Internet Low Bit Rate Codec (iLBC)
RFC 3951

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• 05

Network Working Group                                        S. Andersen
Request for Comments: 3951                            Aalborg University
Category: Experimental                                          A. Duric
                                                                   Telio
                                                               H. Astrom
                                                                R. Hagen
                                                               W. Kleijn
                                                               J. Linden
                                                         Global IP Sound
                                                           December 2004

                   Internet Low Bit Rate Codec (iLBC)

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

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.

Andersen, et al.              Experimental                      [Page 1]
RFC 3951              Internet Low Bit Rate Codec          December 2004

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Outline of the Codec . . . . . . . . . . . . . . . . . . . . .  5
       2.1.  Encoder. . . . . . . . . . . . . . . . . . . . . . . . .  5
       2.2.  Decoder. . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Encoder Principles . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.  Pre-processing . . . . . . . . . . . . . . . . . . . . .  9
       3.2.  LPC Analysis and Quantization. . . . . . . . . . . . . .  9
             3.2.1.  Computation of Autocorrelation Coefficients. . . 10
             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 . . . . . . . . 12
             3.2.5.  Stability Check of LSF Coefficients. . . . . . . 13
             3.2.6.  Interpolation of LSF Coefficients. . . . . . . . 13
             3.2.7.  LPC Analysis and Quantization for 20 ms Frames . 14
       3.3.  Calculation of the Residual. . . . . . . . . . . . . . . 15
       3.4.  Perceptual Weighting Filter. . . . . . . . . . . . . . . 15
       3.5.  Start State Encoder. . . . . . . . . . . . . . . . . . . 15
             3.5.1.  Start State Estimation . . . . . . . . . . . . . 16
             3.5.2.  All-Pass Filtering and Scale Quantization. . . . 17
             3.5.3.  Scalar Quantization. . . . . . . . . . . . . . . 18
       3.6.  Encoding the Remaining Samples . . . . . . . . . . . . . 19
             3.6.1.  Codebook Memory. . . . . . . . . . . . . . . . . 20
             3.6.2.  Perceptual Weighting of Codebook Memory
                     and Target . . . . . . . . . . . . . . . . . . . 22
             3.6.3.  Codebook Creation. . . . . . . . . . . . . . . . 23
                     3.6.3.1. Creation of a Base Codebook . . . . . . 23
                     3.6.3.2. Codebook Expansion. . . . . . . . . . . 24
                     3.6.3.3. Codebook Augmentation . . . . . . . . . 24
             3.6.4.  Codebook Search. . . . . . . . . . . . . . . . . 26
                     3.6.4.1. Codebook Search at Each Stage . . . . . 26
                     3.6.4.2. Gain Quantization at Each Stage . . . . 27
                     3.6.4.3. Preparation of Target for Next Stage. . 28
       3.7.  Gain Correction Encoding . . . . . . . . . . . . . . . . 28
       3.8.  Bitstream Definition . . . . . . . . . . . . . . . . . . 29
   4.  Decoder Principles . . . . . . . . . . . . . . . . . . . . . . 32
       4.1.  LPC Filter Reconstruction. . . . . . . . . . . . . . . . 33
       4.2.  Start State Reconstruction . . . . . . . . . . . . . . . 33
       4.3.  Excitation Decoding Loop . . . . . . . . . . . . . . . . 34
       4.4.  Multistage Adaptive Codebook Decoding. . . . . . . . . . 35
             4.4.1.  Construction of the Decoded Excitation Signal. . 35
       4.5.  Packet Loss Concealment. . . . . . . . . . . . . . . . . 35
             4.5.1.  Block Received Correctly and Previous Block
                     Also Received. . . . . . . . . . . . . . . . . . 35
             4.5.2.  Block Not Received . . . . . . . . . . . . . . . 36