Meter switch: allows the meter to indicate either channels input level, either channels output level or the amount of stereo audio signal compression Sidechain: modifies the control of the compressor with a mono audio A blue LED: gives the user a visual indication that the circuit is active and that the sidechain audio is in control of the compressor The HPF (High Pass Filter) rotary control: allows the sidechain or main audio signal to be equalized to minimize lower frequency audio signals" eff