AAC WebCodecs Registration

W3C Group Draft Note,

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This registration is entered into the [webcodecs-codec-registry]. It describes, for AAC, 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 AAC 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 technical reports index at https://www.w3.org/TR/.

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, public-media-wg@w3.org (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 Draft Note using the Note track.

Group Draft Notes 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 work in progress.

The W3C Patent Policy does not carry any licensing requirements or commitments on this document.

This document is governed by the 2 November 2021 W3C Process Document.

1. Fully qualified codec strings

This codec has multiple possible codec strings:

2. EncodedAudioChunk data

If the bitstream is in adts format, the [[internal data]] of EncodedAudioChunks are expected to be an ADTS frame, as described in section 1.A.3.2 of [iso14496-3].

If the bitstream is in aac format, the [[internal data]] of EncodedAudioChunks are expected to be a raw AAC frame (syntax element raw_data_block()), as described in section of [iso14496-3].

3. AudioDecoderConfig description

If AudioDecoderConfig.description is present, it is assumed to a AudioSpecificConfig as defined in [iso14496-3] section, Table 1.15, and the bitstream is assumed to be in aac.

If the AudioDecoderConfig.description is not present, the bitstream is assumed to be in adts format.

The AudioDecoderConfig.sampleRate and AudioDecoderConfig.numberOfChannels members are ignored.

4. EncodedAudioChunk type

The [[type]] for an EncodedAudioChunk containing AAC is always "key".

NOTE: Once the initialization has succeeded, any AAC 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 {
  AacEncoderConfig aac;

aac, of type AacEncoderConfig
Contains codec specific configuration options for the AAC codec.

5.1. AacEncoderConfig

dictionary AacEncoderConfig {
  AacBitstreamFormat format = "aac";

format, of type AacBitstreamFormat, defaulting to "aac"
Configures the format of output EncodedAudioChunks. See AacBitstreamFormat.

5.2. AacBitstreamFormat

enum AacBitstreamFormat {

The AacBitstreamFormat determines the location of the metadata necessary to decode the encoded audio stream.

The metadata of the encoded audio stream are provided at configuration via AudioDecoderConfig.description.
The metadata of the encoded audio stream are provided in each ADTS frame, and therefore no AudioDecoderConfig.description is necessary.

6. Privacy Considerations

Please refer to the section Privacy Considerations in [WEBCODECS].

7. Security Considerations

Please refer to the section Security Considerations in [WEBCODECS].


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.

Conformant Algorithms

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

Conformance requirements phrased as algorithms or specific steps can be implemented in any manner, so long as the end result is equivalent. In particular, the algorithms defined in this specification are intended to be easy to understand and are not intended to be performant. Implementers are encouraged to optimize.


Terms defined by this specification

Terms defined by reference


Normative References

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
Chris Cunningham; Paul Adenot; Bernard Aboba. WebCodecs. 20 October 2022. WD. URL: https://www.w3.org/TR/webcodecs/

Informative References

ISO/IEC 14496-3:2009 - Information technology — Coding of audio-visual objects — Part 3: Audio. 2009-09. URL: https://www.iso.org/standard/53943.html
Chris Cunningham; Paul Adenot; Bernard Aboba. WebCodecs Codec Registry. 10 October 2022. NOTE. URL: https://www.w3.org/TR/webcodecs-codec-registry/

IDL Index

partial dictionary AudioEncoderConfig {
  AacEncoderConfig aac;

dictionary AacEncoderConfig {
  AacBitstreamFormat format = "aac";

enum AacBitstreamFormat {