Opus is a totally open, royalty-free, highly versatile audio codec. Opus is unmatched for interactive speech and music transmission over the Internet, but also intended for storage and streaming applications. It is standardized by the Internet Engineering Task Force (IETF) as RFC 6716 which incorporated technology from Skype's SILK codec and Xiph.Org's CELT codec.
Technology Opus can handle a wide range of audio applications, including Voice over IP, videoconferencing, in-game chat, and even remote live music performances. It can scale from low bit-rate narrowband speech to very high quality stereo music. Supported features are: Bit-rates from 6 kb/s to 510 kb/s Sampling rates from 8 kHz (narrowband) to 48 kHz (fullband) Frame sizes from 2.5 ms to 60 ms Support for both constant bit-rate (CBR) and variable bit-rate (VBR) Audio bandwidth from narrowband to full-band Support for speech and music Support for mono and stereo Support for up to 255 channels (multistream frames) Dynamically adjustable bitrate, audio bandwidth, and frame size Good loss robustness and packet loss concealment (PLC) Floating point and fixed-point implementation You can read the full specification, including the reference implementation, in RFC 6716.