CV Input Signal Types #16 — Random and S&H CV

Description

Random CV and sample & hold outputs are signals with random or pseudo-random voltage values that jump to a new value on each clock pulse. The voltage remains constant between jumps (stepped).

Use Cases

• Random melodies (random CV → quantizer → oscillator)
• Random modulation (filter cutoff, parameter variation)
• Generative music (probabilistic sequences)
• Quantizer input (random CV as raw material for scale quantization)

Eurorack Examples (Mutable Instruments)

• Marbles: Generates quantized and unquantized random CV

Hardware Implementation

Standard Input Stage: OpAmp → Mux → ADC

• Bipolar: Standard stage (R_in=100k, R_ref=200k, R_fb=33k)
• Unipolar: Clipping at >5V applies

Firmware Requirements (critical!)

• No linear low-pass filtering (IIR/averaging), as this smears the steps and destroys the rhythmic character. However, a short median filter (e.g., 3 samples) is allowed and recommended. This preserves the steep step edges exactly while eliminating 1-sample spikes (e.g., from mux switching or EMI).
• High sample rate: Mux sample rate must be high enough to not miss the jump moment (stepped signals can be clocked at audio rate)
• Synchronized sampling: If the module has a separate trigger input, sampling should occur synchronized shortly after the trigger (e.g., 1–2ms delay), to give the external S&H module time to settle.
• When used as quantizer input: pass ADC value directly (unsmoothed) to quantization algorithm