The FADER knob is the channel fader, with the ability to insert an external fader from the rear panel.
The PAN control is a standard stereo pan circuit with a center detent.
The INSERT button can act as a balanced insert for outboard equipment, or it can be used as a MIC PRE/TAPE PLAY switch for live recording to check the mic pre output and then the output of the machine for confidence checks or post recording playback without repatching anything.
The Polarity switch is a standard phase inverter for the main signal path.
The 5.1 switch turns on the signal to the surround bus, allowing the module to send signal to the aux sends without going to the surround bus.
The LCR button switches the PAN pot from stereo to LCR, and pans from left, through center, to right, or in 2 channel stereo, allowing phantom center.
The 2 sends are internally configurable for PRE or POST fader signal. One of the sends can be used as the LFE send for Surround Mixes.
The MX5.1 has an optional DIRECT transformer output.