FM vs Subtractive vs Wavetable vs Additive Synthesis

Subtractive synthesis starts with a bright, buzzy wave and uses a filter to take harmonics away. FM synthesis starts with plain sine waves and creates new harmonics by letting one oscillator wobble another’s pitch, which is how it makes metallic bell tones a filter cannot. Wavetable synthesis sweeps through a row of stored waveforms, and additive synthesis builds a tone one sine wave at a time. If you are new to synths, learn subtractive first: its knobs match what you hear, and its filter and envelopes turn up on almost every other type.

What is subtractive synthesis?

Subtractive synthesis is the classic analog signal chain. An oscillator makes a wave, a filter shapes its tone, and an amplifier shapes its volume. The wave is picked because it is crowded with harmonics: a sawtooth has every one, and a square has only the odd ones.

Play A2, which vibrates at 110 Hz. A sawtooth on that note contains 110 Hz plus harmonics at 220, 330, 440 Hz and every multiple above, each one quieter than the last. Set a low-pass filter’s cutoff to 660 Hz, the sixth harmonic, and the harmonics below it pass through while everything higher fades away. A 12 dB per octave filter lets that top end fade gently; a 24 dB per octave filter, like the Minimoog’s ladder filter, cuts it off much harder, which is part of why a Minimoog bass sounds so round. Turn up resonance and the harmonics right at the cutoff get louder, the squelch you hear in acid bass lines.

Most of the life in a subtractive patch comes from moving that cutoff. Start the cutoff low, around 300 Hz, and give the filter its own envelope with a big amount: attack 0 ms, decay 200 ms, sustain 0. Every note now opens bright and closes fast. Set the amp envelope to decay 400 ms and sustain 0 as well, and the saw has become a pluck. For a pad, set both attacks to 1.5 seconds, both sustains to about 70 percent and the amp release to 2 seconds, and the same oscillator swells in like a string section. The full tour of cutoff, resonance and drive is in synth filters explained.

Where subtractive struggles is metal. Bells and gongs have overtones that fall between the neat multiples of the note, and a filter can only remove harmonics the wave already has.

FM vs subtractive synthesis, heard side by side

FM synthesis runs in the other direction. It starts with sine waves, which have no overtones at all, and creates them. One oscillator, the modulator, pushes the pitch of another, the carrier, up and down hundreds of times a second, and new frequencies called sidebands appear above and below the carrier. Two controls do most of the work: the ratio between the two oscillators decides where the sidebands land, and the FM amount decides how many of them are loud enough to hear.

Play an FM patch and a subtractive patch side by side and four differences stand out:

• The sweep. Close a low-pass filter on a saw and the brightness drains evenly from the top, the smooth “wow” of a filter sweep. Lower the FM amount instead and the sidebands rise and fall unevenly, some swelling while others fade, so an FM sweep sounds glassy and a little metallic.
• Bells. Set the modulator to an in-between ratio such as 1:1.41 and the sidebands land in the gaps between harmonics. With the carrier at 200 Hz and the modulator at 282 Hz, the first sidebands sit at 82 and 482 Hz, the next at 364 and 764 Hz, and none of them match the harmonics of a 200 Hz note at 400, 600 and 800 Hz. Give that tone a sharp attack and a long fade and it rings like a bell. Whole-number ratios such as 1:1 or 1:2 keep every sideband on a harmonic, so the tone stays pitched.
• The attack. Both can start a note bright and let it mellow. A filter envelope does it smoothly, closing down over the wave’s own harmonics from the top. An envelope on the FM amount can add a cluster of high or off-pitch partials for a tenth of a second, a metallic ping over a note that rings on: the tine sound of the classic DX7 electric piano. In Gelato Synth, our iPhone synth, that control is called Envelope to FM, and it sits next to Amount on the FM page.
• Analog or digital. Subtractive grew up in analog circuits, and a little oscillator drift is part of its warmth. FM needs the two oscillators to hold an exact ratio, so the FM synths that made it famous were digital.

FM counts as its own type of synthesis. The Yamaha DX7 of 1983 had six sine-wave operators per voice (operator is Yamaha’s name for an FM oscillator) and no filter at all; every change in brightness came from the modulators’ levels and envelopes. Many newer FM synths add a filter. The Korg opsix pairs its six operators with a filter section that includes models of the MS-20 and Polysix filters, so one patch can make harmonics with FM and then trim them with a filter.

Wavetable synthesis vs FM and subtractive

A wavetable oscillator stores a series of single-cycle waveforms, often called frames, lined up in order. A position control picks which frame plays, and most synths crossfade between neighbors as you move it. Wolfgang Palm pioneered the technique at PPG in Germany in the late 1970s, and the PPG Wave synths of the early 1980s are the best-known early wavetable instruments. In software, Xfer Records’ Serum and Vital (which has a free version) are two widely used examples.

A wavetable sounds like any fixed wave until something moves the position. Hold a note and sweep from a soft, round frame to a hollow, nasal one over three seconds, and a pad changes character as it sustains. Send an LFO synced to eighth notes to the position, and a bass line growls in rhythm. Neither move touches a filter.

Against FM, the big difference is where the harmonics come from. A wavetable frame is one cycle of a wave, repeated, and a wave that repeats exactly holds only whole-number harmonics of its note, however odd the frame looks. FM at an in-between ratio puts partials between those harmonics, which is why bells and metallic tines are FM territory. A wavetable’s movement is whatever someone stored in the frames, so it is easier to explore by ear and harder to predict before you listen. The two do meet: Serum has an oscillator warp mode labeled FM that lets its second oscillator modulate the first.

Against subtractive, the wavetable is only the oscillator; after it, nearly every wavetable synth has a filter and an amp with envelopes, the same subtractive chain. Even the PPG Wave ran its digital waves through analog filters. Think of a wavetable synth as a subtractive synth with far more starting waves and a knob for moving between them.

Additive vs subtractive synthesis

Additive synthesis comes from the math under all of this. Joseph Fourier showed that a steady, repeating wave can be split into sine waves at whole-number multiples of its pitch, which means you can also build one by adding sines together. Built that way, a sawtooth is the fundamental at full level, the 2nd harmonic at half, the 3rd at a third, and so on up.

The Hammond organ, first sold in 1935, mixes tones with nine drawbars, and each one adds a single pitch to every note you play. The 8’ drawbar is the fundamental, 16’ sits an octave below, and 4’, 2’ and 1’ sit one, two and three octaves above; the others add harmonics in between, such as 2 2/3’, an octave and a fifth above. Pull the first three drawbars all the way out, written 888000000, and you get the registration most associated with Jimmy Smith. That is additive synthesis with nine partials and a row of sliders.

Additive can do things a filter cannot. It can raise the 7th harmonic on its own, place partials at non-harmonic frequencies for a bell, or analyze a recording and rebuild it from sine waves, a process called resynthesis. The price is programming time. For a tone that changes as it plays, every partial needs its own envelope. The Kawai K5000, introduced in 1996, allowed up to 64 harmonics in each of up to six layers, so one sound could hold 384 harmonic levels to set.

Which type of synthesis to learn first

Start with subtractive, where each control does one thing you can hear. Wavetable synths keep its filter and amp, and FM synths give every operator its own envelope, so what you learn carries straight over. How a synthesizer works walks through every knob in order.

Then work through the types in this order, about 20 minutes each:

1. Subtractive: a pluck, then a pad. From an init patch with one saw, build the pluck and the pad from the first section, and play the same phrase on each.
2. Wavetable: change one thing. Keep the pad’s filter and envelopes, switch the oscillator to a wavetable, and send a slow LFO to the position, around 0.2 Hz (one sweep every five seconds). Only the starting wave has changed.
3. FM: open the filter. Turn the cutoff all the way up. Set two sines at 1:1, hold C3 and raise the FM amount slowly: the sine brightens into a brassy tone with every harmonic in it. Then try a ratio of 2, which sounds hollow and reedy, 0.5, which drops the pitch an octave, and 1.41 for the bell. FM synthesis explained has starting settings for bell and bass patches.
4. Additive: if you are curious. On a drawbar organ or an organ plugin, start with only the 8’ drawbar out, then pull out 4’ and 2 2/3’ one at a time and listen to each partial join the tone.