Mercurial > hg > audiostuff
diff 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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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/spandsp-0.0.6pre17/src/spandsp/dc_restore.h Fri Jun 25 15:50:58 2010 +0200 @@ -0,0 +1,93 @@ +/* + * SpanDSP - a series of DSP components for telephony + * + * dc_restore.h - General telephony routines to restore the zero D.C. + * level to audio which has a D.C. bias. + * + * Written by Steve Underwood <steveu@coppice.org> + * + * Copyright (C) 2001 Steve Underwood + * + * All rights reserved. + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU Lesser General Public License version 2.1, + * as published by the Free Software Foundation. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU Lesser General Public License for more details. + * + * You should have received a copy of the GNU Lesser General Public + * License along with this program; if not, write to the Free Software + * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. + * + * $Id: dc_restore.h,v 1.24 2008/09/19 14:02:05 steveu Exp $ + */ + +/*! \file */ + +#if !defined(_SPANDSP_DC_RESTORE_H_) +#define _SPANDSP_DC_RESTORE_H_ + +/*! \page dc_restore_page Removing DC bias from a signal + +\section dc_restore_page_sec_1 What does it do? + +Telecoms signals often contain considerable DC, but DC upsets a lot of signal +processing functions. Placing a zero DC restorer at the front of the processing +chain can often simplify the downstream processing. + +\section dc_restore_page_sec_2 How does it work? + +The DC restorer uses a leaky integrator to provide a long-ish term estimate of +the DC bias in the signal. A 32 bit estimate is used for the 16 bit audio, so +the noise introduced by the estimation can be keep in the lower bits, and the 16 +bit DC value, which is subtracted from the signal, is fairly clean. The +following code fragment shows the algorithm used. dc_bias is a 32 bit integer, +while the sample and the resulting clean_sample are 16 bit integers. + + dc_bias += ((((int32_t) sample << 15) - dc_bias) >> 14); + clean_sample = sample - (dc_bias >> 15); +*/ + +/*! + Zero DC restoration descriptor. This defines the working state for a single + instance of DC content filter. +*/ +typedef struct +{ + int32_t state; +} dc_restore_state_t; + +#if defined(__cplusplus) +extern "C" +{ +#endif + +static __inline__ void dc_restore_init(dc_restore_state_t *dc) +{ + dc->state = 0; +} +/*- End of function --------------------------------------------------------*/ + +static __inline__ int16_t dc_restore(dc_restore_state_t *dc, int16_t sample) +{ + dc->state += ((((int32_t) sample << 15) - dc->state) >> 14); + return (int16_t) (sample - (dc->state >> 15)); +} +/*- End of function --------------------------------------------------------*/ + +static __inline__ int16_t dc_restore_estimate(dc_restore_state_t *dc) +{ + return (int16_t) (dc->state >> 15); +} +/*- End of function --------------------------------------------------------*/ + +#if defined(__cplusplus) +} +#endif + +#endif +/*- End of file ------------------------------------------------------------*/