Constant bitrate audio encoding with MediaRecorder

[sizeak]

Request for Mozilla Position on an Emerging Web Specification

• Specification Title: MediaStream Recording, section 2. Media Recorder API
• Specification or proposal URL: https://w3c.github.io/mediacapture-record/#mediarecorder-api
• Pull request & discussion of additions: Add audioConstantBitRate flag to MediaRecorderOptions. w3c/mediacapture-record#185
• Explainer: https://docs.google.com/document/d/10OheBob6lP7v-fO89g0A5F9BYnsMEU5Nkh4p-Bk0wMg/edit?usp=sharing
• Chromestatus entry: https://chromestatus.com/feature/5730124735447040

Other information

A new enumeration (BitrateMode) has been added for specifying either constant or variable bitrate for audio encoding, MediaRecorderOptions has an extra field called audioBitrateMode for requesting a specific bitrate mode from the MediaRecorder and a field was added to MediaRecorder for retrieving the bitrate mode being used for encoding.

[hsivonen]

I note that neither the explainer nor the other linked documents explain a use case. The person who requested said they "need" this, and later comments took it as a given that CBR for audio is common.

However, it was not explained what use cases are enabled by CBR than now fail due to MediaRecorder producing VBR.

[annevk]

cc @jan-ivar

[sizeak]

I note that neither the explainer nor the other linked documents explain a use case. The person who requested said they "need" this, and later comments took it as a given that CBR for audio is common.

However, it was not explained what use cases are enabled by CBR than now fail due to MediaRecorder producing VBR.

I am the person who needed this and CBR audio is common. I have an open task to update the explainer with more information, but in short, CBR gives me predicable packet and file sizes which can be useful in a number of situations, it can also be less resource intensive to encode and can give higher quality. I have use cases which require CBR encoding, without implementing support in MediaRecorder I would be forced to ship a WASM alternative which would have performance implications.

[sizeak]

The explainer has now been updated.