--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/spandsp-0.0.3/spandsp-0.0.3/src/spandsp/dc_restore.h	Fri Jun 25 16:00:21 2010 +0200
@@ -0,0 +1,126 @@
+/*
+ * 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 General Public License version 2, 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU 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 1.15 2006/12/01 18:00:48 steveu Exp $
+ */
+
+/*! \file */
+
+#if !defined(_DC_RESTORE_H_)
+#define _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;
+
+#ifdef __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 --------------------------------------------------------*/
+
+static __inline__ int16_t saturate(int32_t amp)
+{
+    int16_t amp16;
+
+    /* Hopefully this is optimised for the common case - not clipping */
+    amp16 = (int16_t) amp;
+    if (amp == amp16)
+        return amp16;
+    if (amp > INT16_MAX)
+        return  INT16_MAX;
+    return  INT16_MIN;
+}
+/*- End of function --------------------------------------------------------*/
+
+static __inline__ int16_t fsaturatef(float famp)
+{
+    if (famp > 32767.0)
+        return  INT16_MAX;
+    if (famp < -32768.0)
+        return  INT16_MIN;
+    return (int16_t) rintf(famp);
+}
+/*- End of function --------------------------------------------------------*/
+
+static __inline__ int16_t fsaturate(double damp)
+{
+    if (damp > 32767.0)
+        return  INT16_MAX;
+    if (damp < -32768.0)
+        return  INT16_MIN;
+    return (int16_t) rint(damp);
+}
+/*- End of function --------------------------------------------------------*/
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+/*- End of file ------------------------------------------------------------*/