Voicing the Filter: Picking an SVF for a Bass Envelope Filter
Qingtan's filter section is really an envelope filter — an auto-wah that opens with how hard you play. On bass that's a demanding job: the cutoff moves fast, the resonance is high, and if the low end drops out the whole part loses its foundation. The filter topology decides whether that works.
The three options
- Biquad IIR (RBJ cookbook) — cheap and everywhere. Great for a static EQ, but when you sweep the cutoff quickly the coefficients jump and it can zipper or even blow up at high Q.
- FIR, linear phase — phase-perfect and stable, but expensive, and its long impulse response fights against fast modulation. Wrong tool for a moving filter.
- State-variable filter (SVF) — gives lowpass / bandpass / highpass at once, with independent, smoothly modulatable cutoff and Q. Built for exactly this.
What I chose, and why
The SVF was the obvious pick for a modulated bass filter:
- Stable under fast sweeps. The envelope can slam the cutoff open and the SVF stays well-behaved where a naively-swept biquad would zipper.
- Independent fc and Q. I can push resonance for that vocal "wah" without the cutoff fighting it.
- Bandpass tap for free. The auto-wah voice lives in the bandpass output, and I still have the lowpass around to keep weight.
Protecting the low end
High resonance on a bass is where things go wrong — a resonant peak that sweeps down into the fundamentals can boom and mud. Two decisions kept it tight:
- Blend a highpassed version of the resonance path so the peak adds vocal character up top without amplifying sub-bass.
- Cap the Q / cap how low the cutoff can travel, so the sweep stays in the expressive range and never parks on the fundamental.
The envelope follower driving all this is its own small design (attack/release, gate) — — but the topology choice is what makes the fast, resonant, bass-safe sweep possible in the first place.
The filter shapes; the next log is about adding harmonic weight without losing the sub — drive.