Issue with "Web Audio playbackRate explained": (Note: pitch of the audio track does not change, for video only)

[mattgilbertnet]

MDN URL: https://developer.mozilla.org/en-US/docs/Web/Guide/Audio_and_video_delivery/WebAudio_playbackRate_explained

What information was incorrect, unhelpful, or incomplete?

One of the notes says "The pitch of the audio track does not change when playBackRate is altered." This is true for video elements, like the example provided. However, it's not true for audio elements, like this example.

Ideally, links to examples of both types would be provided, along with a more detailed note.

Specific section or headline?

Notes

What did you expect to see?

The pitch of the audio track changes when playBackRate is altered for audio elements, but not for video elements.

Did you test this? If so, how?

I compared the relationship between pitch and playback rate with these two examples:

• https://jsbin.com/UGIxoJis/1/edit?html,output
• https://mdn.github.io/webaudio-examples/decode-audio-data/

MDN Content page report details

• Folder: en-us/web/guide/audio_and_video_delivery/webaudio_playbackrate_explained
• MDN URL: https://developer.mozilla.org/en-US/docs/Web/Guide/Audio_and_video_delivery/WebAudio_playbackRate_explained
• GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/web/guide/audio_and_video_delivery/webaudio_playbackrate_explained/index.html
• Last commit: 603be04
• Document last modified: 2020-12-30T11:38:24.000Z

[CreaTorAlexander]

I can update this 👍

[mattgilbertnet]

It's possible that this is dependent on the audio format of the video file, but I'm not sure.

[wbamberg]

I think this is not correct.

The spec is clear that the <audio>and <video> elements should (by default) preserve the pitch when playbackRate is changed: https://html.spec.whatwg.org/multipage/media.html#dom-media-playbackrate . There's no suggestion that it should be different for audio and for video, or that the format should make a difference.

I think what's happening with the example quoted: https://mdn.github.io/webaudio-examples/decode-audio-data/ is that this example isn't using the HTMLMediaElement interface (which is the relevant interface described in https://developer.mozilla.org/en-US/docs/Web/Guide/Audio_and_video_delivery/WebAudio_playbackRate_explained). It's using the AudioBufferSourceNode interface, which says nothing about pitch preserving.

If I use the same audio file in an <audio> element, I can adjust playbackRate and preserve pitch: https://codepen.io/willbamberg/pen/JjMRbbQ?editors=1111.

One real issue is that https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/playbackRate says:

The pitch of the audio is corrected by default and is the same for every speed. Some browsers implement the non-standard HTMLMediaElement.preservesPitch property to control this.

It looks like preservesPitch() is now standard (not supported in Firefox, but it is in Chrome) and probably deserves a page. (More info: https://bugzilla.mozilla.org/show_bug.cgi?id=1652950).

Maybe we should file a separate issue for that though?

[wbamberg]

It turns out that we have BCD for preservesPitch, which makes things easier: https://github.com/mdn/browser-compat-data/blob/2a845c8742b274407cad0011cfd836dddd04061d/api/HTMLMediaElement.json#L2664.

[wbamberg]

It looks like preservesPitch() is now standard (not supported in Firefox, but it is in Chrome) and probably deserves a page. (More info: https://bugzilla.mozilla.org/show_bug.cgi?id=1652950).

Maybe we should file a separate issue for that though?

I filed #14183 to add this.