A NAND-gate Schmitt-trigger oscillator can be gated on/off by a control input, letting one oscillator modulate another

The 4093 (CD4093) Quad NAND Gate has a Schmitt trigger on each of its four gates, so it can build the same RC relaxation oscillator as the 74C14 but with an extra control input. Each gate has two inputs: one carries the RC feedback that oscillates, the other is a control line. Because NAND logic only produces a changing output when both inputs are high, holding the control high (+V, e.g. +9 V) enables oscillation while pulling it to ground forces the output high and stops it — this is what the plain 74C14 cannot do. Connecting the output of a slow oscillator to the control input of a second, faster oscillator gates the second on and off at the first’s tempo, producing rhythmic patterns. As the controlling oscillator’s frequency rises into the audio range, the on/off gating turns into frequency modulation and a tone cluster. Cascading four such stages yields evolving polyrhythms or FM-like timbres.

Examples

Oscillator 1 (e.g. 4.7 µF cap, 1 MΩ pot) runs near metronome/1 Hz rate and gates Oscillator 2 (0.1 µF, photoresistor) on and off in sync. Replace Oscillator 1’s capacitor with successively smaller values, sweeping it up into audio range, and the rhythmic gating gives way to flanging and then a tone cluster. Cascading four stages creates complex polyrhythmic or spectral textures.

Assessment

Explain what a listener hears as a gating oscillator running at ~1 Hz is swept up to ~1 kHz, moving through successively smaller capacitor values. Why does the 4093 allow gating while the 74C14 does not?