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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