Introduction to Speex

The Speex codec (http://www.speex.org/) exists because there is a need for a speech codec that is open-source and free from software patent royalties. These are essential conditions for being usable by any open-source software. In essence, Speex is to speech what Vorbis is to audio/music. Unlike many other speech codecs, Speex is not designed for mobile phones but rather for packet networks and voice over IP (VoIP) application. File-based compression is of course also supported.

The Speex codec is designed to be very flexible and support a wide range of speech quality and bit-rate. Support for very good quality speech also means that Speex can encode wideband speech (16 kHz sampling rate) in addition to narrowband speech (telephone quality, 8 kHz sampling rate).

Designing for VoIP instead of mobile phones means that Speex is robust to lost packets, but not to corrupted ones. This is based on the assumption that in VoIP, packets either arrive unaltered or don't arrive at all. Because Speex is targeted at a wide range of devices, it has modest complexity (variable) and memory footprint.

All the design goals led to the choice of CELP as the encoding technique. One of the main reasons is that CELP has long proved that it could work reliably and scale well to both low bit-rates (e.g. DoD CELP @ 4.8 kbps) and high bit-rates (e.g. G.728 @ 16 kbps).

Getting help

As for many open source projects, there are many ways to get help with Speex. These include:

• This manual
• Other documentation on the Speex website (http://www.speex.org/)
• Mailing list: Discuss any Speex-related topic on speex-dev@xiph.org (not just for developers)
• IRC: The main channel is #speex on irc.freenode.net. Note that due to time differences, it may take a while to get someone, so please be patient.
• Email the author privately at jean-marc.valin@usherbrooke.ca only for private/delicate topics you do not wish to discuss publically.

Before asking for help (mailing list or IRC), it is important to first read this manual. It is generally considered rude to ask on a mailing list about topics that are clearly detailed in the documentation. On the other hand, it's perfectly OK (and encouraged) to ask for clarifications about something covered in the manual. This manual does not (yet) cover everything about Speex, so everyone is encouraged to ask questions, send comments, feature requests, or just let us know how Speex is being used.

Here are some additional guidelines related to the mailing list. Before reporting bugs in Speex to the list, it is strongly recommended (if possible) to first test whether these bugs can be reproduced using the speexenc and speexdec (see Section 4) command-line utilities. Bugs reported based on 3rd party code are both harder to find and far too often caused by errors that have nothing to do with Speex.

About this document

This document is divided in the following way. Section 2 describes the different Speex features and defines many basic terms that are used throughout this manual. Section 4 documents the standard command-line tools provided in the Speex distribution. Section 5 includes detailed instructions about programming using the libspeex API. Section 6 has some information related to Speex and standards.

The three last sections describe the algorithms used in Speex. These sections require signal processing knowledge, but are not required for merely using Speex. They are intended for people who want to understand how Speex really works and/or want to do research based on Speex. Section 7 explains the general idea behind CELP, while sections 8 and 9 are specific to Speex.