A subtractive oscillator offers three or four shapes: sine, triangle, sawtooth, pulse. A wavetable oscillator can hold sixty-four or more, because they are not made by a circuit but stored as numbers, one cycle each, in a table. Play one position and you have a fixed tone; move along the table while the note sounds and the tone changes shape as it goes.
How it works
Digitise one cycle of a wave as a list of amplitude values and an oscillator can read that list round and round at the pitch you ask for: table-lookup. Wavetable synthesis stores many such single-cycle waves side by side and lets you choose where in the row the oscillator reads. Jump between neighbouring waves and you hear a step; interpolate between them and the timbre glides. Sweep the position with an envelope or an LFO and the wave animates over the note.
| What it does | Knob |
|---|---|
| Which row of waves is loaded | Table |
| Where in the row the oscillator reads; the main sound-shaping knob | Position |
| Whether moving between waves steps or glides | Interpolation |
| An envelope, LFO or velocity moving the position while the note plays | Modulation |
A table can imitate older tricks: a row of square waves with different duty cycles, swept, sounds like pulse-width modulation. Or it can be something no circuit would make: the Waldorf Microwave shipped a table cut from a voice synthesizer saying "nineteen twenty", and different start points and lengths in it gave very different sounds.
Where it came from
| What | When |
|---|---|
| Hal Chamberlin discusses the technique in Byte | Sept 1977 |
| Michael McNabb uses his own version in the composition Dreamsong; PPG's Wavecomputer 360 ships with 30 wavetables of 64 waves each, but no filter, and sounds thin | 1978 |
| Wolfgang Palm's method is published, in a paper at the AES convention in Brussels | 1979 |
| The PPG Wave series: the digital wavetable engine run through analogue amplifiers and 24 dB per octave filters, eight voices, 8-bit waves (the Wave 2.3 could take 12-bit samples). Priced at 7,000 to 10,000 US dollars in its first years | 1981–1987 |
| Waldorf's Microwave, a rack unit with the PPG wavetables and analogue filters, the start of the company | 1989 |
Palm's insight was the combination: a digital oscillator with far more shapes than analogue could make, then the familiar filter and amplifier to warm it up. The Wavecomputer without a filter was buzzy; the Wave with one was a hit with a-ha, Alphaville, Trevor Horn and Propaganda, among others.
Why producers reach for it
A wavetable does movement that a filter cannot. A low-pass sweep only ever darkens or brightens; a position sweep can turn a hollow wave into a reedy one into a bright one, and the shape of that journey is the sound. The same controls that made PPG pads in the 1980s make bass that growls and leads that morph today, in any synth whose oscillator has a position knob.
It also plays well with lesson 1. The wave that comes out of the table is just an oscillator signal, so it goes through the same filter, amplifier and envelope as a sawtooth would. That is exactly what PPG added in 1981 and what the Wavecomputer of 1978 lacked, and it is why a wavetable patch still has a cutoff knob: the table shapes the wave, the filter shapes the tone, and the two sweeps together are more than either alone.
Sources
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