Audio as CV (Exciter)
| Property | Value |
|---|---|
| Category | Bipolar / Special |
| Voltage Range | -5V..+5V |
| Polarity | Bipolar |
| Bandwidth | DC..20kHz+ |
Description
Some modules use short, hard audio transients or complete audio signals as excitation (exciter) for physical models or as modulation sources. Unlike FM/AM, the audio signal is not used to modulate a parameter, but as a direct sonic input to an algorithm.
All Eurorack CV inputs are DC-coupled (no coupling capacitor). A DC-blocker filter can be implemented in software if needed.
Use Cases
- Karplus-Strong exciter (short noise burst or audio transient excites a string model)
- Wavefolder input (audio signal through waveshaper)
- Granular synthesis (external audio as grain source)
- Vocoder carrier or modulator
Eurorack Examples (Mutable Instruments)
- Rings: IN input — external audio as exciter for the resonator (alternative to internal noise burst), uses internal STM32 ADCs
- Clouds: IN L/R — audio input for granular processing
Hardware Implementation
Dedicated Audio Input Required
Audio-rate signals cannot be meaningfully processed via the multiplexed ADS8866.
Implementation Options
- Internal MCU ADCs (STM32G4): Dedicated, non-multiplexed input directly on an internal ADC pin. The internal 12-bit ADCs (up to 4 Msps, with hardware oversampling to 16-bit) are perfectly adequate for Karplus-Strong exciters — exactly how MI Rings does it.
- Audio codec with input (WM8731, CS4270) via I2S/SAI — for hi-fi audio, bidirectional
- Dedicated external ADC without mux
Input Stage for Audio
- Leave DC-coupled (no coupling capacitor in hardware) — maximum flexibility
- DC blocker in software: y[n] = x[n] - x[n-1] + 0.995 × y[n-1]
- Op-amp choice: MCP6004 (1MHz BW, 0.6V/μs slew) is borderline for audio. Faster buffer (TL072, MCP6022) before the ADC pin recommended.
- Impedance: 100kΩ input impedance (as inverting summing amplifier) does not load Eurorack outputs