165 lines
6.9 KiB
C
165 lines
6.9 KiB
C
/*
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* SpanDSP - a series of DSP components for telephony
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*
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* plc.h
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*
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* Written by Steve Underwood <steveu@coppice.org>
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*
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* Copyright (C) 2004 Steve Underwood
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*
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* All rights reserved.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*
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* This version may be optionally licenced under the GNU LGPL licence.
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* This version is disclaimed to DIGIUM for inclusion in the Asterisk project.
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*/
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/*! \file */
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#if !defined(_PLC_H_)
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#define _PLC_H_
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#ifdef SOLARIS
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#include <sys/int_types.h>
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#else
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#ifndef _MSC_VER
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#include <inttypes.h>
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#else
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typedef short int16_t;
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#endif
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#endif
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/*! \page plc_page Packet loss concealment
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\section plc_page_sec_1 What does it do?
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The packet loss concealment module provides a suitable synthetic fill-in signal,
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to minimise the audible effect of lost packets in VoIP applications. It is not
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tied to any particular codec, and could be used with almost any codec which does not
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specify its own procedure for packet loss concealment.
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Where a codec specific concealment procedure exists, the algorithm is usually built
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around knowledge of the characteristics of the particular codec. It will, therefore,
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generally give better results for that particular codec than this generic concealer will.
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\section plc_page_sec_2 How does it work?
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While good packets are being received, the plc_rx() routine keeps a record of the trailing
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section of the known speech signal. If a packet is missed, plc_fillin() is called to produce
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a synthetic replacement for the real speech signal. The average mean difference function
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(AMDF) is applied to the last known good signal, to determine its effective pitch.
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Based on this, the last pitch period of signal is saved. Essentially, this cycle of speech
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will be repeated over and over until the real speech resumes. However, several refinements
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are needed to obtain smooth pleasant sounding results.
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- The two ends of the stored cycle of speech will not always fit together smoothly. This can
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cause roughness, or even clicks, at the joins between cycles. To soften this, the
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1/4 pitch period of real speech preceeding the cycle to be repeated is blended with the last
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1/4 pitch period of the cycle to be repeated, using an overlap-add (OLA) technique (i.e.
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in total, the last 5/4 pitch periods of real speech are used).
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- The start of the synthetic speech will not always fit together smoothly with the tail of
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real speech passed on before the erasure was identified. Ideally, we would like to modify
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the last 1/4 pitch period of the real speech, to blend it into the synthetic speech. However,
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it is too late for that. We could have delayed the real speech a little, but that would
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require more buffer manipulation, and hurt the efficiency of the no-lost-packets case
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(which we hope is the dominant case). Instead we use a degenerate form of OLA to modify
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the start of the synthetic data. The last 1/4 pitch period of real speech is time reversed,
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and OLA is used to blend it with the first 1/4 pitch period of synthetic speech. The result
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seems quite acceptable.
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- As we progress into the erasure, the chances of the synthetic signal being anything like
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correct steadily fall. Therefore, the volume of the synthesized signal is made to decay
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linearly, such that after 50ms of missing audio it is reduced to silence.
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- When real speech resumes, an extra 1/4 pitch period of sythetic speech is blended with the
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start of the real speech. If the erasure is small, this smoothes the transition. If the erasure
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is long, and the synthetic signal has faded to zero, the blending softens the start up of the
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real signal, avoiding a kind of "click" or "pop" effect that might occur with a sudden onset.
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\section plc_page_sec_3 How do I use it?
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Before audio is processed, call plc_init() to create an instance of the packet loss
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concealer. For each received audio packet that is acceptable (i.e. not including those being
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dropped for being too late) call plc_rx() to record the content of the packet. Note this may
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modify the packet a little after a period of packet loss, to blend real synthetic data smoothly.
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When a real packet is not available in time, call plc_fillin() to create a sythetic substitute.
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That's it!
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*/
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#define SAMPLE_RATE 8000
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/*! Minimum allowed pitch (66 Hz) */
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#define PLC_PITCH_MIN 120
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/*! Maximum allowed pitch (200 Hz) */
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#define PLC_PITCH_MAX 40
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/*! Maximum pitch OLA window */
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#define PLC_PITCH_OVERLAP_MAX (PLC_PITCH_MIN >> 2)
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/*! The length over which the AMDF function looks for similarity (20 ms) */
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#define CORRELATION_SPAN 160
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/*! History buffer length. The buffer much also be at leat 1.25 times
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PLC_PITCH_MIN, but that is much smaller than the buffer needs to be for
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the pitch assessment. */
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#define PLC_HISTORY_LEN (CORRELATION_SPAN + PLC_PITCH_MIN)
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typedef struct
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{
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/*! Consecutive erased samples */
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int missing_samples;
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/*! Current offset into pitch period */
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int pitch_offset;
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/*! Pitch estimate */
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int pitch;
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/*! Buffer for a cycle of speech */
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float pitchbuf[PLC_PITCH_MIN];
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/*! History buffer */
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int16_t history[PLC_HISTORY_LEN];
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/*! Current pointer into the history buffer */
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int buf_ptr;
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} plc_state_t;
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#ifdef __cplusplus
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extern "C" {
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#endif
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/*! Process a block of received audio samples.
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\brief Process a block of received audio samples.
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\param s The packet loss concealer context.
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\param amp The audio sample buffer.
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\param len The number of samples in the buffer.
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\return The number of samples in the buffer. */
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int plc_rx(plc_state_t *s, int16_t amp[], int len);
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/*! Fill-in a block of missing audio samples.
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\brief Fill-in a block of missing audio samples.
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\param s The packet loss concealer context.
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\param amp The audio sample buffer.
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\param len The number of samples to be synthesised.
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\return The number of samples synthesized. */
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int plc_fillin(plc_state_t *s, int16_t amp[], int len);
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/*! Process a block of received V.29 modem audio samples.
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\brief Process a block of received V.29 modem audio samples.
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\param s The packet loss concealer context.
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\param amp The audio sample buffer.
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\param len The number of samples in the buffer.
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\return A pointer to the he packet loss concealer context. */
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plc_state_t *plc_init(plc_state_t *s);
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#ifdef __cplusplus
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}
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#endif
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#endif
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/*- End of file ------------------------------------------------------------*/
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