SPANDSP_ECHO_H_
+
+/*! \page echo_can_page Line echo cancellation for voice
+
+\section echo_can_page_sec_1 What does it do?
+This module aims to provide G.168-2002 compliant echo cancellation, to remove
+electrical echoes (e.g. from 2-4 wire hybrids) from voice calls.
+
+\section echo_can_page_sec_2 How does it work?
+The heart of the echo cancellor is FIR filter. This is adapted to match the echo
+impulse response of the telephone line. It must be long enough to adequately cover
+the duration of that impulse response. The signal transmitted to the telephone line
+is passed through the FIR filter. Once the FIR is properly adapted, the resulting
+output is an estimate of the echo signal received from the line. This is subtracted
+from the received signal. The result is an estimate of the signal which originated
+at the far end of the line, free from echos of our own transmitted signal. 
+
+The least mean squares (LMS) algorithm is attributed to Widrow and Hoff, and was
+introduced in 1960. It is the commonest form of filter adaption used in things
+like modem line equalisers and line echo cancellers. There it works very well.
+However, it only works well for signals of constant amplitude. It works very poorly
+for things like speech echo cancellation, where the signal level varies widely.
+This is quite easy to fix. If the signal level is normalised - similar to applying
+AGC - LMS can work as well for a signal of varying amplitude as it does for a modem
+signal. This normalised least mean squares (NLMS) algorithm is the commonest one used
+for speech echo cancellation. Many other algorithms exist - e.g. RLS (essentially
+the same as Kalman filtering), FAP, etc. Some perform significantly better than NLMS.
+However, factors such as computational complexity and patents favour the use of NLMS.
+
+A simple refinement to NLMS can improve its performance with speech. NLMS tends
+to adapt best to the strongest parts of a signal. If the signal is white noise,
+the NLMS algorithm works very well. However, speech has more low frequency than
+high frequency content. Pre-whitening (i.e. filtering the signal to flatten
+its spectrum) the echo signal improves the adapt rate for speech, and ensures the
+final residual signal is not heavily biased towards high frequencies. A very low
+complexity filter is adequate for this, so pre-whitening adds little to the
+compute requirements of the echo canceller.
+
+An FIR filter adapted using pre-whitened NLMS performs well, provided certain
+conditions are met: 
+
+    - The transmitted signal has poor self-correlation.
+    - There is no signal being generated within the environment being cancelled.
+
+The difficulty is that neither of these can be guaranteed.
+
+If the adaption is performed while transmitting noise (or something fairly noise
+like, such as voice) the adaption works very well. If the adaption is performed
+while transmitting something highly correlative (typically narrow band energy
+such as signalling tones or DTMF), the adaption can go seriously wrong. The reason
+is there is only one solution for the adaption on a near random signal - the impulse
+response of the line. For a repetitive signal, there are any number of solutions
+which converge the adaption, and nothing guides the adaption to choose the generalised
+one. Allowing an untrained canceller to converge on this kind of narrowband
+energy probably a good thing, since at least it cancels the tones. Allowing a well
+converged canceller to continue converging on such energy is just a way to ruin
+its generalised adaption. A narrowband detector is needed, so adapation can be
+suspended at appropriate times.
+
+The adaption process is based on trying to eliminate the received signal. When
+there is any signal from within the environment being cancelled it may upset the
+adaption process. Similarly, if the signal we are transmitting is small, noise
+may dominate and disturb the adaption process. If we can ensure that the
+adaption is only performed when we are transmitting a significant signal level,
+and the environment is not, things will be OK. Clearly, it is easy to tell when
+we are sending a significant signal. Telling, if the environment is generating a
+significant signal, and doing it with sufficient speed that the adaption will
+not have diverged too much more we stop it, is a little harder. 
+
+The key problem in detecting when the environment is sourcing significant energy
+is that we must do this very quickly. Given a reasonably long sample of the
+received signal, there are a number of strategies which may be used to assess
+whether that signal contains a strong far end component. However, by the time
+that assessment is complete the far end signal will have already caused major
+mis-convergence in the adaption process. An assessment algorithm is needed which
+produces a fairly accurate result from a very short burst of far end energy. 
+
+\section echo_can_page_sec_3 How do I use it?
+The echo cancellor processes both the transmit and receive streams sample by
+sample. The processing function is not declared inline. Unfortunately,
+cancellation requires many operations per sample, so the call overhead is only a
+minor burden. 
+*/
+
+#include "fir.h"
+
+/* Mask bits for the adaption mode */
+enum
+{
+    ECHO_CAN_USE_ADAPTION = 0x01,
+    ECHO_CAN_USE_NLP = 0x02,
+    ECHO_CAN_USE_CNG = 0x04,
+    ECHO_CAN_USE_CLIP = 0x08,
+    ECHO_CAN_USE_SUPPRESSOR = 0x10,
+    ECHO_CAN_USE_TX_HPF = 0x20,
+    ECHO_CAN_USE_RX_HPF = 0x40,
+    ECHO_CAN_DISABLE = 0x80
+};
+
+/*!
+    G.168 echo canceller descriptor. This defines the working state for a line
+    echo canceller.
+*/
+typedef struct echo_can_state_s echo_can_state_t;
+
+#if defined(__cplusplus)
+extern "C"
+{
+#endif
+
+/*! Create a voice echo canceller context.
+    \param len The length of the canceller, in samples.
+    \return The new canceller context, or NULL if the canceller could not be created.
+*/
+SPAN_DECLARE(echo_can_state_t *) echo_can_init(int len, int adaption_mode);
+
+/*! Release a voice echo canceller context.
+    \param ec The echo canceller context.
+    \return 0 for OK, else -1.
+*/
+SPAN_DECLARE(int) echo_can_release(echo_can_state_t *ec);
+
+/*! Free a voice echo canceller context.
+    \param ec The echo canceller context.
+    \return 0 for OK, else -1.
+*/
+SPAN_DECLARE(int) echo_can_free(echo_can_state_t *ec);
+
+/*! Flush (reinitialise) a voice echo canceller context.
+    \param ec The echo canceller context.
+*/
+SPAN_DECLARE(void) echo_can_flush(echo_can_state_t *ec);
+
+/*! Set the adaption mode of a voice echo canceller context.
+    \param ec The echo canceller context.
+    \param adaption_mode The mode.
+*/
+SPAN_DECLARE(void) echo_can_adaption_mode(echo_can_state_t *ec, int adaption_mode);
+
+/*! Process a sample through a voice echo canceller.
+    \param ec The echo canceller context.
+    \param tx The transmitted audio sample.
+    \param rx The received audio sample.
+    \return The clean (echo cancelled) received sample.
+*/
+SPAN_DECLARE(int16_t) echo_can_update(echo_can_state_t *ec, int16_t tx, int16_t rx);
+
+/*! Process to high pass filter the tx signal.
+    \param ec The echo canceller context.
+    \param tx The transmitted auio sample.
+    \return The HP filtered transmit sample, send this to your D/A.
+*/
+SPAN_DECLARE(int16_t) echo_can_hpf_tx(echo_can_state_t *ec, int16_t tx);
+
+SPAN_DECLARE(void) echo_can_snapshot(echo_can_state_t *ec);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
+/*- End of file ------------------------------------------------------------*/