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Internet Low Bit Rate Codec (Ilbc)

By Andersen, S.

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Book Id: WPLBN0000693593
Format Type: PDF eBook
File Size: 376.46 KB.
Reproduction Date: 2005
Full Text

Title: Internet Low Bit Rate Codec (Ilbc)  
Author: Andersen, S.
Volume:
Language: English
Subject: Modern Regression
Collections: Technical eBooks Collection
Historic
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Andersen, S. (n.d.). Internet Low Bit Rate Codec (Ilbc). Retrieved from http://hawaiilibrary.net/


Description
Technical Reference Publication

Excerpt
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.

Table of Contents
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 3

 

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