Its low-pass filters are analog (CEM 3379), but digitally controlled, and thus can be modulated with a significant number of sources, including LFO, envelopes, velocity, aftertouch, modulation wheel, etc. Even though the filter resonance can be driven to extreme effect, the resonance setting features only 32 steps, and the filters do not fully self-oscillate. The ESQ-1's keybed doesn't feature aftertouch, but the synthesis engine is capable of processing polyphonic aftertouch over MIDI, such as the polyphonic aftertouch implementation of the SQ-80's keyboard.