Opus WebCodecs Registration W3C Group Note Draft , 8 June 2026 More details about this document This version: https://www.w3.org/TR/2026/DNOTE-webcodecs-opus-codec-registration-20260608/ Latest published version: https://www.w3.org/TR/webcodecs-opus-codec-registration/ Editor's Draft: https://w3c.github.io/webcodecs/opus_codec_registration.html Previous Versions: https://www.w3.org/TR/2025/DNOTE-webcodecs-opus-codec-registration-20250514/ History: https://www.w3.org/standards/history/webcodecs-opus-codec-registration/ Feedback: GitHub Editors: Paul Adenot ( Mozilla ) Eugene Zemtsov ( Google LLC ) Former Editors: Bernard Aboba ( Microsoft Corporation ) Chris Cunningham ( Google LLC ) Participate: Git Repository. File an issue. Version History: https://github.com/w3c/webcodecs/commits Copyright © 2026 World Wide Web Consortium . W3C ® liability , trademark and permissive document license rules apply. Abstract This registration is entered into the [webcodecs-codec-registry] . It describes, for Opus, the (1) fully qualified codec strings , (2) the codec-specific EncodedAudioChunk [[internal data]] bytes, (3) the AudioDecoderConfig.description bytes, (4) the values of EncodedAudioChunk [[type]] , and (5) the codec-specific extensions to AudioEncoderConfig The registration is not intended to include any information on whether a codec format is encumbered by intellectual property claims. Implementers and authors are advised to seek appropriate legal counsel in this matter if they intend to implement or use a specific codec format. Implementers of WebCodecs are not required to support the Opus codec. This registration is non-normative. Status of this document This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C standards and drafts index . Feedback and comments on this specification are welcome. GitHub Issues are preferred for discussion on this specification. Alternatively, you can send comments to the Media Working Group’s mailing-list, [email protected] ( archives ). This draft highlights some of the pending issues that are still to be discussed in the working group. No decision has been taken on the outcome of these issues including whether they are valid. This document was published by the Media Working Group as a Group Note Draft using the Note track . Group Note Drafts are not endorsed by W3C nor its Members. This is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to cite this document as other than a work in progress. The W3C Patent Policy does not carry any licensing requirements or commitments on this document. This document is governed by the 18 August 2025 W3C Process Document . Table of Contents 1 Fully qualified codec strings 2 EncodedAudioChunk data 3 AudioDecoderConfig description 4 EncodedAudioChunk type 5 AudioEncoderConfig extensions 5.1 OpusEncoderConfig 5.2 OpusBitstreamFormat 5.3 OpusSignal 5.4 OpusApplication 6 Privacy Considerations 7 Security Considerations Conformance Document conventions Index Terms defined by this specification Terms defined by reference References Normative References Non-Normative References IDL Index 1. Fully qualified codec strings The codec string is "opus" . 2. EncodedAudioChunk data An EncodedAudioChunk containing Opus can be in two different formats. If the bitstream is in opus format, EncodedAudioChunk have to be Opus packets, as described in section 3 of [OPUS] If the bitstream is in ogg format, EncodedAudioChunk have to be audio data packets, as described in section 3 of [OPUS-IN-OGG] . 3. AudioDecoderConfig description description can be optionally set to an Identification Header, described in section 5.1 of [OPUS-IN-OGG] . If a description has been set, the bitstream is assumed to be in ogg format. If a description has not been set, the bitstream is assumed to be in opus format. 4. EncodedAudioChunk type The [[type]] for an EncodedAudioChunk containing Opus is always " key ". NOTE: Once the initialization has succeeded, any packet can be decoded at any time without error, but this might not result in the expected audio output. 5. AudioEncoderConfig extensions partial dictionary AudioEncoderConfig { OpusEncoderConfig opus ; }; opus , of type OpusEncoderConfig Contains codec specific configuration options for the Opus codec. 5.1. OpusEncoderConfig dictionary OpusEncoderConfig { OpusBitstreamFormat format = "opus"; OpusSignal signal = "auto"; OpusApplication application = "audio"; [ EnforceRange ] unsigned long long frameDuration = 20000; [ EnforceRange ] unsigned long complexity ; [ EnforceRange ] unsigned long packetlossperc = 0; boolean useinbandfec = false ; boolean usedtx = false ; }; To check if an OpusEncoderConfig is valid, run these steps: If frameDuration is not a valid frame duration, which is described section 2.1.4 of [RFC6716] , return false . If complexity is specified and not within the range of 0 and 10 inclusively, return false . If packetlossperc is specified and not within the range of 0 and 100 inclusively, return false . Return true . format , of type OpusBitstreamFormat , defaulting to "opus" Configures the format of output EncodedAudioChunk s. See OpusBitstreamFormat . signal , of type OpusSignal , defaulting to "auto" Specificies the type of audio signal being encoded. See OpusSignal . application , of type OpusApplication , defaulting to "audio" Specificies the encoder’s intended application. See OpusApplication . frameDuration , of type unsigned long long , defaulting to 20000 Configures the frame duration, in microseconds, of output EncodedAudioChunk s. complexity , of type unsigned long Configures the encoder’s computational complexity, as described in section 2.1.9. of [RFC6716] . The valid range is 0 to 10 , with 10 representing the highest complexity. If no value is specificied, the default value is platform-specific: User Agents SHOULD set a default of 5 for mobile platforms, and a default of 9 for all other platforms. packetlossperc , of type unsigned long , defaulting to 0 Configures the encoder’s expected packet loss percentage. The valid range is 0 to 100 . NOTE: The packet loss percentage might be updated over the course of an encoding, and it is recommended for User Agents to support these reconfigurations. useinbandfec , of type boolean , defaulting to false Specifies whether the encoder provides Opus in-band Forward Error Correction (FEC), as described by section 2.1.7. of [RFC6716] . usedtx , of type boolean , defaulting to false Specifies if the encoder uses Discontinuous Transmission (DTX), as described by section 2.1.9. of [RFC6716] . 5.2. OpusBitstreamFormat enum OpusBitstreamFormat { "opus" , "ogg" , }; The OpusBitstreamFormat determines if extra-data are necessary to decode the encoded audio stream. opus No metadata are necessary to decode the encoded audio stream. ogg The metadata of the encoded audio stream are provided at configuration via AudioDecoderConfig.description . 5.3. OpusSignal enum OpusSignal { "auto" , "music" , "voice" , }; The OpusSignal indicates the default value for the type of signal being encoded. auto The audio signal is not specified to be of a particular type. music The audio signal is music. voice The audio signal is voice or speech. 5.4. OpusApplication enum OpusApplication { "voip" , "audio" , "lowdelay" , }; The OpusApplication indicates the default value for the encoder’s intended application. voip Process signal for improved speech intelligibility. audio Favor faithfulness to the original input. lowdelay Configure the minimum possible coding delay by disabling certain modes of operation. 6. Privacy Considerations Please refer to the section Privacy Considerations in [WEBCODECS] . 7. Security Considerations Please refer to the section Security Considerations in [WEBCODECS] . Conformance Document conventions Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification. All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119] Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example" , like this: This is an example of an informative example. Informative notes begin with the word “Note” and are set apart from the normative text with class="note" , like this: Note, this is an informative note. Index Terms defined by this specification application , in § 5.1 "audio" , in § 5.4 audio , in § 5.4 "auto" , in § 5.3 auto , in § 5.3 complexity , in § 5.1 format , in § 5.1 frameDuration , in § 5.1 "lowdelay" , in § 5.4 lowdelay , in § 5.4 "music" , in § 5.3 music , in § 5.3 "ogg" , in § 5.2 ogg , in § 5.2 "opus" , in § 5.2 opus dict-member for AudioEncoderConfig , in § 5 enum-value for OpusBitstreamFormat , in § 5.2 OpusApplication , in § 5.4 OpusBitstreamFormat , in § 5.2 OpusEncoderConfig , in § 5.1 OpusSignal , in § 5.3 packetlossperc , in § 5.1 signal , in § 5.1 usedtx , in § 5.1 useinbandfec , in § 5.1 "voice" , in § 5.3 voice , in § 5.3 "voip" , in § 5.4 voip , in § 5.4 Terms defined by reference [WEBCODECS] defines the following terms: "key" AudioEncoderConfig EncodedAudioChunk [[internal data]] [[type]] description [WEBIDL] defines the following terms: EnforceRange boolean unsigned long unsigned long long References Normative References [RFC2119] S. Bradner. Key words for use in RFCs to Indicate Requirement Levels . March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119 [WEBCODECS] Paul Adenot; Eugene Zemtsov. WebCodecs . 5 May 2026. WD. URL: https://www.w3.org/TR/webcodecs/ [WEBIDL] Edgar Chen; Timothy Gu. Web IDL Standard . Living Standard. URL: https://webidl.spec.whatwg.org/ Non-Normative References [OPUS] RFC 6716: Definition of the Opus Audio Codec . September 2012. URL: https://datatracker.ietf.org/doc/html/rfc6716 [OPUS-IN-OGG] RFC 7845: Ogg Encapsulation for the Opus Audio Codec . April 2016. URL: https://datatracker.ietf.org/doc/html/rfc7845 [RFC6716] JM. Valin; K. Vos; T. Terriberry. Definition of the Opus Audio Codec . September 2012. Proposed Standard. URL: https://www.rfc-editor.org/rfc/rfc6716 [WEBCODECS-CODEC-REGISTRY] Paul Adenot; Eugene Zemtsov. WebCodecs Codec Registry . 12 February 2026. DRY. URL: https://www.w3.org/TR/webcodecs-codec-registry/ IDL Index partial dictionary AudioEncoderConfig { OpusEncoderConfig opus ; }; dictionary OpusEncoderConfig { OpusBitstreamFormat format = "opus"; OpusSignal signal = "auto"; OpusApplication application = "audio"; [ EnforceRange ] unsigned long long frameDuration = 20000; [ EnforceRange ] unsigned long complexity ; [ EnforceRange ] unsigned long packetlossperc = 0; boolean useinbandfec = false ; boolean usedtx = false ; }; enum OpusBitstreamFormat { "opus" , "ogg" , }; enum OpusSignal { "auto" , "music" , "voice" , }; enum OpusApplication { "voip" , "audio" , "lowdelay" , };