[Webkit-unassigned] [Bug 193667] New: Distortion when playing audio samples on old iOS devices

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=193667

            Bug ID: 193667
           Summary: Distortion when playing audio samples on old iOS
                    devices
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: iPhone / iPad
                OS: iOS 12
            Status: NEW
          Severity: Major
          Priority: P2
         Component: Web Audio
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: corentin at tutteo.com

Reproduction URL : https://corentingurtner.github.io/ios-distortion/

Steps to reproduce:
1. Visit the URL
2. Click play using an iPhone or iPad older than an iPhone 7s (We have tested with an iPhone 6, iPhone 6s, iPhone 7, iPad Air 4, iPhone X and iPad Pro, only the first 4 devices are reproducing the issue)
3. Distortion should randomly occur in form of glitches/crackles

Observed result:
The playback is severely distorted making it very hard to listen.

Expected result:
It should play it fine, without any distortion.


Corentin here, from the https://flat.io team

It seems to be an issue due to the lack of resources of the device, since the older the device is, the more critical the distortion is, but it seems that even when playing a single note the distortion happens randomly.
We first thought it was related to this https://bugs.webkit.org/show_bug.cgi?id=154538 , since our samples have 44100 sample rate and the iPhones were upsampling them, but the given workarounds are not working for us, and the iPad Air 4 is keeping a 44.1k sample rate.


We are wondering if the sample format we are using is triggering some heavy computations making the Web Audio thread overloaded. 
We still tried with different sample rate to try to match the DAC sample rate (22.05khz, 48khz, 96khz), there is an improvement with 22.05khz files but it is still not perfect, especially on iPhones older than 7.

A very basic, 1 instrument song has been loaded in the Reproduction site, considering that we often need to load more than 10 instruments in a single score and to keep it smooth. 
Although sometimes distortion-free on an iPhone 7, this score very frequently produces multiple crackles with our testing iPhone 6.
The distortion is often more critical with the whole UI loaded at https://flat.io

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