At present, most mobile phones on the market can support the FiiO music App, but the USB audio standard of various phone brands is not uniform, so we cannot guarantee full compatibility. 2. The BTR5 2021 supports a switch to 'USB Audio Adapter Mode' by just one press. When a USB connection is detected the BTR5 2021, it will automatically prompt you to turn on 'USB Audio Adapter' mode. At this time, just press any button so that it will turn off the charging function and enter the 'USB Audio Adapter Mode'.