Speex

Speex is an open-source audio compression format designed for speech, developed by the Xiph.Org Foundation to provide high-quality speech at low bitrates for voice over IP and telephony. It is released under a permissive BSD-like license and aims to be patent-unencumbered.

Speex uses a code-excited linear prediction (CELP) based encoding algorithm with perceptual weighting to reduce bitrate while preserving intelligibility. It supports multiple operating modes, including narrowband for conventional telephone bandwidth, wideband for improved clarity, and ultra-wideband for higher fidelity. Bitrates can be chosen flexibly, enabling configurations from a few kilobits per second up to tens of kilobits per second, depending on mode and quality requirements. The codec also includes features such as voice activity detection, discontinuous transmission, and comfort noise generation. The Speex DSP library (libspeexdsp) provides additional speech processing with echo cancellation and denoising.

Speex data can be stored or streamed in the Ogg container as Ogg Speex (commonly with the .spx extension) or used in raw form. It has been implemented in libspeex and integrated into various open-source media stacks and VoIP projects.

Development of Speex has slowed in recent years as Opus emerged as the preferred open standard for interactive audio, combining general-purpose audio and speech capabilities. Speex remains in use in legacy systems and projects that require a small, low-complexity speech codec; Opus is generally recommended for new designs.