Definition of the Opus Audio Codec

This RFC 6716 was published in 2012.

Abstract

This document defines the Opus interactive speech and audio codec.
Opus is designed to handle a wide range of interactive audio applications, including Voice over IP, videoconferencing, in-game chat, and even live, distributed music performances.
It scales from low bitrate narrowband speech at 6 kbit/s to very high quality stereo music at 510 kbit/s.
Opus uses both Linear Prediction (LP) and the Modified Discrete Cosine Transform (MDCT) to achieve good compression of both speech and music.

RFC 6716 introduction

The Opus codec is a real-time interactive audio codec designed to meet the requirements described in [REQUIREMENTS].
It is composed of a layer based on Linear Prediction (LP) [LPC] and a layer based on the Modified Discrete Cosine Transform (MDCT) [MDCT].
The main idea behind using two layers is as follows: in speech, linear prediction techniques (such as Code-Excited Linear Prediction, or CELP) code low frequencies more efficiently than transform (e.g., MDCT) domain techniques, while the situation is reversed for music and higher speech frequencies.
Thus, a codec with both layers available can operate over a wider range than either one alone and can achieve better quality by combining them than by using either one individually.

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