Yamaha TG500 Owner's Manual page 134

VOICE EDIT MODE / 3: Full Edit. 3-3: Filter
Cof (Cutoff frequency)
Res (Resonance)
122 3-3-01: PARAMETER
Sets the cutoff frequency of the selected filter.
Lower cutoff values produce a lower cutoff frequency and higher values
produce a higher cutoff frequency.
With an LPF response (selected by the "Type" parameter, above), a lower
cutoff frequency reduces the range of high frequencies passed, making the
sound "darker" or "rounder."
With a HPF response, a higher cutoff frequency reduces the range of low
frequencies passed, making the sound "thinner" or "sharper."
Determines the degree of filter resonance when the "LPF" filter type is
selected ("--" appears in place of the resonance parameter when any other filter
type is selected).
This parameter has a similar effect to the "resonance" settings on traditional
analog synthesizer filters — i.e. it determines the height of a peak in the filter
response at the cutoff frequency.