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5
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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 * dc_restore.h - General telephony routines to restore the zero D.C.
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5 * level to audio which has a D.C. bias.
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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) 2001 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: dc_restore.h,v 1.15 2006/12/01 18:00:48 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(_DC_RESTORE_H_)
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32 #define _DC_RESTORE_H_
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33
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34 /*! \page dc_restore_page Removing DC bias from a signal
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35
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36 \section dc_restore_page_sec_1 What does it do?
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37
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38 Telecoms signals often contain considerable DC, but DC upsets a lot of signal
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39 processing functions. Placing a zero DC restorer at the front of the processing
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40 chain can often simplify the downstream processing.
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41
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42 \section dc_restore_page_sec_2 How does it work?
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43
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44 The DC restorer uses a leaky integrator to provide a long-ish term estimate of
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45 the DC bias in the signal. A 32 bit estimate is used for the 16 bit audio, so
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46 the noise introduced by the estimation can be keep in the lower bits, and the 16
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47 bit DC value, which is subtracted from the signal, is fairly clean. The
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48 following code fragment shows the algorithm used. dc_bias is a 32 bit integer,
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49 while the sample and the resulting clean_sample are 16 bit integers.
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50
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51 dc_bias += ((((int32_t) sample << 15) - dc_bias) >> 14);
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52 clean_sample = sample - (dc_bias >> 15);
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53 */
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54
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55 /*!
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56 Zero DC restoration descriptor. This defines the working state for a single
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57 instance of DC content filter.
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58 */
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59 typedef struct
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60 {
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61 int32_t state;
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62 } dc_restore_state_t;
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63
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64 #ifdef __cplusplus
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65 extern "C" {
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66 #endif
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67
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68 static __inline__ void dc_restore_init(dc_restore_state_t *dc)
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69 {
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70 dc->state = 0;
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71 }
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72 /*- End of function --------------------------------------------------------*/
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73
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74 static __inline__ int16_t dc_restore(dc_restore_state_t *dc, int16_t sample)
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75 {
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76 dc->state += ((((int32_t) sample << 15) - dc->state) >> 14);
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77 return (int16_t) (sample - (dc->state >> 15));
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78 }
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79 /*- End of function --------------------------------------------------------*/
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80
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81 static __inline__ int16_t dc_restore_estimate(dc_restore_state_t *dc)
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82 {
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83 return (int16_t) (dc->state >> 15);
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84 }
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85 /*- End of function --------------------------------------------------------*/
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86
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87 static __inline__ int16_t saturate(int32_t amp)
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88 {
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89 int16_t amp16;
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90
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91 /* Hopefully this is optimised for the common case - not clipping */
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92 amp16 = (int16_t) amp;
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93 if (amp == amp16)
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94 return amp16;
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95 if (amp > INT16_MAX)
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96 return INT16_MAX;
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97 return INT16_MIN;
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98 }
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99 /*- End of function --------------------------------------------------------*/
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100
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101 static __inline__ int16_t fsaturatef(float famp)
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102 {
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103 if (famp > 32767.0)
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104 return INT16_MAX;
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105 if (famp < -32768.0)
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106 return INT16_MIN;
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107 return (int16_t) rintf(famp);
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108 }
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109 /*- End of function --------------------------------------------------------*/
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110
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111 static __inline__ int16_t fsaturate(double damp)
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112 {
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113 if (damp > 32767.0)
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114 return INT16_MAX;
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115 if (damp < -32768.0)
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116 return INT16_MIN;
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117 return (int16_t) rint(damp);
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118 }
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119 /*- End of function --------------------------------------------------------*/
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120
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121 #ifdef __cplusplus
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122 }
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123 #endif
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124
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125 #endif
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126 /*- End of file ------------------------------------------------------------*/
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