Dave Smith Instruments TEMPEST - Manual (Page 41)

31 Tempest Operation Manual Highpass
Highpass
Frequency: 0...127 —Sets the filter cutoff frequency for the
2-pole high-pass filter. A setting of 0 passes all frequencies.
Turning the control up attenuates frequencies below the cutoff
at a slope of -12dB/octave.
Key > Freq: 0...127 —Sets the amount of modulation from the note number—
from either the beat, the pads, or via MIDI—to the filter cutoff frequency.
Sometimes called key tracking in keyboard synths, because the cutoff frequency
“tracks” notes played on the keyboard.