Mercurial > hg > audiostuff
comparison spandsp-0.0.6pre17/src/spandsp/dc_restore.h @ 4:26cd8f1ef0b1
import spandsp-0.0.6pre17
author | Peter Meerwald <pmeerw@cosy.sbg.ac.at> |
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date | Fri, 25 Jun 2010 15:50:58 +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 Lesser General Public License version 2.1, | |
15 * as 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 Lesser General Public License for more details. | |
21 * | |
22 * You should have received a copy of the GNU Lesser General Public | |
23 * License 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 1.24 2008/09/19 14:02:05 steveu Exp $ | |
27 */ | |
28 | |
29 /*! \file */ | |
30 | |
31 #if !defined(_SPANDSP_DC_RESTORE_H_) | |
32 #define _SPANDSP_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 #if defined(__cplusplus) | |
65 extern "C" | |
66 { | |
67 #endif | |
68 | |
69 static __inline__ void dc_restore_init(dc_restore_state_t *dc) | |
70 { | |
71 dc->state = 0; | |
72 } | |
73 /*- End of function --------------------------------------------------------*/ | |
74 | |
75 static __inline__ int16_t dc_restore(dc_restore_state_t *dc, int16_t sample) | |
76 { | |
77 dc->state += ((((int32_t) sample << 15) - dc->state) >> 14); | |
78 return (int16_t) (sample - (dc->state >> 15)); | |
79 } | |
80 /*- End of function --------------------------------------------------------*/ | |
81 | |
82 static __inline__ int16_t dc_restore_estimate(dc_restore_state_t *dc) | |
83 { | |
84 return (int16_t) (dc->state >> 15); | |
85 } | |
86 /*- End of function --------------------------------------------------------*/ | |
87 | |
88 #if defined(__cplusplus) | |
89 } | |
90 #endif | |
91 | |
92 #endif | |
93 /*- End of file ------------------------------------------------------------*/ |