SEM Filter

A smooth, creamy 12 dB/oct multimode filter modelled on the Oberheim SEM — the voice behind a thousand lush pads. Where an aggressive filter tears at a sound, this one flatters it: gentle, musical, and warm. Its signature is a single Morph knob that continuously sweeps the response from low-pass, through a hollow notch, to high-pass.

How it works

It's a Cytomic zero-delay-feedback state-variable filter that computes its low-pass, band-pass and high-pass taps all at once and keeps them perfectly in tune. The Morph control crossfades the main output through those taps — the notch is simply the low-pass and high-pass summed — so turning one knob walks the filter smoothly from dark to hollow to bright. The resonance is deliberately civil: it can lean right up toward self-oscillation for a singing peak, but a gentle saturation rounds it off so it stays clean and bounded instead of screaming.

Controls

  • Cutoff — the corner frequency, exponential from roughly 20 Hz to near Nyquist. Tracks 1V/oct, so it can be played like a pitch.
  • Res — emphasis at the cutoff. Smooth and musical; turn it up for a creamy resonant peak that leans toward self-oscillation without ever turning harsh.
  • Morph — sweeps the out response continuously: LP at one end, through a true Notch at center, to HP at the other (0→LP, 0.5→Notch, 1→HP). The bandpass tap is always available separately.
  • Drive — a mild input drive into a clean soft-clip; subtle warmth at the top, never gritty.

Inputs & outputs

  • In — the audio (or CV) signal to filter.
  • V/Oct — 1V/oct pitch control of the cutoff, for keyboard- or sequencer-tracked filtering.
  • FM — exponential cutoff modulation; depth and polarity set by the FM attenuverter (centred is off).
  • Res — resonance modulation, summed straight into the resonance amount.
  • Out — the morphed LP → Notch → HP response set by the Morph knob.
  • Bp — the bandpass tap, always available regardless of the Morph setting.

Tips

  • Park Morph at center for a notch and sweep Cutoff for smooth, vocal phaser-like motion.
  • Send a slow LFO to FM with a high Res for classic SEM pad shimmer.
  • Take Bp alone for soft, rounded band-pass tones while Out handles the morph elsewhere.