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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. Despite its advantages, VBR has three main drawbacks: first, by only specifying quality, there is no guarantee about the final average bit-rate. For this reason, VBR can achieve lower bit rate for the same quality, or a better quality for a certain bit rate. s and f sounds) can be coded adequately with fewer bits. 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. Variable bit-rate (VBR) Variable bit-rate (VBR) allows a codec to change its bit rate dynamically to adapt to the 'difficulty' of the audio being encoded. 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 tones, or if encoding is not in real-time. For normal use, the noise level at complexity 1 is between 1 and 2В dB higher than at complexity 10, but the CPU requirements for complexity 10 is about five times higher than for complexity 1. This is done by controlling how the search is performed with an integer ranging from 1 to 10 in a way similar to the -1 to -9 options to gzip compression utilities. Complexity (variable) With Speex, it is possible to vary the complexity allowed for the encoder. In constant bit-rate (CBR) operation, the quality parameter is an integer, while for variable bit-rate (VBR), the parameter is a real ( floating point) number. Quality Speex encoding is controlled most of the time by a quality parameter that ranges from 0 to 10. These are respectively referred to as narrowband, wideband and ultra-wideband.

Ultra-wideband mode at 32В kHz (up to 48В kHz).ДЇeatures Sampling rate Speex is mainly designed for three different sampling rates: 8В kHz (the same sampling rate to transmit telephone calls), 16В kHz, and 32В kHz.

Voice activity detection (VAD, integrated with VBR) (not working from version 1.2).

Dynamic bit rate switching and variable bit-rate (VBR).

Wide range of bit rates available (from 2В kbit/s to 44В kbit/s).

Integration of narrowband and wideband in the same bit-stream.

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Free software/ open-source, patent and royalty-free.The main characteristics can be summarized as follows: 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 4.8В kbit/s) and high bit rates (as with G.728 16В kbit/s).

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All this led to the choice of code excited linear prediction (CELP) as the encoding technique to use for Speex. Since Speex was designed for VoIP instead of cell phone use, the codec must be robust to lost packets, but not to corrupted ones. To achieve this the codec uses multiple bit rates, and supports ultra-wideband (32 kHz sampling rate), wideband (16В kHz sampling rate) and narrowband (telephone quality, 8В kHz sampling rate). The design goals have been to make a codec that would be optimized for high quality speech and low bit rate. Speex is targeted at voice over IP (VoIP) and file-based compression.

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Xiph.Org now considers Speex obsolete its successor is the more modern Opus codec, which uses the SILK format under license from Microsoft and surpasses its performance in most areas except at the lowest sample rates. The last stable version of Speex encoder and decoder is 1.2.0. Speex 1.0 was announced on March 24, 2003, after a year of development. 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. The Speex project was created on February 13, 2002. quality is permanently degraded to reduce file size. The Speex designers see their project as complementary to the Vorbis general-purpose audio compression project. It may also be used with the FLV container format. It may be used with the Ogg container format or directly transmitted over UDP/ RTP. Speex claims to be free of any patent restrictions and is licensed under the revised (3-clause) BSD license. It is based on the CELP speech coding algorithm.

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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.