Opus (Opus Audio Codec)
The open codec built for real-time voice and music that outperforms every rival at low bitrates.
| Full name | Opus Audio Codec |
| Extension | .opus |
| MIME type | audio/opus |
| Developer | IETF Codec Working Group (Xiph.Org, Mozilla, Microsoft/Skype, Broadcom, Google, Octasic) |
| Released | 2012 (RFC 6716, September 11) |
| Type | Lossy audio compression |
| Bitrate range | 6 kbps (narrowband speech) to 510 kbps (high-quality stereo) |
| Latency | As low as 2.5 ms algorithmic delay; 26.5 ms in default voice mode |
What is a Opus file?
Opus is an open, royalty-free audio codec standardized by the Internet Engineering Task Force in September 2012 as RFC 6716. It was designed from the start for real-time communication over the internet, covering everything from low-bitrate voice calls to high-quality music streaming in a single format. No licensing fees apply, and the reference implementation is released under a BSD license.
Opus works by combining two different compression technologies depending on the audio content: Linear Prediction for speech and MDCT (Modified Discrete Cosine Transform) for music. This lets the codec switch modes on the fly, which is why it performs well on calls that mix talking and background music. It supports bitrates from 6 kbps up to 510 kbps, frame sizes from 2.5 ms to 60 ms, and up to 255 audio channels. The format is almost always stored inside an Ogg container, giving it the .opus file extension.
History
Development started around 2007 through separate tracks: Skype was building SILK for voice calls, while Xiph.Org was building CELT for low-latency music. The IETF Codec Working Group formed in February 2010 and brought these two teams together. By July 2010 they had a working hybrid prototype, and by March 2011 it was already beating AAC and Vorbis in blind listening tests. The codec was standardized as RFC 6716 on September 11, 2012, and Opus 1.0.1 was released the same day.
How it works
Opus audio is nearly always wrapped in an Ogg transport container, which segments the bitstream into pages and provides seeking and synchronization. Each Ogg page holds one or more Opus packets, and each packet can carry between one and 120 ms of audio using one of three internal modes: SILK (voice), CELT (music), or a hybrid of both. The first Ogg page contains a special OpusHead header with channel count and pre-skip samples, and the second page contains OpusTags metadata. This structure is defined in RFC 7845, separate from the codec spec itself.
What it is used for
- Voice over IP and video conferencing (WebRTC uses Opus as a mandatory codec)
- Discord, Zoom, and browser-based calling apps that need low-latency audio
- Streaming music and podcasts at lower file sizes than MP3 or AAC
- Game voice chat and in-game audio where bandwidth and delay both matter
How to open it
VLC, Firefox, Chrome, and most modern media players open .opus files without any plugins. On Windows, the built-in Media Player app does not support Opus natively, but VLC or foobar2000 with the appropriate component will play it without issues.
Pros and cons
Strengths
- Royalty-free and open: no patent licensing costs for developers or users
- Outperforms MP3, AAC, and Vorbis at equivalent bitrates, especially below 64 kbps
- Very low algorithmic delay makes it practical for real-time voice and live streaming
- Adapts dynamically between speech and music modes within the same file or stream
Trade-offs
- Not supported natively in Apple ecosystem: Safari added support late, iTunes and Apple Music still do not play .opus files
- Older hardware devices and car stereos rarely recognize the format
- Lossy compression means some audio information is permanently discarded on encoding
- Less tooling support for editing compared to MP3 or AAC in consumer audio software
Convert Opus files
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Opus FAQ
Is Opus better than MP3?
At the same bitrate, Opus generally produces noticeably better audio quality than MP3, particularly at 64 kbps and below. At 128 kbps most listeners hear little difference between the two, but Opus has a clear technical advantage at low bitrates and adds much lower latency for real-time use.
Can I play .opus files on an iPhone or Mac?
Safari added Opus support in version 11 (macOS High Sierra, iOS 11), so you can play Opus in the browser on Apple devices. However, the native Music app and iTunes do not support .opus files. You need a third-party app such as VLC to play them outside the browser on Apple hardware.
Why does Discord use Opus?
Discord uses Opus because it is royalty-free, has very low algorithmic delay (around 26.5 ms in voice mode), and performs well at the 64 kbps range typical for voice calls. The WebRTC standard that Discord builds on mandates Opus as a required codec, so every browser and client already supports it.
What is the difference between .opus and .ogg?
Ogg is a container format, not an audio codec. A .ogg file usually holds audio encoded with the Vorbis codec. An .opus file is also an Ogg container, but the audio inside is encoded with the Opus codec. The two are not interchangeable: a player that handles .ogg/Vorbis files may not handle .opus unless it explicitly supports the Opus codec.