Session 14: Sample & Hold and Slew Limiter

Objective: Use the Sample & Hold to generate stepped random voltages and the Slew Limiter to smooth them, creating generative modulation for filter cutoff and pitch.

What Are Sample & Hold and Slew Limiting?

Sample & Hold (S&H) takes a "snapshot" of whatever voltage is at its input each time it receives a trigger pulse. It holds that voltage at its output until the next trigger arrives. If the input is noise (a rapidly changing random signal), each snapshot captures a different random voltage -- creating a staircase of random steps. This is the classic "computer beeping" sound when routed to pitch, or random filter jumps when routed to a filter.

Slew limiting smooths voltage transitions by limiting how fast the output can change. Feed it a staircase of abrupt voltage steps and it turns them into gentle slopes and curves. Applied to the S&H output, it transforms harsh random jumps into fluid, wandering modulation -- like the difference between a pinball bouncing between bumpers (S&H) and a leaf floating on a breeze (S&H through slew).

The combination of S&H and Slew is one of the most powerful generative modulation tools in modular synthesis: randomness that you can dial from abrupt and chaotic to smooth and organic.

Warm-Up (2 min)

Remove all cables. Set all knobs and sliders to noon/center. Play a MIDI note -- you should hear the normalled default tone. Turn LFO RATE to ~30% and raise PW MOD to ~60% -- recall the PWM effect from Session 13. Return PW MOD and RATE to noon.

Setup

From the normalled default:

• Mixer SAW at ~60%, all other Mixer sliders at 0%
• VCO A OCTAVE at 4
• VCF FREQ at ~45%, Q at ~25% (slight resonance to highlight cutoff changes)
• VCF FM 3 at 0%
• Slew RATE at ~30%
• Slew DIRECTION at Both (center)
• Slew SHAPE at LIN

Exercises

Exercise 1: S&H Driving the Filter (8 min)

This exercise requires one cable.

1. Patch Cable 1: S&H OUT -> VCF FM 3 IN. Set VCF FM 3 to ~30%
2. If you have a MIDI clock running, play notes -- on each clock tick, the filter cutoff should jump to a new random brightness. Each note has a different filter character. If no MIDI clock is available, tap the PUSH GATE button rhythmically while holding a note -- each press triggers a new sample
3. Raise FM 3 to ~50% -- the random jumps become more extreme, swinging between dark and bright. Each clock tick is a surprise
4. Lower FM 3 to ~15% -- the randomness becomes subtle, adding slight unpredictable variation to the filter. This is useful for adding "life" to repetitive sequences
5. Try different clock rates if available. Faster clock = more frequent random changes. Slower clock = the filter holds each random value longer

Exercise 2: Smoothing with the Slew Limiter (8 min)

1. Keep Cable 1 patched. The signal chain is now: noise -> S&H -> Slew (normalled) -> and S&H OUT also goes to VCF FM 3 (via cable). You are hearing the un-slewed S&H output on the filter
2. Now add a second cable:

Remove Cable 1 from VCF FM 3 IN. Patch Cable 2: SLEW OUT -> VCF FM 3 IN. Now the filter receives the slew-smoothed version of the S&H output

3. Set Slew RATE to ~20% and play notes. Instead of abrupt jumps, the filter cutoff now glides smoothly between random values. The "staircase" has become "rolling hills." You should hear a wandering, organic filter movement
4. Raise Slew RATE to ~60% -- the smoothing is more extreme. Each random value takes longer to reach, creating slow, gentle drifts. Very ambient and organic
5. Lower Slew RATE to ~5% -- nearly instant. The output is close to the raw S&H steps, with just a tiny rounding on the edges
6. Try Slew DIRECTION switch positions:
   • Up Only (up position): Rising voltages are slewed but falling voltages snap instantly. You should hear slow filter openings but instant drops -- a "breathing in" quality
   • Down Only (down position): The opposite -- instant rises, slow falls. Filter snaps bright then slowly darkens
   • Both (center): Both directions slewed equally. Smooth in all directions

7. Try SHAPE switch: EXP produces curves that start fast and decelerate (exponential approach). LIN produces constant-rate slopes. EXP sounds more natural for most musical applications

Exercise 3: Smoothed Random to Pitch (5 min)

1. Keep Cable 2 patched (SLEW OUT -> VCF FM 3). Set FM 3 to ~20% for subtle filter variation
2. Add one more cable:

3. Patch Cable 3: SLEW OUT -> VCO A FM 1 IN. Set VCO A FM 1 to ~10% (very subtle)
4. Set Slew RATE to ~50%, SHAPE to EXP. Play and hold notes -- the pitch should wander gently, drifting sharp and flat in a slow, organic way. Combined with the random filter movement, the sound has a living, breathing quality
5. Reduce FM 1 to ~5% for barely perceptible pitch drift -- just enough to make a sustained note feel alive without sounding out of tune. This is a classic ambient synthesis technique
6. Remove all cables

Exploration (optional, hyperfocus days)

• Patch an LFO instead of noise into S&H IN (LFO X OUT -> S&H IN). Now instead of random voltages, the S&H samples the LFO at clock rate, creating quantized stepped versions of the LFO wave
• Try the Envelope Follower: flip the ENV FOLLOW switch to ON and patch an audio signal into SLEW/FOLLOW IN. The output tracks the amplitude of the input signal -- useful for making one sound's dynamics control another parameter
• Route S&H OUT to VCO A pitch (FM 1) with no slew for classic "random computer beeps" -- the signature sound of early electronic music

Output Checklist

• Heard S&H generating stepped random voltages on the filter cutoff
• Heard the Slew Limiter smooth S&H steps into organic wandering modulation
• Tried different Slew directions (rise only, fall only, both) and heard the difference
• Applied smoothed random modulation to pitch for gentle drift
• Session logged in Obsidian daily note

Key Takeaways

• Sample & Hold captures random voltage snapshots at a clock rate -- stepped, unpredictable modulation that works with zero patching on Cascadia thanks to normalled noise and MIDI clock
• The Slew Limiter smooths abrupt voltage changes into gradual transitions, turning harsh random steps into organic wandering
• Direction and shape controls on the Slew Limiter give precise control over how voltages transition -- enabling effects from "breathing" one-directional smoothing to full bidirectional drift

Next Session Preview

Session 15 explores the Mixuverter -- Cascadia's voltage processing utility that can attenuate, invert, double, and offset control voltages. You will learn to scale modulation depth, create bipolar modulation from unipolar envelopes, and combine multiple CV sources.