Session 10: VCF Modes and Resonance

Objective: Explore Cascadia's 8 VCF filter modes and understand how resonance shapes the harmonic content of a signal, from subtle emphasis to self-oscillation.

What Is a Multimode Filter?

A filter removes frequencies from a sound. The simplest filter is a low-pass -- it lets low frequencies through and cuts the highs, making a bright sound duller. But that is only one way to carve a spectrum. A multimode filter offers several filter types in one module, each cutting a different slice of the frequency spectrum.

The main filter types are: low-pass (cuts highs, warm and dark), high-pass (cuts lows, thin and bright), band-pass (cuts both extremes, keeping a narrow band -- nasal and vocal), notch (cuts a narrow band in the middle, keeping everything else -- hollow), and phaser (shifts phase relationships to create sweeping cancellations). Filters also have slope -- measured in dB per octave -- which determines how aggressively they cut. A 6dB slope (1-pole) is gentle; a 24dB slope (4-pole) is steep and dramatic.

Resonance (also called Q) boosts the frequencies right at the cutoff point. Low resonance adds subtle emphasis; high resonance creates a ringing peak; maximum resonance causes self-oscillation where the filter generates its own sine wave with no audio input at all.

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. Now set Envelope A to a percussive shape (Attack ~0%, Decay ~20%, Sustain ~0%) and flip VCA CONTROL to UP -- recall the LPG bongo from Session 9. Flip VCA CONTROL back to DOWN and reset Envelope A to noon/center for this session.

Setup

From the normalled default:

• Mixer SAW at ~75%, all other Mixer sliders at 0%
• VCO A OCTAVE at 4
• VCF FREQ at ~50% (center)
• VCF Q at 0%
• VCF LEVEL at ~50% (unity, no overdrive)
• VCF MODE selector at LP4 (4-pole low-pass)
• Envelope B in ENV mode, all sliders at noon

Exercises

Exercise 1: Tour the 8 Filter Modes (8 min)

1. Play and hold a note in the middle of your keyboard. With VCF FREQ at ~50%, you should hear a moderately bright sawtooth -- the LP4 filter is removing some high harmonics
2. Slowly rotate the MODE selector one click at a time. At each position, play and hold a note. Listen for how the timbre changes:
   • LP1 (6dB/oct): Very gentle rolloff. Barely different from unfiltered -- you should hear most of the brightness still present
   • LP2 (12dB/oct): Moderate rolloff. Noticeably darker than LP1 but still open
   • LP4 (24dB/oct): Steep rolloff. The classic "fat" low-pass sound -- warm, round, rich
   • BP2 (12dB/oct): Both lows and highs are cut. You should hear a thinner, nasal, vocal-like quality
   • BP4 (24dB/oct): Narrower bandpass -- even more nasal and resonant
   • HP4 (24dB/oct): Lows are removed. Thin, bright, almost tinny
   • NT2 (notch): A narrow scoop in the middle. Sounds slightly hollow, like a phaser frozen at one position
   • PHZR (phaser): Phase cancellations create a sweepy, jet-like quality

3. Return MODE to LP4. Now sweep VCF FREQ slowly from ~0% to ~100% -- you should hear the timbre go from very dark and muffled (low cutoff) to fully bright (high cutoff, filter wide open). This is the classic filter sweep

Exercise 2: Resonance -- From Subtle to Self-Oscillation (8 min)

1. Set MODE to LP4, VCF FREQ at ~40%. Play and hold a note. Slowly raise VCF Q from 0% toward ~30% -- you should hear a subtle nasal emphasis appear around the cutoff frequency. The filter is boosting a narrow band
2. Continue raising Q to ~60% -- the resonant peak becomes a pronounced ringing. The sound becomes thinner overall as the resonance dominates. You should hear an almost vocal "wah" quality
3. Raise Q to ~85% -- the ringing is now very strong. Short notes may produce an audible "ping" as the filter rings after the note ends
4. Push Q to ~100% -- the filter begins to self-oscillate. You should hear a pure sine tone even between notes. This sine comes from the filter itself, not from VCO A. Sweep FREQ slowly and the pitch of the sine changes -- the filter is now acting as a sine oscillator

5. With Q still at ~100%, set FREQ to ~50%. Now play MIDI notes -- you should hear both VCO A's sawtooth and the filter's sine tone. The sine does not track your keyboard by default. To make it track, raise VCF FM 2 to ~50% (this uses the normalled MIDI PITCH signal)
6. Set Q back to ~25% for a musically useful amount of resonance. This is a good starting point for most patches

Exercise 3: LEVEL Pre-Gain Distortion (5 min)

1. Set MODE to LP4, FREQ at ~50%, Q at ~25%. Play a note -- clean filtered sawtooth
2. Raise VCF LEVEL from ~50% toward ~75%. You should hear the signal get louder and slightly grittier -- you are pushing the signal harder into the filter input
3. Push LEVEL to ~100%. The sound should be noticeably distorted -- crunchy and aggressive. The LEVEL LED lights red indicating clipping. This is pre-filter distortion: the signal clips before the filter shapes it, which produces a different character than distortion after the filter

4. Return LEVEL to ~50% for clean operation

Exploration (optional, hyperfocus days)

• With Q at ~100% (self-oscillating), try each of the 8 modes. Notice how NT2 and PHZR self-oscillation sounds different from the low-pass modes
• Patch VCO A SAW OUT -> VCF IN (overriding MIXER normalling) to isolate just the sawtooth. Compare the sound of each mode with only a raw saw as input
• Try moderate Q (~50%) with LEVEL at ~80% -- the combination of resonance and drive creates aggressive, characterful tones

Output Checklist

• Heard all 8 VCF filter modes and can identify the general character of each
• Heard resonance (Q) from subtle to self-oscillation
• Heard the filter self-oscillate and produce a sine tone
• Heard pre-filter distortion using the LEVEL knob
• Session logged in Obsidian daily note

Key Takeaways

• A multimode filter offers multiple ways to carve a spectrum -- Cascadia's VCF provides 8 modes with slopes from 6dB to 24dB per octave
• Resonance (Q) boosts frequencies at the cutoff; at maximum it causes self-oscillation, turning the filter into a sine oscillator
• The VCF LEVEL knob adds pre-filter distortion for gritty character, a separate tool from resonance or post-filter effects

Next Session Preview

Session 11 explores filter frequency modulation -- using envelopes, LFOs, and audio-rate signals to move the filter cutoff dynamically, creating everything from subtle envelope sweeps to aggressive FM metallic tones.