FLAC WebCodecs Registration

Abstract

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, and (4) the values of EncodedAudioChunk [[type]].

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 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.

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This document is governed by the 2 November 2021 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

AudioDecoderConfig.description is required, and has to be the following:

The bytes 0x66 0x4C 0x61 0x43 ("fLaC" in ASCII)
A METADATA_BLOCK of type STEAMINFO as described in [FLAC]
Optionaly other METADATA_BLOCK, that are not used by the specification

The AudioDecoderConfig.sampleRate and AudioDecoderConfig.numberOfChannels members are overridden by what the decoder finds in the AudioDecoderConfig.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. Privacy Considerations

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

6. 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.

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.