What is SPX?

Audio Format Description

Speex is a lossy format targeted at VoIP (Voice over IP), and not at cell phones as most other speech codecs are. This means that Speex is adapted to Internet applications and has certain features not available in other speech codecs.

Some of Speex’s features include:

• Designed for narrowband (8 kHz), wideband (16 kHz), and ultra-wideband (32kHz) in the same bit stream.
• Encoding controlled by parameters of 0 to 10.
• Variable complexity for different noise levels.
• Variable bit rate (VBR) allowing for adaptation to different sound qualities.
• VAD (Voice Activity Detection) detects whether sound is speech or background sound for Speex to encode accordingly.
• DTX (Discontinuous Transmission) stops transmission when background sound is stationary.
• Perceptual enhancement – reduces noise produced by the coding/decoding process.

These features make Speex well suited for VoIP, audio streaming over the internet (like teleconferencing and video games), voice mail, and audio books. Speex usually uses Ogg container format to store its bit streams as files for transmission. But Speex is not tied to Ogg, it can also be used without the container. For example for VoIP, Ogg is not used at all.

Speex has a large application base, including Net Meeting on Microsoft Windows and OpenH323 on Linux. KSP Sound Player, foobar2000, Apple iPod, and iRiver H10 support Speex. There are also plugins for Winamp and XMMS player. Other than these, Speex is also used by Microsoft Xbox Live headsets, Half-Life 1engine and mods, United States Army’s Land Warrior system for VoIP over EPLRs radio, and Rockbox for its voice interface.