LTB Audio Systems USB Pro 5.1 Surround Sound Headphone System - Manual (Page 21)

6. Special Buttons Function (Optional)
HID Function Button
USB Audio provided 2 special digital control buttons for user control from
external device without mouse click. These buttons can let user directly control
volume up/volume down. User can easily control volume promptly without
operating mouse and GUI.
1.Volume Up Pressing button can directly synchronizing control host side mixer
to increase volume.
2.Volume Down Pressing button can directly synchronizing control host side
mixer to decrease volume.
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