CV Input Signal Types #14 — Audio as CV (Exciter)

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

1. 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.
2. Audio codec with input (WM8731, CS4270) via I2S/SAI — for hi-fi audio, bidirectional
3. 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