Internet Draft                                        S. V. Andersen
   Document: draft-ietf-avt-ilbc-codec-05.txt        Aalborg University
   Category: Experimental                                      A. Duric
   May 29th, 2004                                                 Telio
   Expires: November 29th, 2004                               H. Astrom
                                                               R. Hagen
                                                           W. B. Kleijn
                                                              J. Linden
                                                        Global IP Sound
                                               
                                               



                        Internet Low Bit Rate Codec



Status of this Memo


   This document specifies an Internet experimental standards track
   protocol for the Internet community, and requests discussion and
   suggestions for improvements. Please refer to the current edition of
   the "Internet Official Protocol Standards" (STD 1) for the
   standardization state and status of this protocol. Distribution of
   this memo is unlimited.
   
   By submitting this Internet-Draft, we certify that any applicable
   patent or other IPR claims of which we am aware have been disclosed,
   and any of which we become aware will be disclosed, in accordance
   with RFC 3668.


Copyright Notice
   
   Copyright (C) The Internet Society (2004). All Rights Reserved.
   
Abstract
   
   This document specifies a speech codec suitable for robust voice
   communication over IP. The codec is developed by Global IP Sound
   (GIPS). It is designed for narrow band speech and results in a
   payload bit rate of 13.33 kbit/s for 30 ms frames and 15.20 kbit/s
   for 20 ms frames. The codec enables graceful speech quality
   degradation in the case of lost frames, which occurs in connection
   with lost or delayed IP packets.












   
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Table of Contents
   
   Status of this Memo................................................1
   Copyright Notice...................................................1
   Abstract...........................................................1
   Table of Contents..................................................2
   1. INTRODUCTION....................................................5
   2. OUTLINE OF THE CODEC............................................5
   2.1 Encoder........................................................6
   2.2 Decoder........................................................7
   3. ENCODER PRINCIPLES..............................................8
   3.1 Pre-processing.................................................9
   3.2 LPC Analysis and Quantization..................................9
   3.2.1 Computation of Autocorrelation Coefficients..................9
   3.2.2 Computation of LPC Coefficients.............................11
   3.2.3 Computation of LSF Coefficients from LPC Coefficients.......11
   3.2.4 Quantization of LSF Coefficients............................11
   3.2.5 Stability Check of LSF Coefficients.........................12
   3.2.6 Interpolation of LSF Coefficients...........................12
   3.2.7 LPC Analysis and Quantization for 20 ms frames..............13
   3.3 Calculation of the Residual...................................14
   3.4 Perceptual Weighting Filter...................................14
   3.5 Start State Encoder...........................................15
   3.5.1 Start State Estimation......................................15
   3.5.2 All-Pass Filtering and Scale Quantization...................16
   3.5.3 Scalar Quantization.........................................17
   3.6 Encoding the remaining samples................................17
   3.6.1 Codebook Memory.............................................19
   3.6.2 Perceptual Weighting of Codebook Memory and Target..........20
   3.6.3 Codebook Creation...........................................21
   3.6.3.1 Creation of a Base Codebook...............................21
   3.6.3.2 Codebook Expansion........................................22
   3.6.3.3 Codebook Augmentation.....................................22
   3.6.4 Codebook Search.............................................23
   3.6.4.1 Codebook Search at Each Stage.............................24
   3.6.4.2 Gain Quantization at Each Stage...........................24
   3.6.4.3 Preparation of Target for Next Stage......................26
   3.7 Gain Correction Encoding......................................26
   3.8 Bitstream Definition..........................................27
   4. DECODER PRINCIPLES.............................................30
   4.1 LPC Filter Reconstruction.....................................30
   4.2 Start State Reconstruction....................................31
   4.3 Excitation Decoding Loop......................................31
   4.4 Multistage Adaptive Codebook Decoding.........................32
   4.4.1 Construction of the Decoded Excitation Signal...............32
   4.5 Packet Loss Concealment.......................................33
   4.5.1 Block Received Correctly and Previous Block also Received...33


   
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   4.5.2 Block Not Received..........................................33
   4.5.3 Block Received Correctly When Previous Block Not Received...34
   4.6 Enhancement...................................................34
   4.6.1 Estimating the pitch........................................36
   4.6.2 Determination of the Pitch-Synchronous Sequences............36
   4.6.3 Calculation of the smoothed excitation......................37
   4.6.4 Enhancer criterion..........................................38
   4.6.5 Enhancing the excitation....................................38
   4.7 Synthesis Filtering...........................................39
   4.8 Post Filtering................................................39
   5. IANA CONSIDERATIONS............................................39
   6. SECURITY CONSIDERATIONS........................................39
   7. EVALUATION OF THE ILBC IMPLEMENTATIONS.........................39
   8. REFERENCES.....................................................40
   8.1 Normative.....................................................40
   8.2 Informative...................................................40
   9. ACKNOWLEDGEMENTS...............................................40
   10. AUTHOR'S ADDRESSES............................................41
   Full Copyright Statement..........................................42
   Intellectual Property.............................................42
   APPENDIX A REFERENCE IMPLEMENTATION...............................43
   A.1 iLBC_test.c...................................................44
   A.2 iLBC_encode.h.................................................49
   A.3 iLBC_encode.c.................................................50
   A.4 iLBC_decode.h.................................................59
   A.5 iLBC_decode.c.................................................60
   A.6 iLBC_define.h.................................................71
   A.7 constants.h...................................................74
   A.8 constants.c...................................................76
   A.9 anaFilter.h...................................................89
   A.10 anaFilter.c..................................................89
   A.11 createCB.h...................................................90
   A.12 createCB.c...................................................91
   A.13 doCPLC.h.....................................................95
   A.14 doCPLC.c.....................................................96
   A.15 enhancer.h..................................................101
   A.16 enhancer.c..................................................101
   A.17 filter.h....................................................113
   A.18 filter.c....................................................114
   A.19 FrameClassify.h.............................................117
   A.20 FrameClassify.c.............................................118
   A.21 gainquant.h.................................................120
   A.22 gainquant.c.................................................120
   A.23 getCBvec.h..................................................122
   A.24 getCBvec.c..................................................123
   A.25 helpfun.h...................................................126
   A.26 helpfun.c...................................................128


   
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   A.27 hpInput.h...................................................133
   A.28 hpInput.c...................................................134
   A.29 hpOutput.h..................................................135
   A.30 hpOutput.c..................................................135
   A.31 iCBConstruct.h..............................................136
   A.32 iCBConstruct.c..............................................137
   A.33 iCBSearch.h.................................................139
   A.34 iCBSearch.c.................................................140
   A.35 LPCdecode.h.................................................148
   A.36 LPCdecode.c.................................................149
   A.37 LPCencode.h.................................................152
   A.38 LPCencode.c.................................................152
   A.39 lsf.h.......................................................156
   A.40 lsf.c.......................................................157
   A.41 packing.h...................................................162
   A.42 packing.c...................................................163
   A.43 StateConstructW.h...........................................166
   A.44 StateConstructW.c...........................................166
   A.45 StateSearchW.h..............................................168
   A.46 StateSearchW.c..............................................169
   A.47 syntFilter.h................................................172
   A.48 syntFilter.c................................................173






























   
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1. INTRODUCTION 


   This document contains the description of an algorithm for the
   coding of speech signals sampled at 8 kHz. The algorithm, called
   iLBC, uses a block-independent linear-predictive coding (LPC)
   algorithm and has support for two basic frame lengths: 20 ms at 15.2
   kbit/s and 30 ms at 13.33 kbit/s. When the codec operates at block
   lengths of 20 ms, it produces 304 bits per block which SHOULD be
   packetized as in [1]. Similarly, for block lengths of 30 ms it
   produces 400 bits per block which SHOULD be packetized as in [1].
   The two modes for the different frame sizes operate in a very
   similar way. When they differ it is explicitly stated in the text,
   usually with the notation x/y, where x refers to the 20 ms mode and
   y refers to the 30 ms mode.
   
   The described algorithm results in a speech coding system with a
   controlled response to packet losses similar to what is known from
   pulse code modulation (PCM) with packet loss concealment (PLC), such
   as the ITU-T G.711 standard [4] which operates at a fixed bit rate
   of 64 kbit/s. At the same time, the described algorithm enables
   fixed bit rate coding with a quality-versus-bit rate tradeoff close
   to state-of-the-art. A suitable RTP payload format for the iLBC
   codec is specified in [1].
   
   Some of the applications for which this coder is suitable are: real
   time communications such as telephony and videoconferencing,
   streaming audio, archival, and messaging.
   
   Cable Television Laboratories (CableLabs(R)) intends to adapt iLBC
   as a PacketCable(TM) audio codec standard for VoIP over Cable
   applications [3].
   
   This document is organized as follows. In Section 2 a brief outline
   of the codec is given. The specific encoder and decoder algorithms
   are explained in Sections 3 and 4, respectively. A c-code reference
   implementation is provided in Appendix A.
   
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC 2119 [2].


   
2. OUTLINE OF THE CODEC


   The codec consists of an encoder and a decoder described in Section
   2.1 and 2.2, respectively.
   
   The essence of the codec is LPC and block based coding of the LPC
   residual signal. For each 160/240 (20ms/30 ms) sample block, the
   following major steps are performed: A set of LPC filters are