manual.lyx

一个开源的sip源代码

#LyX 1.4.4 created this file. For more info see http://www.lyx.org/
\lyxformat 245
\begin_document
\begin_header
\textclass scrbook
\language english
\inputencoding auto
\fontscheme pslatex
\graphics default
\paperfontsize 10
\spacing onehalf
\papersize letterpaper
\use_geometry true
\use_amsmath 2
\cite_engine basic
\use_bibtopic false
\paperorientation portrait
\leftmargin 2cm
\topmargin 2cm
\rightmargin 2cm
\bottommargin 2cm
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\defskip medskip
\quotes_language english
\papercolumns 1
\papersides 1
\paperpagestyle headings
\tracking_changes false
\output_changes true
\end_header

\begin_body

\begin_layout Title
The Speex Codec Manual
\newline
(version 1.2-beta2)
\end_layout

\begin_layout Author
Jean-Marc Valin
\end_layout

\begin_layout Standard

\newpage
Copyright (c) 2002-2006 Jean-Marc Valin/Xiph.org Foundation
\end_layout

\begin_layout Standard
Permission is granted to copy, distribute and/or modify this document under
 the terms of the GNU Free Documentation License, Version 1.1 or any later
 version published by the Free Software Foundation; with no Invariant Section,
 with no Front-Cover Texts, and with no Back-Cover.
 A copy of the license is included in the section entitled "GNU Free Documentati
on License".
 
\end_layout

\begin_layout Standard

\newpage

\begin_inset LatexCommand \tableofcontents{}

\end_inset


\newpage

\end_layout

\begin_layout Standard
\begin_inset FloatList table

\end_inset


\newpage

\end_layout

\begin_layout Chapter
Introduction to Speex
\end_layout

\begin_layout Standard
The Speex project (
\family typewriter
http://www.speex.org/
\family default
) has been started because there was a need for a speech codec that was
 open-source and free from software patents.
 These are essential conditions for being used by any open-source software.
 There is already Vorbis that does general audio, but it is not really suitable
 for speech.
 Also, unlike many other speech codecs, Speex is not targeted at cell phones
 but rather at voice over IP (VoIP) and file-based compression.
 
\end_layout

\begin_layout Standard
As design goals, we wanted to have a codec that would allow both very good
 quality speech and low bit-rate (unfortunately not at the same time!),
 which led us to developing a codec with multiple bit-rates.
 Of course very good quality also meant we had to do wideband (16 kHz sampling
 rate) in addition to narrowband (telephone quality, 8 kHz sampling rate).
\end_layout

\begin_layout Standard
Designing for VoIP instead of cell phone use means that Speex must be robust
 to lost packets, but not to corrupted ones since packets either arrive
 unaltered or don't arrive at all.
 Also, the idea was to have a reasonable complexity and memory requirement
 without compromising too much on the efficiency of the codec.
\end_layout

\begin_layout Standard
All this led us to the choice of CELP
\begin_inset LatexCommand \index{CELP}

\end_inset

 as the encoding technique to use for Speex.
 One of the main reasons is that CELP has long proved that it could do the
 job and scale well to both low bit-rates (think DoD CELP @ 4.8 kbps) and
 high bit-rates (think G.728 @ 16 kbps).
 
\end_layout

\begin_layout Standard
This document is divided in the following way.
 Section 
\begin_inset LatexCommand \ref{sec:Feature-description}

\end_inset

 describes the different Speex features and defines some terms that will
 be used in later sections.
 Section 
\begin_inset LatexCommand \ref{sec:Command-line-encoder/decoder}

\end_inset

 provides information about the standard command-line tools, while 
\begin_inset LatexCommand \ref{sec:Programming-with-Speex}

\end_inset

 contains information about programming using the Speex API.
 Section 
\begin_inset LatexCommand \ref{sec:Formats-and-standards}

\end_inset

 has some information related to Speex and standards.
 The three last sections describe the internals of the codec and require
 some signal processing knowledge.
 Section 
\begin_inset LatexCommand \ref{sec:Introduction-to-CELP}

\end_inset

 explains the general idea behind CELP, while sections 
\begin_inset LatexCommand \ref{sec:Speex-narrowband-mode}

\end_inset

 and 
\begin_inset LatexCommand \ref{sec:Speex-wideband-mode}

\end_inset

 are specific to Speex.
 Note that if you are only interested in using Speex, those three last sections
 are not required.
\end_layout

\begin_layout Standard

\newpage

\end_layout

\begin_layout Chapter
Codec description
\begin_inset LatexCommand \label{sec:Feature-description}

\end_inset


\end_layout

\begin_layout Standard
This section describes the main features provided by Speex.
\end_layout

\begin_layout Section
Concepts
\end_layout

\begin_layout Standard
Before introducing all the Speex features, here are some concepts in speech
 coding that help better understand the rest of the manual.
 Emphasis is placed on Speex.
\end_layout

\begin_layout Subsection*
Sampling rate
\begin_inset LatexCommand \index{sampling rate}

\end_inset


\end_layout

\begin_layout Standard
Speex is mainly designed for three different sampling rates: 8 kHz, 16 kHz,
 and 32 kHz.
 These are respectively refered to as narrowband
\begin_inset LatexCommand \index{narrowband}

\end_inset

, wideband
\begin_inset LatexCommand \index{wideband}

\end_inset

 and ultra-wideband
\begin_inset LatexCommand \index{ultra-wideband}

\end_inset

.
 For a sampling rate of 
\begin_inset Formula $F_{s}$
\end_inset

 kHz, the highest frequency that can be represented is equal to 
\begin_inset Formula $F_{s}/2$
\end_inset

 kHz.
 This is a consequence of Nyquist's sampling theorem (and 
\begin_inset Formula $F_{s}/2$
\end_inset

 is known as the Nyquist frequency).
\end_layout

\begin_layout Subsection*
Quality
\begin_inset LatexCommand \index{quality}

\end_inset


\end_layout

\begin_layout Standard
Speex encoding is controlled most of the time by a quality parameter that
 ranges from 0 to 10.
 In constant bit-rate
\begin_inset LatexCommand \index{constant bit-rate}

\end_inset

 (CBR) operation, the quality parameter is an integer, while for variable
 bit-rate (VBR), the parameter is a float.
 
\end_layout

\begin_layout Subsection*
Complexity
\begin_inset LatexCommand \index{complexity}

\end_inset

 (variable)
\end_layout

\begin_layout Standard
With Speex, it is possible to vary the complexity allowed for the encoder.
 This is done by controlling how the search is performed with an integer
 ranging from 1 to 10 in a way that's similar to the -1 to -9 options to
 
\emph on
gzip
\emph default
 and 
\emph on
bzip2
\emph default
 compression utilities.
 For normal use, the noise level at complexity 1 is between 1 and 2 dB higher
 than at complexity 10, but the CPU requirements for complexity 10 is about
 5 times higher than for complexity 1.
 In practice, the best trade-off is between complexity 2 and 4, though higher
 settings are often useful when encoding non-speech sounds like DTMF
\begin_inset LatexCommand \index{DTMF}

\end_inset

 tones.
\end_layout

\begin_layout Subsection*
Variable Bit-Rate
\begin_inset LatexCommand \index{variable bit-rate}

\end_inset

 (VBR)
\end_layout

\begin_layout Standard
Variable bit-rate (VBR) allows a codec to change its bit-rate dynamically
 to adapt to the 
\begin_inset Quotes eld
\end_inset

difficulty
\begin_inset Quotes erd
\end_inset

 of the audio being encoded.
 In the example of Speex, sounds like vowels and high-energy transients
 require a higher bit-rate to achieve good quality, while fricatives (e.g.
 s,f sounds) can be coded adequately with less bits.
 For this reason, VBR can achive lower bit-rate for the same quality, or
 a better quality for a certain bit-rate.
 Despite its advantages, VBR has two main drawbacks: first, by only specifying
 quality, there's no guaranty about the final average bit-rate.
 Second, for some real-time applications like voice over IP (VoIP), what
 counts is the maximum bit-rate, which must be low enough for the communication
 channel.
\end_layout

\begin_layout Subsection*
Average Bit-Rate
\begin_inset LatexCommand \index{average bit-rate}

\end_inset

 (ABR)
\end_layout

\begin_layout Standard
Average bit-rate solves one of the problems of VBR, as it dynamically adjusts
 VBR quality in order to meet a specific target bit-rate.
 Because the quality/bit-rate is adjusted in real-time (open-loop), the
 global quality will be slightly lower than that obtained by encoding in
 VBR with exactly the right quality setting to meet the target average bit-rate.
\end_layout

\begin_layout Subsection*
Voice Activity Detection
\begin_inset LatexCommand \index{voice activity detection}

\end_inset

 (VAD)
\end_layout

\begin_layout Standard
When enabled, voice activity detection detects whether the audio being encoded
 is speech or silence/background noise.
 VAD is always implicitly activated when encoding in VBR, so the option
 is only useful in non-VBR operation.
 In this case, Speex detects non-speech periods and encode them with just
 enough bits to reproduce the background noise.
 This is called 
\begin_inset Quotes eld
\end_inset

comfort noise generation
\begin_inset Quotes erd
\end_inset

 (CNG).
\end_layout

\begin_layout Subsection*
Discontinuous Transmission
\begin_inset LatexCommand \index{discontinuous transmission}

\end_inset

 (DTX)
\end_layout

\begin_layout Standard
Discontinuous transmission is an addition to VAD/VBR operation, that allows
 to stop transmitting completely when the background noise is stationary.
 In file-based operation, since we cannot just stop writing to the file,
 only 5 bits are used for such frames (corresponding to 250 bps).
\end_layout

\begin_layout Subsection*
Perceptual enhancement
\begin_inset LatexCommand \index{perceptual enhancement}

\end_inset


\end_layout

\begin_layout Standard
Perceptual enhancement is a part of the decoder which, when turned on, tries
 to reduce (the perception of) the noise produced by the coding/decoding
 process.
 In most cases, perceptual enhancement make the sound further from the original
 
\emph on
objectively
\emph default
 (if you use SNR), but in the end it still 
\emph on
sounds
\emph default
 better (subjective improvement).
\end_layout

\begin_layout Subsection*
Algorithmic delay
\begin_inset LatexCommand \index{algorithmic delay}

\end_inset


\end_layout

\begin_layout Standard
Every speech codec introduces a delay in the transmission.
 For Speex, this delay is equal to the frame size, plus some amount of 
\begin_inset Quotes eld
\end_inset

look-ahead
\begin_inset Quotes erd
\end_inset

 required to process each frame.
 In narrowband operation (8 kHz), the delay is 30 ms, while for wideband
 (16 kHz), the delay is 34 ms.
 These values don't account for the CPU time it takes to encode or decode
 the frames.
\end_layout

\begin_layout Section
Codec
\end_layout

\begin_layout Standard
The main characteristics of Speex can be summarized as follows:
\end_layout

\begin_layout Itemize
Free software/open-source
\begin_inset LatexCommand \index{open-source}

\end_inset

, patent
\begin_inset LatexCommand \index{patent}

\end_inset

 and royalty-free
\end_layout

\begin_layout Itemize
Integration of narrowband
\begin_inset LatexCommand \index{narrowband}

\end_inset

 and wideband
\begin_inset LatexCommand \index{wideband}

\end_inset

 using an embedded bit-stream
\end_layout

\begin_layout Itemize
Wide range of bit-rates available (from 2.15 kbps to 44 kbps)
\end_layout

\begin_layout Itemize
Dynamic bit-rate switching (AMR) and Variable Bit-Rate
\begin_inset LatexCommand \index{variable bit-rate}

\end_inset

 (VBR) operation
\end_layout

\begin_layout Itemize
Voice Activity Detection
\begin_inset LatexCommand \index{voice activity detection}

\end_inset

 (VAD, integrated with VBR) and discontinuous transmission (DTX)
\end_layout

\begin_layout Itemize
Variable complexity
\begin_inset LatexCommand \index{complexity}

\end_inset


\end_layout

\begin_layout Itemize
Embedded wideband structure (scalable sampling rate)
\end_layout

\begin_layout Itemize
Ultra-wideband mode at 32 kHz
\end_layout

\begin_layout Itemize
Intensity stereo encoding option
\end_layout

\begin_layout Itemize
Fixed-point implementation (work in progress)
\end_layout

\begin_layout Section
Preprocessor
\end_layout

\begin_layout Standard
This part refers to the preprocessor module introduced in the 1.1.x branch.
 The preprocessor is designed to be used on the audio 
\emph on
before
\emph default
 running the encoder.
 The preprocessor provides three main functionalities:
\end_layout

\begin_layout Itemize
noise suppression
\end_layout

\begin_layout Itemize
automatic gain control (AGC)
\end_layout

\begin_layout Itemize
voice activity detection (VAD)
\end_layout

\begin_layout Standard
The denoiser can be used to reduce the amount of background noise present
 in the input signal.
 This provides higher quality speech whether or not the denoised signal
 is encoded with Speex (or at all).
 However, when using the denoised signal with the codec, there is an additional
 benefit.
 Speech codecs in general (Speex included) tend to perform poorly on noisy
 input, which tends to amplify the noise.
 The denoiser greatly reduces this effect.
\end_layout

\begin_layout Standard
Automatic gain control (AGC) is a feature that deals with the fact that
 the recording volume may vary by a large amount between different setups.
 The AGC provides a way to adjust a signal to a reference volume.
 This is useful for voice over IP because it removes the need for manual
 adjustment of the microphone gain.
 A secondary advantage is that by setting the microphone gain to a conservative
 (low) level, it is easier to avoid clipping.
\end_layout

\begin_layout Standard
The voice activity detector (VAD) provided by the preprocessor is more advanced
 than the one directly provided in the codec.
 
\end_layout

\begin_layout Section
Adaptive Jitter Buffer
\end_layout

\begin_layout Standard
When transmitting voice (or any content for that matter) over UDP or RTP,
 packet may be lost, arrive with different delay, or even out of order.
 The purpose of a jitter buffer is to reorder packets and buffer them long
 enough (but no longer than necessary) so they can be sent to be decoded.
 
\end_layout

\begin_layout Section
Acoustic Echo Canceller
\end_layout

\begin_layout Standard