Session 02: Pulse Width Modulation & Sub-Oscillator

Objective: Understand pulse width modulation and sub-oscillators as two techniques for creating thicker, more animated tones, using Cascadia's normalled LFO and built-in sub-oscillator.

What Is Pulse Width Modulation?

A pulse wave is a waveform that switches between two voltage levels -- high and low. The pulse width (also called duty cycle) is the percentage of time the wave spends in the high state. At 50%, it is a perfect square wave with a hollow, woody character. At 25% or 75%, it becomes a narrow pulse with a thin, nasal quality. Changing the pulse width changes the harmonic content: a square wave has only odd harmonics, while asymmetric pulse widths introduce even harmonics.

Pulse width modulation (PWM) means continuously changing the pulse width over time, usually with an LFO. The shifting harmonic content creates a rich, animated, chorus-like effect -- one oscillator sounds like two because the timbre is constantly evolving. PWM is one of the most efficient ways to create movement in a sound without adding complexity to the patch.

What Is a Sub-Oscillator?

A sub-oscillator is a secondary tone generator derived from the main oscillator, running at a lower octave (typically one or two octaves down). Because it is mathematically derived from the main oscillator's signal, it tracks pitch perfectly -- no tuning drift, no beating. Sub-oscillators add weight and fullness to bass sounds and anchor higher-register patches with low-end foundation. They are standard on most analog synthesizers and are especially effective when blended at moderate levels rather than dominating the mix.

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 sawtooth tone from Session 1. Now slowly move the VCF FREQ slider from ~75% down to ~25% and back up -- recall how the filter shapes brightness. Return VCF FREQ to ~75%.

Setup

From the normalled default:

• Ensure VCO A OCTAVE is set to a comfortable range (4 or 5)
• Ensure Mixer SAW slider is at ~50%
• Ensure Mixer PULSE slider is at 0% (we will raise it during exercises)
• All other Mixer sliders at 0%

Exercises

Exercise 1: Explore Pulse Width (8 min)

1. Raise the Mixer PULSE slider to ~75% and lower the Mixer SAW slider to 0% -- you should hear a hollow, square-wave tone (the default pulse width is ~50%)
2. Slowly move the VCO A PW slider from ~50% down to ~25% -- you should hear the tone become thinner and more nasal as the pulse narrows
3. Move PW to ~75% -- the tone should sound similar to ~25% (symmetric around 50%) but with a subtly different phase character
4. Return PW to ~50% -- the full, hollow square wave is back

Exercise 2: Add Pulse Width Modulation (8 min)

1. With Mixer PULSE at ~75% and SAW at 0%, slowly raise the VCO A PW MOD slider from 0% to ~50% -- you should hear the tone start to shimmer and move as LFO Y modulates the pulse width
2. Raise PW MOD to ~75% -- the movement becomes more dramatic, almost chorus-like
3. Raise PW MOD to ~100% -- at full depth, the sound thins out dramatically at the extremes of each LFO cycle
4. Set PW MOD to ~50% for a balanced effect. Now adjust the LFO RATE knob -- slower rates create a gentle sweep, faster rates create a vibrato-like flutter

5. Set LFO RATE to a slow setting (~25%) for a smooth, evolving PWM effect

Exercise 3: Add the Sub-Oscillator (7 min)

1. Return PW MOD to 0%. Set Mixer PULSE to ~50% and Mixer SAW to ~50% -- you should hear both waveforms blended
2. Raise the Mixer SUB slider to ~50% -- you should hear a strong low-end tone one octave below the main pitch appear underneath

3. Toggle the SUB TYPE switch to the top position (SUB -1) -- one octave down, a solid bass foundation
4. Toggle to the middle position (OR) -- two octaves down with a distinctive hollow character
5. Toggle to the bottom position (SUB -2) -- two octaves down, a deeper square wave
6. Set SUB TYPE to SUB -1 and SUB level to ~40% for a balanced low-end addition. Play notes across the keyboard -- the sub tracks perfectly

Exercise 4: Combine PWM and Sub (2 min)

1. With SUB at ~40% (SUB -1), raise PW MOD back to ~50%
2. Play a sustained chord or single note -- you should hear a thick, animated sound: the PWM adds movement to the upper harmonics while the sub provides a stable low-end anchor
3. This combination -- PWM for animation plus sub for weight -- is a classic technique for rich, full sounds on any analog synthesizer

Exploration (optional, hyperfocus days)

• Try raising both SAW and PULSE to ~50% with PWM active -- hear how the sawtooth stays steady while the pulse shimmers around it
• Blend the Mixer IN 2 slider to ~25% to mix in VCO A's sine wave (normalled to IN 2) for a softer fundamental underneath the harmonics
• Set SUB TYPE to OR and SUB to ~75% for an aggressive, thick bass tone -- play in the C1-C2 range

Output Checklist

• Can hear the difference between narrow and wide pulse widths
• Can activate PWM using the PW MOD slider (no cable needed)
• Can hear the three sub-oscillator types and describe their character
• Session logged in Obsidian daily note

Key Takeaways

• Pulse width modulation creates animated, chorus-like movement from a single oscillator by continuously changing the harmonic content
• The sub-oscillator adds low-end weight that tracks pitch perfectly -- blend it at moderate levels for best results
• Both PWM and the sub-oscillator are available on Cascadia with zero cables, thanks to normalled connections

Next Session Preview

Next time you will explore the filter envelope, wave folding, and get a taste of FM synthesis and external effects -- completing your tour of Cascadia's normalled signal path. You will save your first patch.