CV Input Signal Types #13 — Generic Bipolar CV

Description

Generic bipolar CV is a catch-all term for all bipolar modulation signals that are neither V/Oct (calibrated) nor audio-rate (too fast for standard ADC). It encompasses LFO outputs, envelope follower outputs, manual CV (joystick, fader), and arbitrary combinations from attenuverters/mixers.

The bipolar range enables modulation in both directions around a center value — e.g., filter cutoff can be both opened and closed.

Use Cases

• General parameter modulation (filter cutoff, resonance, effect parameters)
• Bipolar sequences (sequencer CV that outputs negative values too)
• Joystick/fader control (center position = 0V)
• Inverted envelope (e.g., for filter closing on attack)

Hardware Implementation

Standard Input Stage: OpAmp → Mux → ADC

Same topology as V/Octave (R_in=100k, R_ref=200k, R_fb=33k, -10V reference), but without the elevated precision requirements:

• 1% resistors sufficient
• MCP6004 suitable

Anti-aliasing: Capacitor across R_fb. Value depends on sample rate:

• At sample rate 2–4kHz: 4.7nF across R_fb → f_c ≈ 1kHz (hardware anti-aliasing matched to sample rate)
• At sample rate 20kHz+ (oversampling): 470pF → f_c ≈ 10kHz (digital decimation then filters to target rate)

Nyquist warning: The analog low-pass must match the sample rate! A 10kHz filter at 2kHz sample rate produces aliasing artifacts (frequencies between 1kHz and 10kHz fold into the digital signal).

Clipping behavior: Eurorack signals can reach ±12V. The 3.3V single-supply MCP6004 clips everything outside 0..3.3V and thus protects the ADS8866.

Firmware Requirements

• Bipolar value range (-1.0..+1.0) in software
• Light smoothing (IIR, α ≈ 0.9) against ADC noise
• Sample rate: 2–4kHz with 4.7nF anti-aliasing, or 20kHz+ oversampling with 470pF and digital decimation