CV Input Signal Types #11 — FM (Audio-Rate)
| Property | Value |
|---|---|
| Category | Bipolar / Modulation |
| Voltage Range | -5V..+5V |
| Polarity | Bipolar |
| Bandwidth | DC..20kHz+ (audio-rate) |
Description
Audio-rate FM is a bipolar signal in the audible frequency range (typically 20Hz–20kHz) that modulates the frequency of an oscillator. FM synthesis generates complex overtone spectra and is one of the most powerful synthesis techniques.
There are two FM variants:
- Linear FM: Modulation depth is frequency-independent (sounds metallic, harmonic at integer ratios)
- Exponential FM: Modulation depth scales with the fundamental frequency (traditional "analog" FM sound, detunes at high modulation depths)
Use Cases
- FM synthesis (classic: sine-wave FM for DX7-style sounds)
- Complex oscillator designs (through-zero FM)
- Tonal coloring through sideband generation
- Percussive sounds (fast FM sweeps)
Eurorack Examples (Mutable Instruments)
- Plaits: FM input with dedicated attenuverter, supports linear or exponential FM
- Warps: Audio-rate cross-modulation entirely digital via internal STM32 ADCs
Hardware Implementation
NOT via Multiplexed ADS8866!
Audio-rate FM requires sample rates of at least 48kHz (preferably 96kHz) per channel. The ADS8866 via multiplexer cannot deliver this — the settling time of the mux and the shared bandwidth make audio-rate sampling impossible.
Implementation Options
- Internal MCU ADCs (STM32G4): The STM32G431KB has fast internal 12-bit ADCs (up to 4 Msps). With hardware oversampling to 16-bit and direct input (without mux), this is the standard solution for audio-rate CV in Mutable Instruments modules (e.g., Plaits, Warps). A dedicated, non-multiplexed input on an internal ADC pin is the most pragmatic solution.
- Dedicated external audio ADC (e.g., second ADS8866 without mux, FM only)
- Audio codec with input (e.g., WM8731, CS4270 via I2S/SAI) — for hi-fi audio
- Analog FM: FM signal fed directly into the oscillator circuit — only relevant if the oscillator were analog
Input Stage for Audio-Rate
The MCP6004 has 1MHz bandwidth and 0.6V/μs slew rate — borderline for hi-fi audio. For an internal ADC pin, a faster op-amp buffer (e.g., TL072, MCP6022) before the pin would be advisable.