CV Input Signal Types #9 — Pressure and Ribbon

Description

Pressure CV (aftertouch) and ribbon controller CV are expressive control signals that come directly from the musician's playing technique. They are continuous but often noisy and dynamically uneven due to the physical interaction (finger pressure, finger position).

Use Cases

• Control vibrato intensity (aftertouch → LFO amount)
• Filter opening through finger pressure
• Pitch bend via ribbon position
• Expression (general dynamic control)
• MPE-like per-voice modulation

Eurorack Examples

• Make Noise Pressure Points: Pressure-dependent CV per pad
• Intellijel Planar 2: X/Y position as CV
• Arturia Keystep: Aftertouch as CV output

Hardware Implementation

Input Stage: OpAmp → Mux → ADC

Identical to macro parameter. Clipping problem at >5V applies.

Firmware Requirements (critical!)

• Low-pass filtering mandatory: These signals are extremely noisy due to finger resistance. Without a software low-pass, the VCA/VCF will click and pop with every finger movement.
• Two-stage filtering recommended:
  1. Median filter (e.g., median of 5 values) — removes jump glitches when the finger briefly lifts
  2. One-pole IIR filter: y[n] = α × x[n] + (1-α) × y[n-1], with α ≈ 0.01–0.05
• Dead zone: Ignore small voltages near 0V (finger not on controller = no signal, not noise)
• Non-linear scaling: Finger pressure is not linear to voltage — use a lookup table or power function (e.g., x^1.5) for a more natural feel