Session 11: Filter FM and Filter as Sound Source

Objective: Use envelopes, LFOs, and audio-rate signals to modulate the VCF cutoff frequency, and explore playing the self-oscillating filter as a keyboard-tracked sine oscillator.

What Is Filter Frequency Modulation?

Filter FM means using a control voltage to move the filter's cutoff frequency over time. Without modulation, the filter sits at a fixed point -- static brightness. With modulation, the filter sweeps, creating movement and life in the sound.

The most common filter FM source is an envelope: the filter opens bright on attack and closes as the note decays, mimicking how acoustic instruments work (a plucked string is brightest at the moment of pluck). An LFO creates rhythmic, repeating filter sweeps -- the classic "wah-wah" effect. An audio-rate oscillator modulating the filter creates metallic, sideband-rich timbres similar to FM synthesis but applied to the filter instead of an oscillator.

The speed and shape of the modulation source determines the character: slow envelopes for expressive sweeps, medium LFOs for rhythmic movement, fast audio-rate signals for timbral complexity.

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. Rotate the VCF MODE selector through 3-4 positions, recalling the different filter characters from Session 10. Return MODE to LP4.

Setup

From the normalled default:

• Mixer SAW at ~75%, all other Mixer sliders at 0%
• VCO A OCTAVE at 4
• VCF FREQ at ~25% (low cutoff -- dark starting point so envelope sweeps are audible)
• VCF Q at ~20% (slight resonance to emphasize the cutoff movement)
• VCF MODE at LP4
• VCF FM 1 at 0%, FM 2 at 0%, FM 3 at 0%
• Envelope B in ENV mode: Attack ~0%, Decay ~40%, Sustain ~20%, Release ~25%

Exercises

Exercise 1: Envelope-to-Filter Sweep (7 min)

1. Play a note -- you should hear a dark, muffled sawtooth because FREQ is low and FM 1 is at 0% (no envelope reaching the filter)
2. Slowly raise VCF FM 1 from 0% to ~30%. Play notes -- you should hear the filter open slightly on each note attack, then close during the decay. The sound has a subtle "bwow" quality
3. Raise FM 1 to ~60%. Play notes -- the envelope sweep is now dramatic. Each note starts bright and closes to dark, like a plucked string. This is the bread-and-butter of subtractive synthesis
4. Raise FM 1 to ~100%. Play notes -- the sweep is extreme, reaching into very bright territory on attack. Try different Envelope B shapes: set Decay to ~15% for a short "pluck", then Decay to ~75% for a slow, gradual close
5. Try changing VCF Q to ~40% while FM 1 is at ~60%. The resonant peak rides the envelope sweep, creating a more vocal, "wah" quality. Return Q to ~20%

Exercise 2: LFO-to-Filter Wobble (7 min)

This exercise requires one cable.

1. Keep FM 1 at ~30% (subtle envelope). Patch Cable 1: LFO X OUT -> VCF FM 3 IN
2. Set VCF FM 3 to ~25%. Set LFO RATE to ~30% (slow). You should hear the filter cutoff gently sweeping up and down in a repeating cycle -- the classic filter wobble
3. Raise FM 3 to ~50% -- the wobble becomes deeper and more dramatic. Raise LFO RATE to ~50% -- the wobble speeds up into a rhythmic pulsing
4. Set LFO Y RATE DIVIDER to div3 and LFO Z RATE DIVIDER to div5. While LFO X modulates the filter, the other two LFOs are now running at related but different rates -- this creates polyrhythmic modulation if you route them to additional destinations later

5. Push LFO RATE to ~80% -- the LFO approaches audio rate and the wobble becomes a buzzy, grainy texture rather than a clean sweep. This is the threshold between modulation and audio-rate FM. Pull RATE back to ~40%
6. Remove Cable 1

Exercise 3: Playing the Self-Oscillating Filter (7 min)

1. Remove all cables. Set VCF FREQ at ~50%, raise VCF Q to ~100%. You should hear the filter self-oscillating -- a pure sine tone
2. Play MIDI notes -- the sine tone does not change pitch because the filter is not tracking your keyboard yet

3. Slowly raise VCF FM 2 from 0% toward ~50%. Play notes across the keyboard -- the self-oscillating filter now tracks your pitch. At ~50%, it should roughly follow your keyboard. You are playing the filter as a sine oscillator
4. Fine-tune FM 2 until playing an octave on the keyboard produces an audible octave from the filter. The exact value depends on your tuning -- typically ~45-55%
5. Set FREQ to tune the filter's base pitch. Lower FREQ = lower starting note. Combined with FM 2 keyboard tracking, you now have a playable sine oscillator with no VCO involved
6. Try reducing Q slightly to ~85% -- the self-oscillation becomes quieter and less pure, blending with the input signal. At moderate Q with keyboard tracking, the resonant filter adds a singing overtone on top of your sawtooth. Return Q to ~25% and FM 2 to 0%

Exploration (optional, hyperfocus days)

• Patch VCO B SINE OUT -> VCF FM 3 IN (1 cable). Set VCO B to audio rate and raise FM 3 gradually -- this is audio-rate filter FM, creating metallic sidebands similar to oscillator FM but applied to the filter's cutoff
• Combine all three FM sources simultaneously: envelope on FM 1 (~40%), keyboard tracking on FM 2 (~50%), LFO on FM 3 (~20%) -- this is how complex, expressive filter behavior is built
• Try self-oscillation in BP4 mode instead of LP4 -- the bandpass self-oscillation has a different, more focused character

Output Checklist

• Heard envelope-to-filter sweep using the normalled Envelope B -> FM 1 path
• Patched LFO X -> VCF FM 3 for rhythmic filter wobble
• Played the self-oscillating filter as a keyboard-tracked sine oscillator via FM 2
• Understand the difference between the 3 VCF FM inputs (envelope, 1V/oct, linear)
• Session logged in Obsidian daily note

Key Takeaways

• Filter FM is the primary way to add movement and expression to subtractive synthesis -- envelopes for per-note sweeps, LFOs for rhythmic wobble, audio-rate signals for metallic complexity
• Cascadia's VCF has 3 dedicated FM inputs with different scaling: FM 1 for envelopes, FM 2 for 1V/oct keyboard tracking, FM 3 for general-purpose linear modulation
• A self-oscillating filter with keyboard tracking becomes a playable sine oscillator, adding a third tone source beyond VCO A and VCO B

Next Session Preview

Session 12 returns to the Low Pass Gate from Session 9, going deeper into percussion techniques. You will explore different source waveforms through the LPG, vary envelope shapes for different timbres, and build a palette of percussion tones.