Generic Bipolar CV
| Property | Value |
|---|---|
| Category | Bipolar / Modulation |
| Voltage Range | -5V..+5V |
| Polarity | Bipolar |
| Bandwidth | DC..~1kHz |
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