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I have a USB Audio Soundcard, it work on ipad and iphone well. Audio can be play through the soundcard. Now, i want to send some controlling command to the soundcard. I think it will be done with the USB HID interface. But I do not know how to do it. Can anybody give me some guide ? My App is designed for IOS, not Mac OS. The app can send volume and mute command to the soundcard, which are the Usb-Audio-class request.