Cutoff: the brightness control

01Why the filter matters so much

Almost every synthesizer sound starts life as something bright and buzzy, and is then sculpted. The oscillators give you a block of raw material full of harmonics. The filter carves away what you don’t want.

This way of working is called subtractive synthesis: start rich, then take away. It is the oldest and still the most widely used method, and it is the heart of KiMiA’s layers. If you understand the filter, you can already reach half of the sounds you know from records.

02What a filter actually does

Remember from Module 1 that every pitched note is a stack of sine waves: the fundamental (the pitch you hear) plus harmonics at 2×, 3×, 4× that frequency and so on. The balance of those harmonics is the sound’s tone color, its timbre.

A saw wave is the richest of the classic waves: it contains every harmonic, each one a little quieter than the one before (the 2nd at half the level, the 3rd at a third, and so on). That is why it sounds so bright and buzzy, and why it is the favorite starting point for filtering.

A low-pass filter lets the low frequencies pass and turns the high ones down. Look at what it does to one saw wave:

Three things to notice in the figure:

• On the right, the lavender lines are the harmonics before filtering, all the way up to 20 kHz. The orange lines are what is left. Below 600 Hz they are almost unchanged. Above it, they shrink further and further, following the curve.
• The curve is the filter itself: KiMiA’s exact filter response, calculated from the same equations the engine runs. Each harmonic’s level is its original height plus the curve at that frequency.
• On the left, the saw’s vertical drop has become a smooth slope. The picture and the sound tell the same story: fewer high harmonics means softer corners and a darker tone.

03Cutoff: where the filter draws the line

The cutoff is the frequency where the filter starts to cut. Harmonics well below the cutoff pass untouched. Near the cutoff they are already a few dB quieter, and above it they are turned down more and more the higher they sit. Move the cutoff and you move the whole curve left or right.

Drag the slider below from bright to dark. The waveform, the harmonics and the curve are all real KiMiA renders at each position. Switch on Listen while you drag to hear each step.The figure below shows the same saw at four cutoffs, from bright to dark. The waveform, the harmonics and the curve are all real KiMiA renders. In the online course this figure is a slider with sound at every position.

What you should notice as you drag:What you should notice as the cutoff falls:

• 18 kHz to about 4 kHz: the change is small but real: the sound loses its “air” and fizz. The harmonics up there are quiet, but your ear notices when they go.
• About 3 kHz to 700 Hz: this is the heart of the brightness range. The sound moves from bright to warm to mellow. Most musical filter settings live here.
• Below 262 Hz, the second harmonic of this C3: only the fundamental is left strong. The saw now looks, and sounds, almost like a sine wave.
• Below the fundamental (131 Hz) the filter starts to cut the note itself, and the sound gets quieter overall.

04The cutoff sweep, the most famous sound in synthesis

Now let the cutoff move while the note plays. To see it, we need a picture that shows harmonics over time: a spectrogram. Time runs from left to right, pitch runs from bottom to top, and the brighter the color, the louder that frequency is at that moment.

Listen for the moment the sound changes most. Near the end, when the cutoff crosses the lowest few harmonics, the note turns from “muffled” to “distant” in a fraction of a second. Then play the second example: the same move on a lush chord, closed → open → closed, is the classic filter-sweep pad heard on countless records.

05Where to set the cutoff

There is no correct cutoff, only the right one for the part you are playing. As a starting map, here is what the cutoff of a 12 dB low-pass does to a bright saw sound in the middle of the keyboard:

One more thing matters a lot: the cutoff is a fixed frequency, but your ear judges brightness relative to the note. At 800 Hz, C2 (65 Hz) keeps about twelve strong harmonics, while C5 (523 Hz) keeps only its fundamental. The same cutoff makes low notes bright and high notes dull. Lesson 5.4 shows how KiMiA fixes that with key tracking.

06Find it in KiMiA

Almost everything in this module lives on one panel: Edit›Filter 1. This lesson used ④ Cutoff. Lesson 5.2 covers ③ Slope, ⑤ Resonance and the first four types on ②; Lesson 5.3 the second filter beside it; Lesson 5.4 ⑥ Key tracking and Touch → brightness, ⑦ Envelope amount and the filter envelope; Lesson 5.5 ① Drive. The other fifteen types and Osc 2 FM have a module of their own, Module 6.

And here is the panel itself, in KiMiA, with a sound playing: the curve at the top moves with the cutoff as you play, and the dot on it is where the sound is now.

Figure 5.1.5

KiMiA · Edit · Filter 1, live

12345678910

1. Filter curve the live curve, and its readout: the cutoff right now (this lesson)
2. Drive pushes the sound into the filter for grit (Module 9)
3. Type nineteen filter types: four in Lesson 5.2, the rest in Module 6
4. Slope 12 dB or 24 dB an octave (Lesson 5.2)
5. Cutoff where the filter draws the line, 30 Hz to 18 kHz (this lesson)
6. Resonance the peak at the cutoff, 0% to 90% (Lesson 5.2)
7. Key tracking moves the cutoff with the note you play (Lesson 5.4)
8. Touch → brightness harder playing opens the filter (Lesson 5.4)
9. Envelope amount how far the filter envelope moves the cutoff, up or down (Lesson 5.4)
10. Osc 2 FM Oscillator 2 shakes the cutoff at audio rate: growl and clang (Module 6)

07Try it

1. Load the starting patch. Import the course patches for this module (see the module page), then choose 05 · Course Init in Your sounds. It is one raw saw with the filter fully open.
2. Hold C3 and sweep. In Edit›Filter 1, drag Cutoff slowly from 18 kHz down to about 150 Hz and back. Say out loud where it turns from bright to warm, and from warm to dark. Look at the value each time.
3. Set three cutoffs by ear. Close your eyes, move the slider, and stop when it sounds “bright”, then “warm”, then “dark”. Now look at the numbers. Compare them with the table above.
4. Compare registers. Set Cutoff to 800 Hz. Play C2, then C4, then C6. Notice how the high note sounds dull and weak and the low note bright.

08Check yourself