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1 /*
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2 * SpanDSP - a series of DSP components for telephony
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3 *
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4 * plc.h
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5 *
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6 * Written by Steve Underwood <steveu@coppice.org>
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7 *
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8 * Copyright (C) 2004 Steve Underwood
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9 *
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10 * All rights reserved.
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11 *
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12 * This program is free software; you can redistribute it and/or modify
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13 * it under the terms of the GNU General Public License version 2, as
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14 * published by the Free Software Foundation.
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15 *
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16 * This program is distributed in the hope that it will be useful,
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17 * but WITHOUT ANY WARRANTY; without even the implied warranty of
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18 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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19 * GNU General Public License for more details.
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20 *
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21 * You should have received a copy of the GNU General Public License
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22 * along with this program; if not, write to the Free Software
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23 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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24 *
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25 * $Id: plc.h,v 1.12 2006/10/24 13:45:28 steveu Exp $
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26 */
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27
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28 /*! \file */
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29
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30 #if !defined(_PLC_H_)
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31 #define _PLC_H_
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32
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33 /*! \page plc_page Packet loss concealment
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34 \section plc_page_sec_1 What does it do?
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35 The packet loss concealment module provides a synthetic fill-in signal, to minimise
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36 the audible effect of lost packets in VoIP applications. It is not tied to any
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37 particular codec, and could be used with almost any codec which does not
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38 specify its own procedure for packet loss concealment.
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39
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40 Where a codec specific concealment procedure exists, that algorithm is usually built
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41 around knowledge of the characteristics of the particular codec. It will, therefore,
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42 generally give better results for that particular codec than this generic concealer will.
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43
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44 The PLC code implements an algorithm similar to the one described in Appendix 1 of G.711.
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45 However, the G.711 algorithm is optimised for 10ms packets. Few people use such small
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46 packets. 20ms is a much more common value, and longer packets are also quite common. The
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47 algorithm has been adjusted with this in mind. Also, the G.711 approach causes an
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48 algorithmic delay, and requires significant buffer manipulation when there is no packet
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49 loss. The algorithm used here avoids this. It causes no delay, and achieves comparable
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50 quality with normal speech.
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51
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52 Note that both this algorithm, and the one in G.711 are optimised for speech. For most kinds
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53 of music a much slower decay on bursts of lost packets give better results.
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54
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55 \section plc_page_sec_2 How does it work?
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56 While good packets are being received, the plc_rx() routine keeps a record of the trailing
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57 section of the known speech signal. If a packet is missed, plc_fillin() is called to produce
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58 a synthetic replacement for the real speech signal. The average mean difference function
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59 (AMDF) is applied to the last known good signal, to determine its effective pitch.
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60 Based on this, the last pitch period of signal is saved. Essentially, this cycle of speech
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61 will be repeated over and over until the real speech resumes. However, several refinements
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62 are needed to obtain smooth pleasant sounding results.
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63
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64 - The two ends of the stored cycle of speech will not always fit together smoothly. This can
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65 cause roughness, or even clicks, at the joins between cycles. To soften this, the
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66 1/4 pitch period of real speech preceeding the cycle to be repeated is blended with the last
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67 1/4 pitch period of the cycle to be repeated, using an overlap-add (OLA) technique (i.e.
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68 in total, the last 5/4 pitch periods of real speech are used).
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69
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70 - The start of the synthetic speech will not always fit together smoothly with the tail of
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71 real speech passed on before the erasure was identified. Ideally, we would like to modify
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72 the last 1/4 pitch period of the real speech, to blend it into the synthetic speech. However,
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73 it is too late for that. We could have delayed the real speech a little, but that would
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74 require more buffer manipulation, and hurt the efficiency of the no-lost-packets case
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75 (which we hope is the dominant case). Instead we use a degenerate form of OLA to modify
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76 the start of the synthetic data. The last 1/4 pitch period of real speech is time reversed,
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77 and OLA is used to blend it with the first 1/4 pitch period of synthetic speech. The result
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78 seems quite acceptable.
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79
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80 - As we progress into the erasure, the chances of the synthetic signal being anything like
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81 correct steadily fall. Therefore, the volume of the synthesized signal is made to decay
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82 linearly, such that after 50ms of missing audio it is reduced to silence.
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83
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84 - When real speech resumes, an extra 1/4 pitch period of synthetic speech is blended with the
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85 start of the real speech. If the erasure is small, this smoothes the transition. If the erasure
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86 is long, and the synthetic signal has faded to zero, the blending softens the start up of the
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87 real signal, avoiding a kind of "click" or "pop" effect that might occur with a sudden onset.
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88
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89 \section plc_page_sec_3 How do I use it?
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90 Before audio is processed, call plc_init() to create an instance of the packet loss
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91 concealer. For each received audio packet that is acceptable (i.e. not including those being
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92 dropped for being too late) call plc_rx() to record the content of the packet. Note this may
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93 modify the packet a little after a period of packet loss, to blend real synthetic data smoothly.
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94 When a real packet is not available in time, call plc_fillin() to create a sythetic substitute.
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95 That's it!
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96 */
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97
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98 /*! Minimum allowed pitch (66 Hz) */
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99 #define PLC_PITCH_MIN 120
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100 /*! Maximum allowed pitch (200 Hz) */
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101 #define PLC_PITCH_MAX 40
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102 /*! Maximum pitch OLA window */
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103 #define PLC_PITCH_OVERLAP_MAX (PLC_PITCH_MIN >> 2)
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104 /*! The length over which the AMDF function looks for similarity (20 ms) */
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105 #define CORRELATION_SPAN 160
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106 /*! History buffer length. The buffer much also be at leat 1.25 times
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107 PLC_PITCH_MIN, but that is much smaller than the buffer needs to be for
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108 the pitch assessment. */
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109 #define PLC_HISTORY_LEN (CORRELATION_SPAN + PLC_PITCH_MIN)
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110
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111 /*!
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112 The generic packet loss concealer context.
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113 */
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114 typedef struct
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115 {
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116 /*! Consecutive erased samples */
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117 int missing_samples;
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118 /*! Current offset into pitch period */
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119 int pitch_offset;
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120 /*! Pitch estimate */
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121 int pitch;
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122 /*! Buffer for a cycle of speech */
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123 float pitchbuf[PLC_PITCH_MIN];
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124 /*! History buffer */
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125 int16_t history[PLC_HISTORY_LEN];
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126 /*! Current pointer into the history buffer */
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127 int buf_ptr;
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128 } plc_state_t;
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129
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130
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131 #ifdef __cplusplus
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132 extern "C" {
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133 #endif
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134
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135 /*! Process a block of received audio samples for PLC.
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136 \brief Process a block of received audio samples for PLC.
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137 \param s The packet loss concealer context.
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138 \param amp The audio sample buffer.
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139 \param len The number of samples in the buffer.
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140 \return The number of samples in the buffer. */
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141 int plc_rx(plc_state_t *s, int16_t amp[], int len);
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142
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143 /*! Fill-in a block of missing audio samples.
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144 \brief Fill-in a block of missing audio samples.
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145 \param s The packet loss concealer context.
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146 \param amp The audio sample buffer.
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147 \param len The number of samples to be synthesised.
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148 \return The number of samples synthesized. */
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149 int plc_fillin(plc_state_t *s, int16_t amp[], int len);
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150
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151 /*! Initialise a packet loss concealer context.
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152 \brief Initialise a PLC context.
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153 \param s The packet loss concealer context.
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154 \return A pointer to the the packet loss concealer context. */
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155 plc_state_t *plc_init(plc_state_t *s);
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156
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157 /*! Free a packet loss concealer context.
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158 \param s The packet loss concealer context.
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159 \return 0 for OK. */
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160 int plc_release(plc_state_t *s);
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161
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162 #ifdef __cplusplus
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163 }
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164 #endif
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165
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166 #endif
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167 /*- End of file ------------------------------------------------------------*/
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