The standard digital filter, available with both additive and subtractive synthesis, is known for its rather extreme self-oscillation at higher resonance settings.
In linear systems, self-oscillation appears as an instability associated with a negative damping term, which causes small perturbations to grow expontentially in amplitude.
Filter cutoff frequency, resonance (to self-oscillation), keyboard tracking (off, half or full) and envelope amount can be programmed, but the envelope can not be inverted.
The system does not require external components to create the self-oscillation (reducing bill of materials) and the capacitor (that defines the frequency of the self-oscillation) can be connected directly.