FLAC WebCodecs Registration

Editor’s Draft,

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Paul Adenot (Mozilla)
Bernard Aboba (Microsoft Corporation)
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Chris Cunningham (Google Inc.)
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This registration is entered into the [webcodecs-codec-registry]. It describes, for FLAC, 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 FLAC 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 an Editor’s Draft. The group does not expect this document to become a W3C Recommendation.

Publication as an Editor’s Draft does not imply endorsement by W3C and its Members.

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

This document is governed by the 03 November 2023 W3C Process Document.

1. Fully qualified codec strings

The codec string is "flac".

2. EncodedAudioChunk data

EncodedAudioChunk [[internal data]] is expected to be a "FRAME" as described in [FLAC].

3. AudioDecoderConfig description

description is required, and has to be the following:

The sampleRate and numberOfChannels members are overridden by what the decoder finds in the description.

NOTE: This corresponds to the beginning of a FLAC bitstream, before the audio frames.

4. EncodedAudioChunk type

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

NOTE: Once the initialization has succeeded, any FLAC 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 {
  FlacEncoderConfig flac;

flac, of type FlacEncoderConfig
Contains codec specific configuration options for the FLAC codec.

5.1. FlacEncoderConfig

dictionary FlacEncoderConfig {
  [EnforceRange] unsigned long blockSize = 0;
  [EnforceRange] unsigned long compressLevel = 5;

To check if an FlacEncoderConfig is valid, run these steps:

  1. If blockSize is not a valid block size, which is described section 5.1 of [FLAC], return false.

  2. If compressLevel is specified and not within the range of 0 (fastest, least compression) and 8 (slowest, most compression) inclusively, return false.

  3. Return true.

blockSize, of type unsigned long, defaulting to 0
Configures the number of samples to use per frame, of output EncodedAudioChunks.

NOTE: Use 0 to let the encoder estimate a blocksize by default.

compressLevel, of type unsigned long, defaulting to 5
Configures the compression level to use while encoding.

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.


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
Paul Adenot; Bernard Aboba; Eugene Zemtsov. WebCodecs. URL: https://w3c.github.io/webcodecs/
Edgar Chen; Timothy Gu. Web IDL Standard. Living Standard. URL: https://webidl.spec.whatwg.org/

Informative References

Free Lossless Audio Codec. URL: https://datatracker.ietf.org/doc/draft-ietf-cellar-flac
Chris Cunningham; Paul Adenot; Bernard Aboba. WebCodecs Codec Registry. URL: https://w3c.github.io/webcodecs/codec_registry.html

IDL Index

partial dictionary AudioEncoderConfig {
  FlacEncoderConfig flac;

dictionary FlacEncoderConfig {
  [EnforceRange] unsigned long blockSize = 0;
  [EnforceRange] unsigned long compressLevel = 5;