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+/*
+ * SpanDSP - a series of DSP components for telephony
+ *
+ * plc.h
+ *
+ * Written by Steve Underwood <steveu@coppice.org>
+ *
+ * Copyright (C) 2004 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 as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 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.
+ *
+ * This version may be optionally licenced under the GNU LGPL licence.
+ * This version is disclaimed to DIGIUM for inclusion in the Asterisk project.
+ */
+
+/*! \file */
+
+#if !defined(_PLC_H_)
+#define _PLC_H_
+
+#ifdef SOLARIS
+#include <sys/int_types.h>
+#else
+#include <stdint.h>
+#endif
+
+/*! \page plc_page Packet loss concealment
+\section plc_page_sec_1 What does it do?
+The packet loss concealment module provides a suitable synthetic fill-in signal,
+to minimise the audible effect of lost packets in VoIP applications. It is not
+tied to any particular codec, and could be used with almost any codec which does not
+specify its own procedure for packet loss concealment.
+
+Where a codec specific concealment procedure exists, the algorithm is usually built
+around knowledge of the characteristics of the particular codec. It will, therefore,
+generally give better results for that particular codec than this generic concealer will.
+
+\section plc_page_sec_2 How does it work?
+While good packets are being received, the plc_rx() routine keeps a record of the trailing
+section of the known speech signal. If a packet is missed, plc_fillin() is called to produce
+a synthetic replacement for the real speech signal. The average mean difference function
+(AMDF) is applied to the last known good signal, to determine its effective pitch.
+Based on this, the last pitch period of signal is saved. Essentially, this cycle of speech
+will be repeated over and over until the real speech resumes. However, several refinements
+are needed to obtain smooth pleasant sounding results.
+
+- The two ends of the stored cycle of speech will not always fit together smoothly. This can
+  cause roughness, or even clicks, at the joins between cycles. To soften this, the
+  1/4 pitch period of real speech preceeding the cycle to be repeated is blended with the last
+  1/4 pitch period of the cycle to be repeated, using an overlap-add (OLA) technique (i.e.
+  in total, the last 5/4 pitch periods of real speech are used).
+
+- The start of the synthetic speech will not always fit together smoothly with the tail of
+  real speech passed on before the erasure was identified. Ideally, we would like to modify
+  the last 1/4 pitch period of the real speech, to blend it into the synthetic speech. However,
+  it is too late for that. We could have delayed the real speech a little, but that would
+  require more buffer manipulation, and hurt the efficiency of the no-lost-packets case
+  (which we hope is the dominant case). Instead we use a degenerate form of OLA to modify
+  the start of the synthetic data. The last 1/4 pitch period of real speech is time reversed,
+  and OLA is used to blend it with the first 1/4 pitch period of synthetic speech. The result
+  seems quite acceptable.
+
+- As we progress into the erasure, the chances of the synthetic signal being anything like
+  correct steadily fall. Therefore, the volume of the synthesized signal is made to decay
+  linearly, such that after 50ms of missing audio it is reduced to silence.
+
+- When real speech resumes, an extra 1/4 pitch period of sythetic speech is blended with the
+  start of the real speech. If the erasure is small, this smoothes the transition. If the erasure
+  is long, and the synthetic signal has faded to zero, the blending softens the start up of the
+  real signal, avoiding a kind of "click" or "pop" effect that might occur with a sudden onset.
+
+\section plc_page_sec_3 How do I use it?
+Before audio is processed, call plc_init() to create an instance of the packet loss
+concealer. For each received audio packet that is acceptable (i.e. not including those being
+dropped for being too late) call plc_rx() to record the content of the packet. Note this may
+modify the packet a little after a period of packet loss, to blend real synthetic data smoothly.
+When a real packet is not available in time, call plc_fillin() to create a sythetic substitute.
+That's it!
+*/
+
+#define SAMPLE_RATE     8000
+
+/*! Minimum allowed pitch (66 Hz) */
+#define PLC_PITCH_MIN           120
+/*! Maximum allowed pitch (200 Hz) */
+#define PLC_PITCH_MAX           40
+/*! Maximum pitch OLA window */
+#define PLC_PITCH_OVERLAP_MAX   (PLC_PITCH_MIN >> 2)
+/*! The length over which the AMDF function looks for similarity (20 ms) */
+#define CORRELATION_SPAN        160
+/*! History buffer length. The buffer much also be at leat 1.25 times
+    PLC_PITCH_MIN, but that is much smaller than the buffer needs to be for
+    the pitch assessment. */
+#define PLC_HISTORY_LEN         (CORRELATION_SPAN + PLC_PITCH_MIN)
+
+typedef struct
+{
+    /*! Consecutive erased samples */
+    int missing_samples;
+    /*! Current offset into pitch period */
+    int pitch_offset;
+    /*! Pitch estimate */
+    int pitch;
+    /*! Buffer for a cycle of speech */
+    float pitchbuf[PLC_PITCH_MIN];
+    /*! History buffer */
+    int16_t history[PLC_HISTORY_LEN];
+    /*! Current pointer into the history buffer */
+    int buf_ptr;
+} plc_state_t;
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*! Process a block of received audio samples.
+    \brief Process a block of received audio samples.
+    \param s The packet loss concealer context.
+    \param amp The audio sample buffer.
+    \param len The number of samples in the buffer.
+    \return The number of samples in the buffer. */
+int plc_rx(plc_state_t *s, int16_t amp[], int len);
+
+/*! Fill-in a block of missing audio samples.
+    \brief Fill-in a block of missing audio samples.
+    \param s The packet loss concealer context.
+    \param amp The audio sample buffer.
+    \param len The number of samples to be synthesised.
+    \return The number of samples synthesized. */
+int plc_fillin(plc_state_t *s, int16_t amp[], int len);
+
+/*! Process a block of received V.29 modem audio samples.
+    \brief Process a block of received V.29 modem audio samples.
+    \param s The packet loss concealer context.
+    \param amp The audio sample buffer.
+    \param len The number of samples in the buffer.
+    \return A pointer to the he packet loss concealer context. */
+plc_state_t *plc_init(plc_state_t *s);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+/*- End of file ------------------------------------------------------------*/