Oberheim SEM 2-Pole State-Variable Filter (SVF)
Tom Oberheim's classic 12dB/octave discrete analog filter offering continuous morphing between Low-Pass, Notch, and High-Pass modes.
# Mathematical Formula & Architecture
Logic SpecificationSimultaneous Low-Pass, Band-Pass, and High-Pass outputs: V_BP = Integral(V_HP), V_LP = Integral(V_BP). Mode blend: V_out = (1-k)*V_LP + k*V_HP.
Oberheim SEM SVF Circuit Block:
Audio In ---> [Summing Node] ---> Integrator 1 (Band-Pass Output)
---> Integrator 2 (Low-Pass Output)
Inverted Feedback creates resonance. Knob crossfades continuously from LP -> Notch -> HP.# Deep Technical Analysis & Historical Context
In 1974, Tom Oberheim designed the Synthesizer Expander Module (SEM) as a companion for early digital sequencers. Unlike the 4-pole transistor ladders of Moog (which produce a sharp -24dB/octave slope and attenuate low frequencies when resonant), the SEM utilized a discrete 2-pole (-12dB/octave) State-Variable Filter topology. Built from standard op-amps and matched transistors, the SEM filter provides simultaneous Low-Pass, High-Pass, and Band-Pass outputs, allowing musicians to continuously sweep from low-pass through a phasing notch filter into razor-sharp high-pass. This airy, open filter character remains legendary and was reincarnated in the modern Oberheim OB-6.
Common Studio Pitfall & Hardware Hazard
Because SEM filters are -12dB/octave rather than -24dB, high-frequency harmonic overtones leak through above the cutoff, requiring careful EQing in dense mixes.
# Hardware Synthesizers Using This Technology
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# Frequently Asked Engineering Questions
What modern synth uses the SEM filter?
The Dave Smith & Tom Oberheim OB-6 incorporates an authentic modern discrete implementation of the 1974 SEM state-variable filter.
Related Engineering Concepts
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