A highpass filter after FM oscillators removes the tonal fundamental, converting FM output into metallic texture

FM between two oscillators produces a spectrum centred on the carrier frequency, with sidebands spreading above and below. The carrier and lower sidebands give the output a pitched, tonal character. For hi-hats and cymbals, this tonality is unwanted — the goal is noise-like texture. A steep highpass filter (HPF) placed after the FM oscillator pair removes the low-frequency fundamental and lower sidebands, leaving only the upper inharmonic partials. The result sounds ‘metallic’ and ‘unpitched’. A bandpass filter is an alternative that achieves a slightly different timbre by carving a window through the spectrum rather than simply cutting below a threshold. This HPF step is the key transformation from ‘dissonant FM drone’ to ‘usable hi-hat/cymbal sound’.

Examples

Signal chain: OSC1 → FM in of OSC2 → HPF (steep, e.g. -24dB/oct) → VCA → output. Adjusting HPF cutoff upward: sound becomes thinner and more ‘closed hat’-like. Cutoff lower: more body, cymbal-like.

Assessment

In your FM hi-hat patch, sweep the HPF cutoff from 200 Hz to 8 kHz while triggering. Describe at which cutoff region the sound transitions from ‘tonal FM drone’ to ‘metallic hat’. Explain what frequencies the HPF is removing at each extreme.