Speex

Speex is an audio compression codec specifically tuned for the reproduction of human speech and also a free software speech codec that may be used on VoIP applications and podcasts. It is based on the CELP speech coding algorithm. Speex claims to be free of any patent restrictions and is licensed under the revised (3-clause) BSD license. It may be used with the Ogg container format or directly transmitted over UDP/RTP. It may also be used with the FLV container format.
The Speex designers see their project as complementary to the Vorbis general-purpose audio compression project.
Speex is a lossy format, "i.e." quality is permanently degraded to reduce file size.
The Speex project was created on February 13, 2002. The first development versions of Speex were released under LGPL license, but as of version 1.0 beta 1, Speex is released under Xiph's version of the (revised) BSD license. Speex 1.0 was announced on March 24, 2003, after a year of development. The last stable version of Speex encoder and decoder is 1.2.0.
Xiph.Org now considers Speex obsolete; its successor is the more modern Opus codec, which surpasses its performance in most areas except at the lowest sample rates.
Description.
Speex is targeted at voice over IP (VoIP) and file-based compression. The design goals have been to make a codec that would be optimized for high quality speech and low bit rate. To achieve this the codec uses multiple bit rates, and supports ultra-wideband (32 kHz sampling rate), wideband (16 kHz sampling rate) and narrowband (telephone quality, 8 kHz sampling rate). Since Speex was designed for VoIP instead of cell phone use, the codec must be robust to lost packets, but not to corrupted ones. All this led to the choice of code excited linear prediction (CELP) as the encoding technique to use for Speex. One of the main reasons is that CELP has long proven that it could do the job and scale well to both low bit rates (as evidenced by DoD CELP @ 4.8 kbit/s) and high bit rates (as with G.728 @ 16 kbit/s).
The main characteristics can be summarized as follows:
Applications.
There are a large base of applications supporting the Speex codec. Examples include:
Most of these are based on the DirectShow filter or OpenACM codec (e.g. Microsoft NetMeeting) on Microsoft Windows, or Xiph.org's reference implementation, libvorbis, on Linux (e.g. Ekiga). There are also plugins for many audio players. See the plugin and software page on the speex.org site for more details.
The media type for Speex is audio/ogg while contained by Ogg, and audio/speex (previously audio/x-speex) when transported through RTP or without container.
The United States Army's Land Warrior system, designed by General Dynamics, also uses Speex for VoIP on an EPLRS radio designed by Raytheon.
The Ear Bible is a single-ear headphone with a built-in Speex player with 1 GB of flash memory, preloaded with a recording of the New American Standard Bible.
ASL Safety & Security's Linux based VIPA OS software which is used in long line public address systems and voice alarm systems at major international air transport hubs and rail networks.
The Rockbox project uses Speex for its voice interface. It can also play Speex files on supported players, such as the Apple iPod or the iRiver H10.
The Vernier LabQuest handheld data acquisition device for science education uses Speex for voice annotations created by students and teachers using either the built-in or an external microphone.
The Google Mobile App for iPhone currently incorporates Speex. It has also been suggested that the new Google voice search iPhone app is using Speex to transmit voice to Google servers for interpretation.
Adobe Flash Player supports Speex starting with Flash Player 10.0.12.36, released in October 2008. Because of some bugs in Flash Player, the first recommended version for Speex support is 10.0.22.87 and later. Speex in Flash Player can be used for both kind of communication, through Flash Media Server or P2P. Speex can be decoded or converted to any format unlike Nellymoser audio, which was the only speech format in previous versions of Flash Player. Speex can be also used in the Flash Video container format (.flv), starting with version 10 of Video File Format Specification (published in November 2008).
The JavaSonics ListenUp voice recorder uses Speex to compress voice messages that are recorded in a browser and then uploaded to a web server. Primary applications are language training, transcription and social networking.
Speex is used as the voice compression algorithm in the Siri voice assistance on the iPhone 4S. Since text-to-speech occurs on Apple's servers, the Speex codec is used to minimize network bandwidth.
Sources.
"This article uses material from the Speex Codec Manual which is copyright © Jean-Marc Valin and licensed under the terms of the ."