Modulation and Movement — LFO, Unison, and Glide

The LFO: Modulation You Feel, Not Hear

An LFO is exactly what the name says: an oscillator that runs at a frequency too low to be heard as a pitch. Instead of producing an audible tone, it produces a control signal — a repeating wave that moves other parameters up and down over time.

The oscillators from Chapter 3 run at audible frequencies: 100 Hz, 440 Hz, 1000 Hz. An LFO typically runs between 0.1 Hz and 20 Hz — too slow to hear as a note, but fast enough to create movement. At 1 Hz, the LFO completes one full cycle per second. At 5 Hz, five cycles per second. You don’t hear the LFO itself. You hear what it does to the parameter it’s controlling.

LFO Destinations

The magic of the LFO is that you can point it at almost anything. The three most common destinations:

Pitch → Vibrato

Route the LFO to the oscillator’s pitch with a small amount and a moderate rate (around 5-7 Hz). The pitch wobbles up and down slightly — that’s vibrato. Every singer does this naturally. Every violinist does it with their finger. The LFO automates it.

Filter Cutoff → Wah

Route the LFO to the filter cutoff. Now the filter opens and closes rhythmically — the sound gets brighter, then darker, then brighter again. This is the classic synth “wobble” that drives entire genres of electronic music. Sync the LFO rate to your project tempo and the filter movement locks to the beat.

Amplitude → Tremolo

Route the LFO to the amplifier. The volume goes up and down rhythmically — that’s tremolo. Think of a guitar amp tremolo effect, or the pulsing sound of a Rhodes piano through a rotary speaker.

The LFO shape matters too. A sine wave LFO produces smooth, even modulation. A square wave LFO snaps between two values — full on, full off, like a rhythmic gate. A sample-and-hold LFO jumps to random values at each cycle — great for glitchy, unpredictable textures.

Phase and Detuning

Here’s something that should connect a few ideas. Remember phase from Chapter 1 — two waves drifting in and out of alignment?

Take two oscillators, both tuned to 1000 Hz. They’re in perfect sync — same frequency, same phase. Now detune the second oscillator to 1001 Hz. The difference is 1 Hz, which means once per second, the two waves drift out of phase and then back in. The result? The combined signal gets louder and quieter once per second — a slow pulsing, just like tremolo.

Now here’s the question: if you heard that pulsing, could you tell whether it was an LFO doing tremolo or two detuned oscillators drifting in and out of phase? The answer is no — they sound identical. And that’s the point. The LFO is modeling the same phenomenon that occurs naturally when two similar frequencies interact. The tremolo effect and the beating of detuned oscillators are the same thing, just arrived at differently.

Understanding this connection between phase, detuning, and modulation will help you everywhere — in synthesis, in mixing (where phase issues between microphones create similar effects), and in understanding effects like chorus and flanging (which are literally built on short delays that drift in and out of phase with the original signal).

Unison: Making One Sound Like Many

If two slightly detuned oscillators create a pleasing thickness, what about four? Or eight? Unison mode duplicates an oscillator’s voice multiple times and detunes the copies slightly from each other. The result is a massive, chorus-like wall of sound — what you hear in big synth leads and thick pads.

The amount of detuning controls the width of the effect. A small detune amount gives you a subtle thickening — sounds “warm” and “full.” A large detune amount gives you a dramatic, almost out-of-tune shimmer. The number of voices controls how dense it sounds.

Unison is also one of the most CPU-intensive features on a synth — each additional voice is another oscillator running in parallel. If your computer starts struggling, this is the first place to cut back.

Glide and Portamento

Most synthesizers jump instantly from one note to the next. Glide (also called portamento) changes that behavior: instead of jumping, the pitch slides between notes. Set a glide time of 100 milliseconds and you’ll hear the pitch swoop from the old note to the new one. Set it to a full second and you get a dramatic, singing slide.

Glide works best in monophonic mode (one note at a time) and with legato playing (overlapping notes). In legato mode, the envelopes don’t retrigger when you play a new note while holding the previous one — the pitch slides but the sound continues smoothly. This creates the classic monophonic lead-synth sound: one continuous, singing tone that glides between pitches.

Pulse Width Modulation (PWM)

One more modulation trick, specific to the square/pulse waveform. Remember from Chapter 3 that a square wave has equal time at the top and bottom. If you change that ratio — spending more time at the top than the bottom, or vice versa — you change the pulse width, which changes the harmonic content of the wave.

Now apply an LFO to the pulse width. The harmonic recipe constantly shifts as the ratio changes — the sound thickens, thins, and shimmers. This is pulse width modulation, and it creates a distinctive, restless quality that’s especially good for pads and leads. It’s a way to get movement and animation from a single oscillator without adding anything else.

What to Practice

• Load a sawtooth oscillator and add vibrato: route the LFO to pitch with a small amount and moderate rate. Then switch the LFO destination to the filter. Then to amplitude. Same LFO, three completely different effects. This is modulation.
• Try the detuning experiment: set up two oscillators at the same pitch, then detune one by a tiny amount. Listen to the beating. Now add a tremolo LFO to a single oscillator and try to match the speed. You’re hearing the same phenomenon.
• If your synth has a unison mode, turn it on and increase the voice count. Listen to how the sound gets wider and thicker. Then try increasing the detune amount — at some point it stops sounding “thick” and starts sounding “broken.” That threshold is useful to know.
• Play with glide: set up a monophonic patch with a medium glide time and play a slow melody. Notice how the sliding pitch adds expression — it’s the synth equivalent of a singer bending between notes.