--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/spandsp-0.0.3/spandsp-0.0.3/src/spandsp/noise.h	Fri Jun 25 16:00:21 2010 +0200
@@ -0,0 +1,133 @@
+/*
+ * SpanDSP - a series of DSP components for telephony
+ *
+ * noise.h - A low complexity audio noise generator, suitable for
+ *           real time generation (current just approx AWGN)
+ *
+ * Written by Steve Underwood <steveu@coppice.org>
+ *
+ * Copyright (C) 2005 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: noise.h,v 1.8 2006/10/24 13:45:28 steveu Exp $
+ */
+
+/*! \file */
+
+#if !defined(_NOISE_H_)
+#define _NOISE_H_
+
+/*! \page noise_page Noise generation
+
+\section noise_page_sec_1 What does it do?
+It generates audio noise. Currently it only generates reasonable quality
+AWGN. It is designed to be of sufficiently low complexity to generate large
+volumes of reasonable quality noise, in real time.
+
+Hoth noise is used to model indoor ambient noise when evaluating communications
+systems such as telephones. It is named after D.F. Hoth, who made the first 
+systematic study of this. The official definition of Hoth noise is IEEE
+standard 269-2001 (revised from 269-1992), "Draft Standard Methods for Measuring
+Transmission Performance of Analog and Digital Telephone Sets, Handsets and Headsets."
+
+The table below gives the spectral density of Hoth noise, adjusted in level to produce
+a reading of 50 dBA.
+
+Freq (Hz)  Spectral     Bandwidth       Total power in
+           density      10 log_f        each 1/3 octave band
+           (dB SPL/Hz)  (dB)            (dB SPL)
+ 100        32.4        13.5            45.9
+ 125        30.9        14.7            45.5
+ 160        29.1        15.7            44.9
+ 200        27.6        16.5            44.1
+ 250        26.0        17.6            43.6
+ 315        24.4        18.7            43.1
+ 400        22.7        19.7            42.3
+ 500        21.1        20.6            41.7
+ 630        19.5        21.7            41.2
+ 800        17.8        22.7            40.4
+1000        16.2        23.5            39.7
+1250        14.6        24.7            39.3
+1600        12.9        25.7            38.7
+2000        11.3        26.5            37.8
+2500         9.6        27.6            37.2
+3150         7.8        28.7            36.5
+4000         5.4        29.7            34.8
+5000         2.6        30.6            33.2
+6300        -1.3        31.7            30.4
+8000        -6.6        32.7            26.0
+
+The tolerance for each 1/3rd octave band is กำ3dB.
+
+\section awgn_page_sec_2 How does it work?
+The central limit theorem says if you add a few random numbers together,
+the result starts to look Gaussian. In this case we sum 8 random numbers.
+The result is fast, and perfectly good as a noise source for many purposes.
+It should not be trusted as a high quality AWGN generator, for elaborate
+modelling purposes.
+*/
+
+enum
+{
+    NOISE_CLASS_AWGN = 1,
+    NOISE_CLASS_HOTH
+};
+
+/*!
+    Noise generator descriptor. This contains all the state information for an instance
+    of the noise generator.
+ */
+typedef struct
+{
+    int class_of_noise;
+    int quality;
+    int32_t rms;
+    uint32_t rndnum;
+    int32_t state;
+} noise_state_t;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*! Initialise an audio noise generator.
+    \brief Initialise an audio noise generator.
+    \param s The noise generator context.
+    \param seed A seed for the underlying random number generator.
+    \param level The noise power level in dBmO.
+    \param class_of_noise The class of noise (e.g. AWGN).
+    \param quality A parameter which permits speed and accuracy of the noise
+           generation to be adjusted.
+    \return A pointer to the noise generator context.
+*/
+noise_state_t *noise_init_dbm0(noise_state_t *s, int seed, float level, int class_of_noise, int quality);
+
+noise_state_t *noise_init_dbov(noise_state_t *s, int seed, float level, int class_of_noise, int quality);
+
+/*! Generate a sample of audio noise.
+    \brief Generate a sample of audio noise.
+    \param s The noise generator context.
+    \return The generated sample.
+*/
+int16_t noise(noise_state_t *s);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+/*- End of file ------------------------------------------------------------*/