FM Synthesis

Create bell-like tones, metallic sounds, and complex timbres using frequency modulation.

The FM synthesis patch

Overview

FM (Frequency Modulation) synthesis uses one oscillator (modulator) to modulate the frequency of another (carrier). This creates complex harmonic and inharmonic spectra without traditional filtering, producing distinctive metallic, bell-like, and electric piano tones.

Character: Bright, metallic, bell-like, electric Good for: Bells, electric piano, brass, bass, experimental

Modules Used

• 2x Oscillator (Carrier + Modulator)
• 1x VCA
• 2x ADSR Envelope
• 1x Keyboard Input
• 1x Audio Output

Optional:

• 1x Attenuverter

Patch Diagram

┌──────────┐     ┌───────────────┐      ┌──────────────┐      ┌─────┐
│ Keyboard │─V/Oct─▶ Modulator   │─Audio─▶   Carrier    │─Audio─▶ VCA │─▶ Output
│          │     │  Oscillator  │  FM   │  Oscillator  │      │     │
└────┬─────┘     │    (Sine)    │       │    (Sine)    │      └──▲──┘
     │           └──────▲───────┘       └──────▲───────┘         │
     │                  │                      │                  │
     │ V/Oct            │                      │ V/Oct            │
     ├──────────────────┤                      │                  │
     │                  │                      │                  │
     │ Gate        ┌────┴─────┐                │           ┌──────┴──────┐
     │             │ ADSR 1   │────────────────┘           │   ADSR 2    │
     └────────────▶│(FM Depth)│                            │ (Amplitude) │
               │   └──────────┘                            └─────────────┘
               │                                                  ▲
               └──────────────────────────────────────────────────┘

Key Concept: Carrier and Modulator

Carrier: The oscillator you hear. Its frequency determines the pitch.

Modulator: Modulates the carrier's frequency. Its frequency determines the timbre complexity.

FM Amount: How much the modulator affects the carrier. More = brighter, more complex.

Frequency Ratios

The ratio between modulator and carrier frequencies determines the harmonic content:

Integer ratios = harmonic sounds Non-integer ratios = inharmonic/metallic sounds

Step-by-Step Setup

1. Add Modules

Position:

1. Keyboard Input (left)
2. Modulator Oscillator (center-left)
3. Carrier Oscillator (center)
4. VCA (center-right)
5. Audio Output (right)
6. Two ADSRs (below)

2. Connect Pitch Control

Both oscillators track the keyboard:

[Keyboard V/Oct] ──▶ [Modulator V/Oct]
                 ──▶ [Carrier V/Oct]

3. Create FM Connection

The modulator modulates the carrier:

[Modulator Audio Out] ──▶ [Carrier FM Input]

4. Audio Output Path

[Carrier Audio Out] ──▶ [VCA Input]
[VCA Output] ──▶ [Audio Output Mono]

5. Amplitude Envelope

[Keyboard Gate] ──▶ [ADSR 2 Gate]
[ADSR 2 Env] ──▶ [VCA CV]

6. FM Depth Envelope (Optional but Recommended)

The FM amount can be envelope-controlled for dynamic timbre:

[Keyboard Gate] ──▶ [ADSR 1 Gate]
[ADSR 1 Env] ──▶ [Carrier FM Amount CV]

Or route modulator through a VCA:

[Modulator] ──▶ [VCA 2] ──▶ [Carrier FM]
[ADSR 1] ──▶ [VCA 2 CV]

Settings for FM Bell

Classic bell sound:

Carrier Oscillator:

• Waveform: Sine
• Frequency: Keyboard controlled

Modulator Oscillator:

• Waveform: Sine
• Frequency: Keyboard controlled
• (Set slightly higher for inharmonic: multiply frequency by 1.41)

Carrier FM Amount: 0.3 - 0.5

ADSR 2 (Amplitude):

ADSR 1 (FM Depth):

The FM depth envelope makes the sound start bright and become pure as it decays.

Variations

Electric Piano (DX7-style)

Ratio: 1:1
Modulator: Sine
Carrier: Sine
FM Amount: 0.3
ADSR 2: A:1ms D:800ms S:0.4 R:300ms
ADSR 1: A:1ms D:200ms S:0.2 R:200ms

Tubular Bells

Ratio: 3.5:1 (inharmonic)
FM Amount: 0.6
ADSR 2: A:1ms D:3000ms S:0.0 R:2000ms
ADSR 1: A:1ms D:1000ms S:0.05 R:500ms

FM Bass

Ratio: 1:1
FM Amount: 0.4
Modulator: Sine (or saw for grit)
ADSR 2: A:1ms D:200ms S:0.6 R:100ms
ADSR 1: A:1ms D:100ms S:0.3 R:50ms

Brass-like

Ratio: 1:1
FM Amount: 0.5
ADSR 2: A:50ms D:100ms S:0.8 R:200ms
ADSR 1: A:30ms D:200ms S:0.5 R:200ms

Harsh Digital

Ratio: 7:3 (complex)
Modulator: Saw
Carrier: Sine
FM Amount: 0.8

Advanced Techniques

Velocity-Controlled FM

Harder playing = brighter:

[Keyboard Velocity] ──▶ [Attenuverter] ──▶ [FM Amount CV]

Multiple Modulators

Add complexity:

[Modulator 1] ──▶ [Mixer] ──▶ [Carrier FM]
[Modulator 2] ──▶ [Mixer]

Feedback FM

Route carrier back to modulate itself:

[Carrier Out] ──▶ [Attenuverter (very low)] ──▶ [Carrier FM]

Creates chaotic, noisy tones. Use sparingly!

Filter After FM

Add subtractive element:

[Carrier] ──▶ [Filter] ──▶ [VCA] ──▶ [Output]

Tuning the Ratio

To set specific frequency ratios:

1. Set both oscillators to same base frequency
2. Multiply modulator frequency by desired ratio
3. Or use V/Oct input with offset

For a 2:1 ratio:

• Carrier at keyboard pitch
• Modulator at keyboard pitch +1 octave

For 3:2 ratio:

• Carrier at keyboard pitch
• Modulator at keyboard pitch +7 semitones (perfect fifth)

Troubleshooting

Too harsh: Lower FM amount, shorten FM envelope decay

Too pure: Increase FM amount, longer FM envelope

Out of tune: Ensure both oscillators track keyboard

No change with FM amount: Check FM connection to carrier

What You've Learned

• Basic FM synthesis architecture
• Relationship between frequency ratios and timbre
• Using envelopes to control FM depth dynamically
• Creating classic FM sounds like bells and electric piano