CV Input Signal Types #20 — Feedback CV

Description

Feedback CV arises when a module's output is patched back into its own input (self-modulation). This creates chaotic, self-reinforcing, or self-oscillating behaviors.

Electrically identical to generic bipolar CV. The challenge lies in latency: the delay through ADC → DSP → DAC determines the feedback behavior.

Use Cases

• Chaotic sound effects (self-oscillating systems)
• Feedback FM (oscillator modulates itself)
• Saturation and compression
• Experimental/generative sound design

Hardware Implementation

Standard Input Stage: OpAmp → Mux → ADC

Identical to generic bipolar CV.

Latency Problem (critical!)

• Total latency: ADC sampling + mux settling + DSP buffer size + DAC output
• Typical: ADS8866 (~1μs) + Mux (~1μs) + DSP (depends on block size, e.g., 1ms at 48kHz/48 samples) + PCM5102A (~0.5ms internal FIR filter) = ~2–3ms total latency
• Effect: >1–2ms latency → feedback behaves not like analog feedback (chaos/saturation), but like a comb filter. At 2ms latency, a fixed resonance appears at 500 Hz (f = 1/0.002s), giving the signal an extremely metallic/hollow, digital character.
• In DSP terms, this corresponds to the z^(-N) delay element. For true chaos-based analog feedback, block processing (buffer size > 1) would need to be abandoned in favor of sample-by-sample processing — very CPU-inefficient.

Strategies

1. Feedback in software: Implement feedback path entirely internally (without DAC→ADC roundtrip) — eliminates hardware latency completely.
2. Minimize latency: Small DSP buffers (e.g., 1 sample), DMA, no mux waiting. Practically hard to achieve below 0.5ms.
3. Latency as feature: Deliberately use the comb filter effect (Karplus-Strong, delay-based synthesis).