[Webkit-unassigned] [Bug 223380] New: Degraded ScriptProcessorNode performance/quality for (14.0.3+ and iOS 14.4.1+)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=223380

            Bug ID: 223380
           Summary: Degraded ScriptProcessorNode performance/quality for
                    (14.0.3+ and iOS 14.4.1+)
           Product: WebKit
           Version: Safari 14
          Hardware: All
                OS: All
            Status: NEW
          Severity: Critical
          Priority: P2
         Component: Web Audio
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jwschaedler at gmail.com
                CC: cdumez at apple.com

After updating to Safari 14.0.3 or iOS 14.4.1, the audio quality on pages that use ScriptProcessorNode has degraded quite dramatically. This can be observed by visiting a page like  https://learningsynths.ableton.com/playground.

To reproduce: Grab one of the sliders and slowly drag it back and forth.

Prior to 14.0.3 and iOS14.4.1, the audio playback on this site was very smooth, even for devices like an iPhone 5s. After updating, desktop Safari (MacBook Pro 2015) crackles and pops intermittently (~every 1-3 seconds). The situation is a bit worse for iOS devices, where the audio is almost constantly glitchy on a 2020 iPhone SE.

An interesting finding:
Increasing the ScriptProcessorNode's buffer size does not mitigate the problem. Rather, at certain buffer sizes the glitching becomes very repetitive and almost periodic, leading one to believe that the issue is likely not due to the DSP failing to complete processing on time, but may have to do with the implementation of ScriptProcessorNode.

The site works well/smoothly on other browsers which support AudioWorklet: Firefox, Chrome, and Safari Tech Preview. Viewing the site in 14.0.3 or iOS 14.4.1 will trigger a fallback to ScriptProcessorNode since AudioWorklet is not yet supported there, and this will lead to the issues described above.

If the fallback to ScriptProcessorNode is forced on Chrome, similar glitching occurs with the latest stable version.

When considering mitigation strategies, it would be helpful to know if there's a timeframe for the release of AudioWorklet in stable builds of Safari. However, I do understand that this sort of information is usually not disclosed.

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