Synthesis Architecture & Modulation

Synthesis Architecture & Modulation

A static synthesizer note sounds cold and boring. Acoustic instruments have natural, organic movement—like a singer adding vibrato (pitch modulation) or a violinist shaking their hand. In synthesis, we recreate this organic movement by routing a Low-Frequency Oscillator (LFO) to automate dials automatically.

Core Subtractive Synthesis Signal Flow

To understand modulation, you must map the signal flow of a subtractive synth:

[Oscillator(s)] ----> [Filter] ----> [Amplifier (Gain)] ----> [Output]
      ^                  ^                 ^
      |                  |                 |
  [Pitch Env]       [Filter Env]       [Amp Env]
                         ^
                     [LFO (Mod)]

• Oscillator (VCO): The sound source (Sine, Saw, Square).
• Filter (VCF): Shapes the frequency spectrum (Low-Pass, Band-Pass).
• Amplifier (VCA): Controls overall volume.
• Envelopes (ADSR): Provide one-shot shapes triggered by hitting a key.

Low-Frequency Oscillators (LFO)

An LFO is an oscillator that vibrates very slowly—usually below $20\text{Hz}$ (20 cycles per second). Because it is so slow, you cannot hear it as a musical pitch. Instead, we use its slow vibration to automate other knobs:

• Modulation Target: The knob being modulated (e.g., Filter Cutoff).
• LFO Rate: The speed of the modulation (measured in Hz or synced to the sequencer BPM, like 1/4 notes or 1/8 notes).
• LFO Depth (Amount): How much the target knob sweeps back and forth.
  • LFO routed to Pitch: Creates Vibrato (pitch wobbling).
  • LFO routed to Volume: Creates Tremolo (volume pulsing).
  • LFO routed to Cutoff: Creates Filter Wobble (bassy "wub-wub" movements).

Your Assignment Task

1. Experiment in the Synth Sandbox:
   • Program a repeating note in the sequencer.
   • Find the LFO Mod panel. Select Filter Cutoff as your target.
   • Turn the LFO Depth slider to $60%$.
   • Sweep the LFO Rate dial. Set it very slow (e.g., $0.5\text{Hz}$ - taking 2 seconds per cycle), then speed it up to $8\text{Hz}$ (creating a rapid flutter).
   • Sync the LFO rate to the tempo (e.g., set to $1/4$ note value) and observe how it pulsates in time with your kick drum.
2. Write and Submit your Analysis:
   • Signal Flow Diagram: Draw or type out a text diagram showing how an LFO connects to a filter. Detail the difference between an Envelope (one-shot trigger) and an LFO (continuous repeating cycle).
   • Acoustic Counterparts: Define Vibrato and Tremolo using traditional music theory. Explain how a woodwind player or vocalist physically produces vibrato.
   • Modulation Settings Log: Describe your favorite sandbox setup. What target did you modulate, what rate did you use, and how did it affect the emotional feel of the loop?