Wavetable synthesis

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Wavetable synthesis is a technique used in certain digital music synthesizers to implement a restricted form of real-time additive synthesis. The technique was first developed by Wolfgang Palm of PPG in the late 1970s, and has since been used in various forms in other synthesizers built by Ensoniq, Access, PPG, Waldorf Music and Dave Smith Instruments. The particular sound associated with wavetable synthesis is unique due to the technique employed: One which harmonics fade in and out over time.

Wavetable synthesis must not be confused with PCM Sample playback synthesis (see explanation of orgin of the confusion later in this article).

Wavetables

Wavetables are a collection of many single cycle waveforms, each one having subtly changing harmonic contents. The wave cycles may be based on a sample of a real instrument or may be constructed using additive synthesis methods, that is, a collection of multiple sine waves; usualy between 64 and 128. Each single cycle waveform is placed in a memory slot (again either 64 or 128) with the next adjacent wave cycle being one who's harmonic content only differs very slightly. This collection of single cycle waves is a Wavetable. Each wave in the wavetable may be used in the same way as a traditional oscillator or wave generator, that is, producing an output waveform with static harmonic content. Alternatively, the entire wavetable may be scanned through from any point, forwards or backwards, so changing the harmonic content over time. This is akin to Pulse Wide Modulation, however, wavetable synthesis is not limited to using only pulse (square) waves.

Wavetable Synthesis is 3 dimensional in that the position in the wavetable can be assigned to the X plane, the harmonic content to the Z plane and the amplitude of the harmonics to the Y plane. However, in the true mathematical sence, a wavetable synthesizer is 4 dimensional as there is usualy more than one wavetable to work with.

Wavetables are not usualy editable and are hard coded into the synthesizers firmware. Notable exceptions to this were PPG Waveterm and CMI's Fairlight Waveframe, however, user definable wavetables were only possible on these machines as both used interpolation to calculate the smooth harmonic transitions between wavetable positions: A processor hungry method compared to predefined single cycle waveforms stored in memory.

A simplified (and user programable) version of wavetable synthesis was utilized on Sequential Circuits' Profet VS, whereby a sequence of waveforms could be chained together and played forwards or backwards (wave sequencing).

Practical Use

During playback, the sound produced can be harmonically changed by moving to another point in the wavetable, usually under the control of an envelope generator or low frequency oscillator but frequently by any number of modulators (matrix modulation). Doing this modifies the harmonic content of the output wave in real time, producing sounds that can imitate acoustic instruments or be totaly abstract, which is where this method of sound creation excells. The technique is especially useful for evolving Synth pads, where the sound changes slowly over time.

It is often necessary to 'audition' each position in a wavetable and to scan through it, forwards and backwards, in order to make good use of it, though selecting random wavetables, start positions, end positions and directions of scan can produce some satifyingly musical results quite randomly. The output waveform is processed by other circuitry / algorithms just the same way as all synthesizers: The pitch and amplitude being processed by envelope generators and LFO's etc.

It is worth noting that most wavetable synthesizers also employ other synthesis methods to further shape the output waveform, for example, Subtractive Synthesis (Filters), Phase Modulation, Frequency Modulation and AM (Ring) Modulation.

Comparison with other digital synthesis techniques

Wavetable synthesis is most similar to additive synthesis, however unlike additive synthesis the user does not have to work with single harmonics, the composite waveforms being presented for use in the wavetables.

Confusion with sample-based synthesis

Starting around 1993, with the introduction of Creative Labs' Sound Blaster AWE32 and Gravis's Ultrasound cards, the term "wavetable" started to be applied to any sound card that used a suite of pcm samples as the basis of sound creation. This is not wavetable synthesis.

The description of wavetable synthesis in previous sections is based on the original meaning of the term, i.e. 3 dimensional sound creation.

Notes

External links

• Wavetable Synthesis 101, A Fundamental Perspective