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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 * noise.h - A low complexity audio noise generator, suitable for
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5 * real time generation (current just approx AWGN)
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6 *
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7 * Written by Steve Underwood <steveu@coppice.org>
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8 *
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9 * Copyright (C) 2005 Steve Underwood
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10 *
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11 * All rights reserved.
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12 *
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13 * This program is free software; you can redistribute it and/or modify
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14 * it under the terms of the GNU General Public License version 2, as
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15 * published by the Free Software Foundation.
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16 *
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17 * This program is distributed in the hope that it will be useful,
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18 * but WITHOUT ANY WARRANTY; without even the implied warranty of
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19 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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20 * GNU General Public License for more details.
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21 *
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22 * You should have received a copy of the GNU General Public License
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23 * along with this program; if not, write to the Free Software
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24 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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25 *
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26 * $Id: noise.h,v 1.8 2006/10/24 13:45:28 steveu Exp $
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27 */
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28
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29 /*! \file */
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30
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31 #if !defined(_NOISE_H_)
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32 #define _NOISE_H_
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33
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34 /*! \page noise_page Noise generation
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35
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36 \section noise_page_sec_1 What does it do?
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37 It generates audio noise. Currently it only generates reasonable quality
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38 AWGN. It is designed to be of sufficiently low complexity to generate large
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39 volumes of reasonable quality noise, in real time.
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40
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41 Hoth noise is used to model indoor ambient noise when evaluating communications
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42 systems such as telephones. It is named after D.F. Hoth, who made the first
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43 systematic study of this. The official definition of Hoth noise is IEEE
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44 standard 269-2001 (revised from 269-1992), "Draft Standard Methods for Measuring
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45 Transmission Performance of Analog and Digital Telephone Sets, Handsets and Headsets."
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46
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47 The table below gives the spectral density of Hoth noise, adjusted in level to produce
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48 a reading of 50 dBA.
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49
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50 Freq (Hz) Spectral Bandwidth Total power in
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51 density 10 log_f each 1/3 octave band
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52 (dB SPL/Hz) (dB) (dB SPL)
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53 100 32.4 13.5 45.9
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54 125 30.9 14.7 45.5
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55 160 29.1 15.7 44.9
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56 200 27.6 16.5 44.1
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57 250 26.0 17.6 43.6
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58 315 24.4 18.7 43.1
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59 400 22.7 19.7 42.3
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60 500 21.1 20.6 41.7
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61 630 19.5 21.7 41.2
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62 800 17.8 22.7 40.4
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63 1000 16.2 23.5 39.7
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64 1250 14.6 24.7 39.3
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65 1600 12.9 25.7 38.7
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66 2000 11.3 26.5 37.8
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67 2500 9.6 27.6 37.2
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68 3150 7.8 28.7 36.5
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69 4000 5.4 29.7 34.8
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70 5000 2.6 30.6 33.2
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71 6300 -1.3 31.7 30.4
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72 8000 -6.6 32.7 26.0
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73
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74 The tolerance for each 1/3rd octave band is กำ3dB.
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75
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76 \section awgn_page_sec_2 How does it work?
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77 The central limit theorem says if you add a few random numbers together,
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78 the result starts to look Gaussian. In this case we sum 8 random numbers.
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79 The result is fast, and perfectly good as a noise source for many purposes.
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80 It should not be trusted as a high quality AWGN generator, for elaborate
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81 modelling purposes.
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82 */
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83
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84 enum
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85 {
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86 NOISE_CLASS_AWGN = 1,
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87 NOISE_CLASS_HOTH
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88 };
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89
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90 /*!
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91 Noise generator descriptor. This contains all the state information for an instance
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92 of the noise generator.
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93 */
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94 typedef struct
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95 {
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96 int class_of_noise;
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97 int quality;
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98 int32_t rms;
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99 uint32_t rndnum;
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100 int32_t state;
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101 } noise_state_t;
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102
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103 #ifdef __cplusplus
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104 extern "C" {
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105 #endif
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106
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107 /*! Initialise an audio noise generator.
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108 \brief Initialise an audio noise generator.
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109 \param s The noise generator context.
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110 \param seed A seed for the underlying random number generator.
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111 \param level The noise power level in dBmO.
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112 \param class_of_noise The class of noise (e.g. AWGN).
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113 \param quality A parameter which permits speed and accuracy of the noise
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114 generation to be adjusted.
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115 \return A pointer to the noise generator context.
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116 */
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117 noise_state_t *noise_init_dbm0(noise_state_t *s, int seed, float level, int class_of_noise, int quality);
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118
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119 noise_state_t *noise_init_dbov(noise_state_t *s, int seed, float level, int class_of_noise, int quality);
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120
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121 /*! Generate a sample of audio noise.
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122 \brief Generate a sample of audio noise.
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123 \param s The noise generator context.
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124 \return The generated sample.
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125 */
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126 int16_t noise(noise_state_t *s);
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127
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128 #ifdef __cplusplus
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129 }
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130 #endif
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131
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132 #endif
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133 /*- End of file ------------------------------------------------------------*/
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