Incompatibility with USB Audio Devices on iOS 18
Incompatibility with USB Audio Devices on iOS 18
- Subject: Incompatibility with USB Audio Devices on iOS 18
- From: Jean-Charles Rousset via Coreaudio-api <email@hidden>
- Date: Fri, 29 Nov 2024 13:38:13 +0100
Dear all,
Our company develops high-end acoustic measurement tools for the Swiss watch
industry, based on XMOS microcontrollers and the asynchronous UAC2 protocol
(2-in, 2-out). Unfortunately, with the release of iOS 18, we have encountered
critical compatibility issues affecting an entire product line. These devices
are no longer detected by our app or any third-party audio apps.
Specifically, when such a device is connected, while the output is routed
correctly to the USB device, the input remains stuck on the iPhone's built-in
microphone, even though the USB device is selected as the preferred input in
AVAudioSession.
After thorough investigation, we identified the cause of the issue: two
descriptors, MIN_VOLUME and MAX_VOLUME, are set to 0x0000 (indicating 0 dB).
Modifying MIN_VOLUME to 0x8100 (-127 dB) resolves the issue. However, this
workaround is not feasible for us due to the deployment of thousands of devices
worldwide, making a recall and reconfiguration impractical.
We were unable to find any documentation or release notes suggesting whether
this behavior is a new limitation introduced in iOS 18 or a bug. We submitted a
bug report a week ago and would greatly appreciate any guidance or
clarification from the community or Apple.
Thank you for your attention and support.
Best regards,
Jean-Charles
h2i.ch <http://h2i.ch/>
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