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 Voice over IP (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:
- Free software/open-source, patent and royalty-free.
- Integration of narrowband and wideband in the same bit-stream.
- Wide range of bit rates available (from 2 kbit/s to 44 kbit/s).
- Dynamic bit rate switching and Variable bit-rate (VBR).
- Voice Activity Detection (VAD, integrated with VBR) (not working from version 1.2).
- Variable complexity.
- Ultra-wideband mode at 32 kHz (up to 48 kHz).
- Intensity stereo encoding option.
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