Wavetable synthesis
Wavetable synthesis is used in some digital 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 [1][2] and published in 1979,[3] and has since been used as the primary synthesis method in synthesizers built by PPG and Waldorf Music and as an auxiliary synthesis method by Sequential Circuits, Ensoniq, Korg, Access Music, Dave Smith Instruments and others.
Principle
Wavetable synthesis is periodic reproduction of a single-cycle waveform.[4] The distinction from other synthesis methods employing single-cycle waveforms is that multiple single-cycle waveforms are used (originally as a table of 64 waveforms with 128 samples each[3]) and some means of amplitude modulation and mixing the single-cycle waveforms is employed.
Both variable and (more commonly) fixed sample rate systems are used [5] and the wave modulation rate is usually significantly smaller (slower) than the sample rate. Depending on the details of the actual implementation, the sound produced by wavetable synthesis may also contain recognizable artifacts, especially aliasing, quantization errors, and phase truncation noise.
A wavetable is a store of single-cycle waveforms. Together with the wave modulation, the wavetable defines the basic sound, which is then often altered by additional post-processing like filtering. The structure of a wavetable, that is, the number and length of entries, depends on the actual implementation. The individual waveforms and their placement in the wavetable have to follow the musical intent as well as the modulation capabilities of the synthesis engine.
Since all waveforms used in wavetable synthesis are periodic, the time-domain and frequency-domain representation are exact equivalents of each other and both can be used simultaneously to define waveforms and wavetables.
Comparison with other digital synthesis techniques
Unlike additive synthesis which generates and adds multiple harmonics, in wavetable synthesis, the waveform is precomputed from the harmonics and stored as wavetables that are used during synthesis.
- Sample-based synthesis uses multiple-cycle waveforms and intricate algorithms for pitch-shifting.
- LA synthesis uses short PCM samples for the attack portion of the sound, with either a digital subtractive synth sound or looped samples (most of them single-cycle loops) for the sustain/release portion of the sound.
- Granular synthesis uses many overlapping windowed samples. While these samples are very short, they are never periodic.
Confusion with table-lookup oscillators
Numerically controlled oscillators often employ digital memory to store samples of single-cycle waveforms. This type of short-memory based oscillator has also become known as a wavetable oscillator.
Confusion with sample-based synthesis
Starting around 1993, with the introduction of Creative Labs' Sound Blaster AWE32 and Gravis' Ultrasound sound cards, the term "wavetable" started to be applied as a marketing term to any sound card that used PCM samples as the basis of sound creation. However, these sound cards did not employ any form of wavetable synthesis, but rather a sampler and subtractive synthesis system based on technology from the E-mu Emulator series of digital sampling keyboards.
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 totally abstract, which is where this method of sound creation excels. 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 also produce satisfyingly musical results. Most wavetable synthesizers also employ other synthesis methods to further shape the output waveform, such as subtractive synthesis (filters), phase modulation, frequency modulation and AM (ring) modulation.
References
- Wikipedia:Wavetable synthesis
- ^ PPG Wave 2.2 owners manual
- ^ Part 4 "Digital Age" on Wolfgang Palm's blog
- ^ a b A New Way in Sound Synthesis by Uwe Andresen, Audio Engineering Society (AES), 62nd AES Convention (Brussels, Belgium), 1979
- ^ Wavetable Synthesis 101, A Fundamental Perspective by Robert Bristow-Johnson, Audio Engineering Society (AES), 101st AES Convention (Los Angeles, California), 1996
- ^ Practical Considerations in the Design of Music Systems using VLSI by J. William Mauchly, Albert J. Charpentier, Audio Engineering Society (AES), AES 5th International Conference: Music and Digital Technology, 1987