The VCF section contains the controls for the filter section and features a zero-delay feedback filter modelled inspired by two hardware SEM filters. One of these were the vintage SEMs contained within our original 8-Voice. The other is from the reissued, more modern SEM.
During development we constantly compared and analysed the signals of both. The differences between the two were probably more down to calibration than anything else, which again is a testament to Tom’s exceptional engineering abilities. However, we should point out that Tom credits the SEM filter to ex-ARP engineer Dennis P Colin, who we were fortunate enough to interview for our Bright Sparks Documentary, shortly before his death in November 2015.
The smaller knob at the lower right of the filter section determines whether the filter will act as a Low-Pass filter (when set to the left), a Notch filter (when set vertically) or a High-Pass filter (when set to the right). The transition between Low-Pass, Notch & High-Pass is continuous, whereas Band-Pass mode is selected by moving the switch situated underneath to the right.
<aside> ⚠️ When Band-Pass Mode is active, moving the Filter Mode Knob will have no effect.
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This determines the cutoff frequency of the filter, effectively acting as a tone control in conjunction with and depending on the position of the aforementioned Filter Mode Knob. If you don’t know what this means you’re probably a drummer.
The original 4-Voice & 8-Voice manual gives you what we think is a superb description of the Resonance Knob and as such we’ve reproduced it verbatim:
The large single knob labelled RESONANCE determines the filter resonance at the selected cutoff frequency. When this knob is fully counter-clockwise the filter resonance is at a minimum; as it is rotated to the right the resonance increases. At maximum resonance the filter strongly amplifies any components of the input signal that lie within a semitone or so of the cutoff frequency, but virtually ignores components that are not near the cutoff frequency. In other words a pronounced peak occurs in the filter frequency response as the resonance is increased, which grows higher in amplitude as it grows narrower in width. When the centre of the peak is at exactly the frequency of an audio input to the filter, a relatively weak signal can drive the filter to produce a very strong output.
The Modulation Knob is a reversible attenuator that applies an amount of modulation to the filter in conjunction with the three position switch underneath.
In its vertical position no modulation is applied. As the knob is rotated to the right the selected control signal modulates the VCF. As the knob is rotated to the left, the selected source modulates the VCF inversely driving the VCF frequency down instead of up.
The switch selects one of the three modulation sources to modulate the filter.
| ENV2 | The shape of ENV2 modulates the filter provided an amount has been applied via the filter MOD Knob. |
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| VCO3 | The settings of VCO3 modulate the filter provided an amount has been applied via the filter MOD Knob. |
| LFO1 | The settings of VCO3 modulate the filter provided an amount has been applied via the filter MOD Knob. |
Unless at least one of these knobs is open (i.e. rotated either to the left or right), the SEM will produce no sound regardless of any of the settings anywhere on the module panel.
The first knob on the left, labelled VCO1, selects either the Sawtooth or Pulse signals from VCO1 as an audio input to the VCF. The second knob does the same for signals from VCO2. The third knob adds the signal from VCO3 when rotated to the left (see VCO3 Waveform Switch) or Pink noise when rotated to the right. Signals from these three oscillators may be mixed in various proportions by opening all three of the VCO waveform attenuators at the same time.