USB microphone input : Mac "Designed for iPad"

My app - natively iOS but built with the "Designed for iPad" option to run on Mac - does not recognise an attached USB microphone when running on a Mac. This line

int32_t items = (int32_t) [[[AVAudioSession sharedInstance] availableInputs] count ];

returns 1, which is the Mac internal mic. On iPad and iPhone it sees both the internal mic and the USB mic. Is this an inherent "Designed for iPad" restriction, and is there some trick I can pull to get the USB microphone to be recognised by the system?

Right. This is werder than I thought. It IS connecting to the USB microphone, but is applying a brickwall filter to the audio, cutting off everything above 30kHz. I need to talk to somebody at Apple about this behaviour.

USB microphone input : Mac "Designed for iPad"
 
 
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