Fri May 26 01:45:35 2006

Asterisk developer's documentation


plc.h

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00001 /*! \file
00002  * \brief SpanDSP - a series of DSP components for telephony
00003  *
00004  * plc.h
00005  *
00006  * \author Steve Underwood <steveu@coppice.org>
00007  *
00008  * Copyright (C) 2004 Steve Underwood
00009  *
00010  * All rights reserved.
00011  *
00012  * This program is free software; you can redistribute it and/or modify
00013  * it under the terms of the GNU General Public License as published by
00014  * the Free Software Foundation; either version 2 of the License, or
00015  * (at your option) any later version.
00016  *
00017  * This program is distributed in the hope that it will be useful,
00018  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00019  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
00020  * GNU General Public License for more details.
00021  *
00022  * You should have received a copy of the GNU General Public License
00023  * along with this program; if not, write to the Free Software
00024  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
00025  *
00026  * This version may be optionally licenced under the GNU LGPL licence.
00027  * This version is disclaimed to DIGIUM for inclusion in the Asterisk project.
00028  */
00029 
00030 
00031 #if !defined(_PLC_H_)
00032 #define _PLC_H_
00033 
00034 #ifdef SOLARIS
00035 #include <sys/int_types.h>
00036 #else
00037 #if defined(__OpenBSD__) || defined( __FreeBSD__)
00038 #include <inttypes.h>
00039 #else
00040 #include <stdint.h>
00041 #endif
00042 #endif
00043 
00044 /*! \page plc_page Packet loss concealment
00045 \section plc_page_sec_1 What does it do?
00046 The packet loss concealment module provides a suitable synthetic fill-in signal,
00047 to minimise the audible effect of lost packets in VoIP applications. It is not
00048 tied to any particular codec, and could be used with almost any codec which does not
00049 specify its own procedure for packet loss concealment.
00050 
00051 Where a codec specific concealment procedure exists, the algorithm is usually built
00052 around knowledge of the characteristics of the particular codec. It will, therefore,
00053 generally give better results for that particular codec than this generic concealer will.
00054 
00055 \section plc_page_sec_2 How does it work?
00056 While good packets are being received, the plc_rx() routine keeps a record of the trailing
00057 section of the known speech signal. If a packet is missed, plc_fillin() is called to produce
00058 a synthetic replacement for the real speech signal. The average mean difference function
00059 (AMDF) is applied to the last known good signal, to determine its effective pitch.
00060 Based on this, the last pitch period of signal is saved. Essentially, this cycle of speech
00061 will be repeated over and over until the real speech resumes. However, several refinements
00062 are needed to obtain smooth pleasant sounding results.
00063 
00064 - The two ends of the stored cycle of speech will not always fit together smoothly. This can
00065   cause roughness, or even clicks, at the joins between cycles. To soften this, the
00066   1/4 pitch period of real speech preceeding the cycle to be repeated is blended with the last
00067   1/4 pitch period of the cycle to be repeated, using an overlap-add (OLA) technique (i.e.
00068   in total, the last 5/4 pitch periods of real speech are used).
00069 
00070 - The start of the synthetic speech will not always fit together smoothly with the tail of
00071   real speech passed on before the erasure was identified. Ideally, we would like to modify
00072   the last 1/4 pitch period of the real speech, to blend it into the synthetic speech. However,
00073   it is too late for that. We could have delayed the real speech a little, but that would
00074   require more buffer manipulation, and hurt the efficiency of the no-lost-packets case
00075   (which we hope is the dominant case). Instead we use a degenerate form of OLA to modify
00076   the start of the synthetic data. The last 1/4 pitch period of real speech is time reversed,
00077   and OLA is used to blend it with the first 1/4 pitch period of synthetic speech. The result
00078   seems quite acceptable.
00079 
00080 - As we progress into the erasure, the chances of the synthetic signal being anything like
00081   correct steadily fall. Therefore, the volume of the synthesized signal is made to decay
00082   linearly, such that after 50ms of missing audio it is reduced to silence.
00083 
00084 - When real speech resumes, an extra 1/4 pitch period of sythetic speech is blended with the
00085   start of the real speech. If the erasure is small, this smoothes the transition. If the erasure
00086   is long, and the synthetic signal has faded to zero, the blending softens the start up of the
00087   real signal, avoiding a kind of "click" or "pop" effect that might occur with a sudden onset.
00088 
00089 \section plc_page_sec_3 How do I use it?
00090 Before audio is processed, call plc_init() to create an instance of the packet loss
00091 concealer. For each received audio packet that is acceptable (i.e. not including those being
00092 dropped for being too late) call plc_rx() to record the content of the packet. Note this may
00093 modify the packet a little after a period of packet loss, to blend real synthetic data smoothly.
00094 When a real packet is not available in time, call plc_fillin() to create a sythetic substitute.
00095 That's it!
00096 */
00097 
00098 #define SAMPLE_RATE     8000
00099 
00100 /*! Minimum allowed pitch (66 Hz) */
00101 #define PLC_PITCH_MIN           120
00102 /*! Maximum allowed pitch (200 Hz) */
00103 #define PLC_PITCH_MAX           40
00104 /*! Maximum pitch OLA window */
00105 #define PLC_PITCH_OVERLAP_MAX   (PLC_PITCH_MIN >> 2)
00106 /*! The length over which the AMDF function looks for similarity (20 ms) */
00107 #define CORRELATION_SPAN        160
00108 /*! History buffer length. The buffer much also be at leat 1.25 times
00109     PLC_PITCH_MIN, but that is much smaller than the buffer needs to be for
00110     the pitch assessment. */
00111 #define PLC_HISTORY_LEN         (CORRELATION_SPAN + PLC_PITCH_MIN)
00112 
00113 typedef struct
00114 {
00115     /*! Consecutive erased samples */
00116     int missing_samples;
00117     /*! Current offset into pitch period */
00118     int pitch_offset;
00119     /*! Pitch estimate */
00120     int pitch;
00121     /*! Buffer for a cycle of speech */
00122     float pitchbuf[PLC_PITCH_MIN];
00123     /*! History buffer */
00124     int16_t history[PLC_HISTORY_LEN];
00125     /*! Current pointer into the history buffer */
00126     int buf_ptr;
00127 } plc_state_t;
00128 
00129 
00130 #ifdef __cplusplus
00131 extern "C" {
00132 #endif
00133 
00134 /*! Process a block of received audio samples.
00135     \brief Process a block of received audio samples.
00136     \param s The packet loss concealer context.
00137     \param amp The audio sample buffer.
00138     \param len The number of samples in the buffer.
00139     \return The number of samples in the buffer. */
00140 int plc_rx(plc_state_t *s, int16_t amp[], int len);
00141 
00142 /*! Fill-in a block of missing audio samples.
00143     \brief Fill-in a block of missing audio samples.
00144     \param s The packet loss concealer context.
00145     \param amp The audio sample buffer.
00146     \param len The number of samples to be synthesised.
00147     \return The number of samples synthesized. */
00148 int plc_fillin(plc_state_t *s, int16_t amp[], int len);
00149 
00150 /*! Process a block of received V.29 modem audio samples.
00151     \brief Process a block of received V.29 modem audio samples.
00152     \param s The packet loss concealer context.
00153     \return A pointer to the he packet loss concealer context. */
00154 plc_state_t *plc_init(plc_state_t *s);
00155 
00156 #ifdef __cplusplus
00157 }
00158 #endif
00159 
00160 #endif
00161 /*- End of file ------------------------------------------------------------*/

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