Mercurial > hg > audiostuff
comparison spandsp-0.0.3/spandsp-0.0.3/src/spandsp/dc_restore.h @ 5:f762bf195c4b
import spandsp-0.0.3
author | Peter Meerwald <pmeerw@cosy.sbg.ac.at> |
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date | Fri, 25 Jun 2010 16:00:21 +0200 |
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1 /* | |
2 * SpanDSP - a series of DSP components for telephony | |
3 * | |
4 * dc_restore.h - General telephony routines to restore the zero D.C. | |
5 * level to audio which has a D.C. bias. | |
6 * | |
7 * Written by Steve Underwood <steveu@coppice.org> | |
8 * | |
9 * Copyright (C) 2001 Steve Underwood | |
10 * | |
11 * All rights reserved. | |
12 * | |
13 * This program is free software; you can redistribute it and/or modify | |
14 * it under the terms of the GNU General Public License version 2, as | |
15 * published by the Free Software Foundation. | |
16 * | |
17 * This program is distributed in the hope that it will be useful, | |
18 * but WITHOUT ANY WARRANTY; without even the implied warranty of | |
19 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
20 * GNU General Public License for more details. | |
21 * | |
22 * You should have received a copy of the GNU General Public License | |
23 * along with this program; if not, write to the Free Software | |
24 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. | |
25 * | |
26 * $Id: dc_restore.h,v 1.15 2006/12/01 18:00:48 steveu Exp $ | |
27 */ | |
28 | |
29 /*! \file */ | |
30 | |
31 #if !defined(_DC_RESTORE_H_) | |
32 #define _DC_RESTORE_H_ | |
33 | |
34 /*! \page dc_restore_page Removing DC bias from a signal | |
35 | |
36 \section dc_restore_page_sec_1 What does it do? | |
37 | |
38 Telecoms signals often contain considerable DC, but DC upsets a lot of signal | |
39 processing functions. Placing a zero DC restorer at the front of the processing | |
40 chain can often simplify the downstream processing. | |
41 | |
42 \section dc_restore_page_sec_2 How does it work? | |
43 | |
44 The DC restorer uses a leaky integrator to provide a long-ish term estimate of | |
45 the DC bias in the signal. A 32 bit estimate is used for the 16 bit audio, so | |
46 the noise introduced by the estimation can be keep in the lower bits, and the 16 | |
47 bit DC value, which is subtracted from the signal, is fairly clean. The | |
48 following code fragment shows the algorithm used. dc_bias is a 32 bit integer, | |
49 while the sample and the resulting clean_sample are 16 bit integers. | |
50 | |
51 dc_bias += ((((int32_t) sample << 15) - dc_bias) >> 14); | |
52 clean_sample = sample - (dc_bias >> 15); | |
53 */ | |
54 | |
55 /*! | |
56 Zero DC restoration descriptor. This defines the working state for a single | |
57 instance of DC content filter. | |
58 */ | |
59 typedef struct | |
60 { | |
61 int32_t state; | |
62 } dc_restore_state_t; | |
63 | |
64 #ifdef __cplusplus | |
65 extern "C" { | |
66 #endif | |
67 | |
68 static __inline__ void dc_restore_init(dc_restore_state_t *dc) | |
69 { | |
70 dc->state = 0; | |
71 } | |
72 /*- End of function --------------------------------------------------------*/ | |
73 | |
74 static __inline__ int16_t dc_restore(dc_restore_state_t *dc, int16_t sample) | |
75 { | |
76 dc->state += ((((int32_t) sample << 15) - dc->state) >> 14); | |
77 return (int16_t) (sample - (dc->state >> 15)); | |
78 } | |
79 /*- End of function --------------------------------------------------------*/ | |
80 | |
81 static __inline__ int16_t dc_restore_estimate(dc_restore_state_t *dc) | |
82 { | |
83 return (int16_t) (dc->state >> 15); | |
84 } | |
85 /*- End of function --------------------------------------------------------*/ | |
86 | |
87 static __inline__ int16_t saturate(int32_t amp) | |
88 { | |
89 int16_t amp16; | |
90 | |
91 /* Hopefully this is optimised for the common case - not clipping */ | |
92 amp16 = (int16_t) amp; | |
93 if (amp == amp16) | |
94 return amp16; | |
95 if (amp > INT16_MAX) | |
96 return INT16_MAX; | |
97 return INT16_MIN; | |
98 } | |
99 /*- End of function --------------------------------------------------------*/ | |
100 | |
101 static __inline__ int16_t fsaturatef(float famp) | |
102 { | |
103 if (famp > 32767.0) | |
104 return INT16_MAX; | |
105 if (famp < -32768.0) | |
106 return INT16_MIN; | |
107 return (int16_t) rintf(famp); | |
108 } | |
109 /*- End of function --------------------------------------------------------*/ | |
110 | |
111 static __inline__ int16_t fsaturate(double damp) | |
112 { | |
113 if (damp > 32767.0) | |
114 return INT16_MAX; | |
115 if (damp < -32768.0) | |
116 return INT16_MIN; | |
117 return (int16_t) rint(damp); | |
118 } | |
119 /*- End of function --------------------------------------------------------*/ | |
120 | |
121 #ifdef __cplusplus | |
122 } | |
123 #endif | |
124 | |
125 #endif | |
126 /*- End of file ------------------------------------------------------------*/ |