The Synthesis section includes a lowpass filter that is used for coarse signal shaping before the signal is more precisely shaped by the individual bands of the formant filter banks.
Cutoff knob: Set the cutoff frequency of the lowpass filter. Turn to the left to remove high-frequency content from the synthesizer signal.
Resonance knob: Boost or cut the signal portion that surrounds the frequency defined by the cutoff knob.
Tip: Set cutoff as high as possible, then adjust resonance to achieve a brighter high-end signal. This is useful for achieving better speech intelligibility.