Eight Voice's filter section represents a departure from the 24 dB "ladder" style filter often seen in vintage synths. Like the Oberheim SEM module it's based on, Eight Voice uses a 12 dB state-variable filter (no, this doesn't mean it can sound like Rhode Island or Montana). This refers to its curves - it can function as a lowpass, bandpass, or highpass filter and features a knob allowing a continuous sweep from lowpass to highpass response (with "notch" filtering in the middle position). This gives it a great deal of flexibility, and the 12 dB curve gives it a brighter overall tonality than a typical ladder filter.
If you're not familiar with how filters work, a lowpass filter allows frequencies below the cutoff frequency setting to pass through, but blocks frequencies above the cutoff frequency. Highpass is the opposite of lowpass mode: high-frequency content remains, but low frequencies are removed as the cutoff frequency increases. Sliding the Bandpass switch enables Bandpass mode combining both lowpass and highpass modes, leaving sound only "in the middle." The cutoff frequency lies roughly halfway between the falloff on each side.
Frequency- Sets the frequency where frequency attenuation begins with its effect dependent upon the currently chosen lowpass/bandpass/notch/highpass/etc. filter mode.
Resonance- Emphasizes sound energy at and around the cutoff frequency by adding feedback from the filter's output back to its input. This is useful for creating commonly heard synth "wah" tones, especially when the cutoff frequency is modulated with an envelope generator or one of the LFO's.
Modulation amount knob and source switch- Applies modulation to the filter cutoff frequency. The mod amount knob is bidirectional; rotating it right from center position adds positive modulation, rotating it left adds negative modulation. Center position is off, i.e. no modulation.
The three-position slide switch selects the filter mod source from three sources:
Env 2- Though it can modulate VCO 2, Env 2 can be thought of as a dedicated filter envelope.
LFO 1- The module's onboard LFO.
LFO 2- LFO 2 in the master section. LFO 2 is a single global LFO for the entire instrument.
LP>HP/Notch control- Continuously varies the filter response from lowpass to highpass as the knob is rotated from left to right. The middle position creates a notch response.
BP (bandpass) switch- Alters the filter response to bandpass when engaged (i.e. combination of lowpass and highpass leaving frequencies "in the middle." The LP>HP knob disappears when the BP switch is enabled.
VCO 1 / VCO 2 Saw/Pulse level- The VCO 1 and VCO 2 knobs adjust the volumes of the saw and pulse waves for VCO 1 and 2, respectively. These are bidirectional knobs as well - rotating them left from center position increase the saw wave level, rotating it right increase the volume of the pulse wave. Center position is off (if you're getting no sound, check these first).
Sub/Noise level- When rotated to the left, sets sub-octave level, when rotated right, sets the level of the noise generator. The sub-octave is a one-octave down square wave derived from VCO 1. Its waveshape and level are not affected by the VCO 1 Saw/Pul knob.