Internet Low Bit Rate Codec (iLBC)
RFC 3951
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Type |
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RFC - Experimental
(December 2004; No errata)
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Last updated |
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2013-03-02
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Replaces |
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draft-andersen-ilbc
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Stream |
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IETF
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Formats |
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plain text
pdf
html
bibtex
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WG state
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(None)
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Document shepherd |
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No shepherd assigned
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| IESG |
IESG state |
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RFC 3951 (Experimental)
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Consensus Boilerplate |
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Unknown
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Telechat date |
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Responsible AD |
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Allison Mankin
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Send notices to |
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<csp@csperkins.org>, <magnus.westerlund@ericsson.com>, <mankin@psg.com>
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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
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