OPUS audio format

What is Opus? Who created it?

Opus is a completely free, free universal audio codec. First of all, it is designed for interactive voice and music transmission over the Internet, but is also applicable for storage and streaming (streaming). It combines the technologies of the SILK codec from Skype and the CELT codec from Xiph.Org. Opus is standardized by the Internet Engineering Task Force (IETF) as RFC 6716.

How does Opus compare itself to other codecs?

Opus stands out among other high-quality audio formats (AAC, Vorbis, MP3) due to its low latency, but from other low latency codecs (G.711, GSM, Speex) it is distinguished by support for high encoding quality. The quality of its coding is comparable or even exceeds the quality of existing codecs in a wide range of bit rates, at the same time it has lower delays than almost any existing compression format. In addition, the Opus format itself, as well as its basic implementation, is available under a free, free license, which simplifies the adaptation of the codec, makes it compatible with free software and is suitable for use as part of the basic infrastructure of the Internet. More detailed information can be found on the comparison page.

Can Opus replace all other codecs?

Theoretically, yes. From a technical point of view (loss, delay, bit rates, etc.), it can replace Vorbis, and Speex, and even existing proprietary codecs.

Will Opus Vorbis replace in video files?

This is possible for OGG Theora video files, but the resulting space savings will be minimal, and this will lose support with existing players. For WebM video files, this applies, but requires additional changes in the specification, since its current version allows using only VP8 and Vorbis codecs for video and audio, respectively.

How do I use Opus? What programs support Opus?

Opus decoding support is currently included in many programs, in particular Firefox, foobar2000, as well as in frameworks like Gstreamer and FFmpeg. The best way to encode is to use the opussenc console application from the opus-tools package. For real-time applications, Opus support will soon be available on Google WebRTC.

What program can be used to convert opus to mp3?

We recommend using for this online service opus-to-mp3.com

Does Opus support high sampling rates – 96, 192 kHz?

Yes and no. Encoding programs like opusenc can easily encode files with a sampling frequency of 96 or 192 kHz. However, the input files will be converted to 48 kHz on the fly, after which only frequencies up to 20 kHz will be encoded. The meaning of these manipulations is easy to explain: lossy codecs are designed to preserve the sound detailing and at the same time discard unnecessary information. Since a person’s ear can only hear frequencies below 20 kHz (at best), higher frequencies are the first thing to exclude. Read more in the article Downloads in 24/192 format – why they do not make sense.

What are the licensing requirements?

The original Opus source code is distributed under the BSD three-point license, which is highly permissive. Commercial use and distribution (inclusion in proprietary software) is permitted, subject to certain basic requirements specified in this license. Also, Opus is subject to certain patents for which free use is guaranteed under conditions that the author believes are compatible with most open-source licenses, including the GPL (v2 and v3). See the license page for more details.

Why is Opus made free?

On the Internet, standards for protocols and codecs are part of the overall infrastructure, on the basis of which everything else is built. Most of the high quality standards are innovation and the interaction provided by the systems built on them. When several participants have a monopoly on the monetization of the standard, the infrastructure becomes less common – for others, there is more reason to use their own solution, which increases the cost and reduces efficiency. Imagine a road system where every car can only drive on the road of its own manufacturer. We will all win if we live in a world where all roads are interconnected. That’s why Opus, unlike many codecs, is free.

Is the SILK component in Opus compatible with the SILK implementation in Skype?

Not. The SILK codec transmitted by Skype to the IETF was significantly modified during the integration process into Opus. The changes are so significant that it is impossible to just write a “translator”; even the mutual replacement of parts of the code between Opus and the “old SILK” is far from trivial.

Why not develop SILK and CELT codecs separately?

Opus is more than two independent switching codecs. In addition to the SILK mode with linear prediction and MDCT, the CELT mode also has a hybrid mode in which voice frequencies up to 8 kHz are encoded with linear prediction and frequencies higher with MDCT. That is why Opus shows such good results for a voice bitrate of about 32 kbps. Another advantage of integration is the ability to instantly switch between these modes, without any artifacts or frequency range distortions.

Now that Opus is standardized, will its development stop or will it be further developed?

Unlike most ITU-T codecs, Opus is defined only in terms of decoding. The encoder can continue to develop as much as necessary until the stream encoded by it can be decoded by the reference decoder. It was this moment that allowed MP3 coders to significantly advance with the first implementations of l3enc and dist10. Although it is unlikely that the same awesome revolution awaits Opus, we still hope that future versions will greatly outperform the first release. In fact, already in libopus 1.1 we see significant quality improvements.

Will all subsequent releases of Opus be compatible with the Opus specification?

Yes.

How is Opus optimized for the internet?

Optimizing Opus for the Internet obviously means that it has good packet loss resistance, but that’s not all. The first goal we set in developing Opus was to obtain a truly adaptable flow rate, since we never know what speed will be available. This requires not only a wide range of bit rates, but also the possibility of fine adjustment (with small increments). This is why Opus covers a range of 6 to 512 kbps in 0.4 kbps increments (a 1 byte surcharge for 20-millisecond frames). The reason Opus has more than 1200 possible bitrates, spending 11 bits to specify the bitrate, is because UDP already encodes packet sizes. The final aspect is the ease of transfer via RTP, as can be seen from the Opus RTP payload format. For example, you can decode RTP packets without SDP and telephone signaling (out-of-band signaling).

What Android apps can play Opus?

At the moment there are not many of them, but the list is quickly growing. Please read this question on android.stackexchange.com. You can offer your own applications.

When will the next version be released?

When it is ready. Seriously. We do not know. Opus is not a big project with fixed release periodicals. Our preliminary releases and even versions from the git repository are quite stable, suitable for testing and distribution. However, note that the API of the new functional (which has not yet been included in stable releases) may change.

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