Linear PCM WebCodecs Registration

W3C Group Draft Note,

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This registration is entered into the [webcodecs-codec-registry]. It describes, for Linear PCM, the (1) fully qualified codec strings, (2) the codec-specific EncodedAudioChunk [[internal data]] bytes, (3) the AudioDecoderConfig.description, and (4) the values of EncodedAudioChunk [[type]].

Linear PCM is the raw audio format used in WebCodecs and does not require decoding. The motivation for registering the Linear PCM codec is to enable passthrough "decoding" of PCM content as an architectural simpliciation for WebCodecs applications.

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.

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

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, (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’s string begins with the prefix "pcm-", followed by a suffix that indicates the sample format. The complete list of strings and associated formats is as follows.

NOTE: [WEBCODECS] does not define a 24-bit AudioSampleFormat. 24-bit samples are permitted within an EncodedAudioChunk, but such samples will be "decoded" in AudioData objects as either s32 of f32. Please see [WEBCODECS] section on sample magnitude for addtional details.

2. EncodedAudioChunk data

Linear pulse code modulation (linear PCM) describes a format where the audio values are sampled at a regular interval, and where the quantization levels between two successive values are linearly uniform.

EncodedAudioChunk [[internal data]] is expected to be a sequence of bytes of arbitrary length, with a sample occuring every N bits, where N is defined by the codec string. For multichannel PCM, samples from different channels are interleaved.

3. AudioDecoderConfig description

An AudioDecoderConfig.description is expected to be omitted from the AudioDecoderConfig.

4. EncodedAudioChunk type

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

5. Privacy and Security Considerations

Please refer to the Privacy Considerations and Security Considerations sections 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 reference


Normative References

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL:
Chris Cunningham; Paul Adenot; Bernard Aboba. WebCodecs. 9 February 2022. WD. URL:

Informative References

Chris Cunningham; Paul Adenot; Bernard Aboba. WebCodecs Codec Registry. 9 February 2022. NOTE. URL: