Session 08: FM Synthesis Basics

Objective: Use frequency modulation between the digital oscillators to create metallic, bell-like, and complex timbres that are impossible with analog oscillators alone.

Warm-Up (2 min)

Load the basic patch. Set Osc 1+2 Level to 0. Set Osc 3 to your favorite waveshape from Session 07, Level 50. Play a few notes across the range and listen to the digital character, including any aliasing at high notes. Today you will transform these timbres further with FM.

Setup

From the basic patch:

• Set Osc 1 Level to 0 and Osc 2 Level to 0
• Set Osc 3 Shape to 1 (sine wave), Level to 50, Frequency to C0
• Set Osc 4 Shape to 1 (sine wave), Level to 0 (modulator -- we do not want to hear it directly), Frequency to C0

Exercises

Exercise 1: Basic FM -- Sine on Sine (5 min)

1. Play a few notes to hear the pure sine wave from Osc 3 -- you should hear a simple, clean tone
2. Set FM 4->3 to 5 -- you should hear the sine start to develop harmonics, becoming slightly brighter
3. Increase FM 4->3 to 12 -- you should hear a noticeably metallic, bell-like quality. "That is not a sine wave anymore, is it?" (Anu Kirk p.27)
4. Increase to 25 -- you should hear more complex, harsh harmonics
5. Increase to 50 -- you should hear a very complex, almost noisy timbre
6. Return to 12 -- this is the sweet spot for many FM sounds

"Very small changes in value can produce very big effects on the timbre" -- Anu Kirk p.27

See Anu Kirk p.26-27 ("Exercise 1: A Simple FM Sound")

Exercise 2: Different Waveshapes as Modulator (5 min)

The Evolver allows any waveshape as carrier or modulator, unlike the DX-7 which only used sine waves. This makes Evolver FM more aggressive and versatile.

1. Keep FM 4->3 at 12, Osc 3 Shape at 1 (sine carrier)
2. Set Osc 4 Shape to 2 (sawtooth modulator) -- you should hear a much more complex result than sine-on-sine (Anu Kirk p.27)
3. Try Osc 4 Shape to 9 -- you should hear a different harmonic character
4. Try Osc 4 Shape to 36 -- you should hear yet another timbre
5. Try Osc 4 Shape to 70 -- you should hear increasingly wild results
6. Now change the carrier too: set Osc 3 Shape to 2 (sawtooth) with Osc 4 at 1 (sine modulating a saw) -- you should hear a fuzzy, harmonically rich tone
7. Try Osc 3 Shape to 15 -- you should hear even more complexity

"Unlike the DX-7 which only use sine waves as carriers and modulators, Evolver lets you use any of the digital waveshapes" -- Anu Kirk p.27

See Anu Kirk p.27-28 ("Exercise 2: New Waves")

Exercise 3: FM Ratios -- The Key to Usable Sounds (8 min)

The frequency ratio between carrier and modulator determines the harmonic character of the FM sound:

1. Reset: Osc 3 Shape = 1 (sine), Osc 4 Shape = 1 (sine), FM 4->3 = 12
2. Verify Osc 3 Frequency = C0, Osc 4 Frequency = C0 (1:1 ratio) -- play a note
3. Set Osc 4 Frequency to C1 (one octave higher, 2:1 ratio) -- you should hear a distinctly different timbre
4. Set Osc 4 Frequency to G1 (19 semitones up), Osc 4 Fine to +2 (3:1 ratio) -- you should hear another character (Anu Kirk p.29)
5. Set Osc 4 Frequency to C2 (two octaves up, 4:1 ratio) -- you should hear yet another harmonic flavor
6. Try E2 with Fine -14 (5:1 ratio) -- this should sound more bell-like
7. Try C3 (8:1 ratio) -- at high ratios you should hear the modulator as harmonics rather than distinct pitch

Non-integer ratios produce inharmonic, bell-like, or metallic timbres: 8. Set Osc 4 Frequency to C#1 (not a simple ratio) -- you should hear clangorous, bell-like tones 9. Try Osc 4 Frequency to F#1 -- you should hear noisy, metallic results

"If your ratios aren't whole numbers, you're going to get non-harmonic partials -- stuff that sounds like bells, or noise" -- Anu Kirk p.29

See Anu Kirk p.28-30 ("Exercise 3: A Certain Ratio")

Exercise 4: FM Bell Patch (5 min)

Combine what you have learned to create a classic FM bell:

1. Osc 3 Shape = 1 (sine carrier), Frequency = C0, Level = 50
2. Osc 4 Shape = 1 (sine modulator), Frequency = E2, Fine = -14, Level = 0
3. FM 4->3 = 18
4. VCA Envelope (ENV 2): Attack = 0, Decay = 70, Sustain = 0, Release = 45
5. Filter Envelope (ENV 1): Attack = 0, Decay = 55, Sustain = 20, Release = 30, Env Amount = 60, LPF Frequency = 80

Play a note. You should hear a bell-like tone that rings and decays naturally. Play in the mid to upper range for the most bell-like quality. Lower notes will sound more like a marimba.

Modulating the FM amount makes it even more expressive: 6. Set ENV 3 Destination to FM 4->3, Amount = 29, Decay = 46

Now each note starts with more FM harmonics that quickly decay -- the bell attack is brighter, then mellows. "ENV 3 is modulating the amount of FM -- starting it a little bit higher and dropping it fairly quickly" (Anu Kirk p.30).

Save this patch as your FM bell.

See Anu Kirk p.30-31 ("Exercise 4: Modulating FM!")

Exploration (optional, hyperfocus days)

• Try bidirectional FM: set both FM 4->3 to 8 AND FM 3->4 to 8. "You can do both at the same time... small changes have big results when doing this" (Anu Kirk p.33)
• Mix in an analog oscillator: bring Osc 1 Level to 30 (Saw) to add analog warmth underneath the FM digital tone
• FM can make sounds go out of tune. Use Fine on Osc 3 to correct the intonation (Anu Kirk p.31)

Output Checklist

• FM bell/metallic patch saved
• Understand how FM amount, waveshape choice, and ratio affect the timbre
• Can create both harmonic (integer ratio) and inharmonic (non-integer ratio) FM sounds
• Session logged in Obsidian daily note

Key Takeaways

• FM synthesis modulates the frequency of a carrier oscillator with a modulator oscillator, creating harmonics that neither oscillator could produce alone
• Small changes in FM amount have large effects on timbre -- start low and increase gradually
• The ratio between carrier and modulator frequencies is the most important factor: integer ratios produce musical harmonics, non-integer ratios produce bells and metallic textures

Next Session Preview

Next time you will explore ring modulation -- another way to combine digital oscillators that produces sum and difference frequencies, creating metallic, clangorous textures perfect for percussive and atmospheric sounds.