Wavetable synthesis

Wavetable synthesis

Wavetable synthesis is a technique used in certain digital music synthesizers to implement real-time additive synthesis and direct digital synthesis with a minimum of hardware. The technique was first developed by Wolfgang Palm in the late 1970s, and has since been used in other synthesizers built by Yamaha, Korg and Waldorf Music.

Theory of operation

To begin the process, the sound of an existing instrument (a single note) is sampled and processed using a spectrum analyzer, producing a graph of overtones contained in the sample. This graph is then parsed into a sequence of samples or wavetables, each having one period or cycle per table, generated by adding together the partials at each parse point. A set of wavetables with user specified harmonic content can also be generated mathematically. These wavetables are typically arranged one after the other in a random-access memory, allowing for easy access and nearly instant start-point changes; when the data is played back, a phase accumulator looping at a set rate (determined by the sample rate and the frequency of the note desired) feeds the samples to a digital-to-analog converter, creating a continuous waveform at the given frequency.

During playback, the waveform produced can be changed by switching to a different starting point in memory, usually on command from an envelope generator or low frequency oscillator. Doing this modifies the spectral characteristics of the output wave in real time, producing sounds that can imitate certain analog instruments (such as organs, pianos, harpsichords and reed instruments) acceptably without requiring the use of a pulse code modulation technique, which requires much more memory and higher sample rates for good results. The technique is also useful for evolving pads, where the waveform changes slowly over time and can reverse itself or loop back to an arbitrary point.

Since a wavetable oscillator can generate arbitrary waveforms, it is also possible to load simple sine wave, square wave and sawtooth wave tables and use the synthesizer like an analog synthesizer, using subtractive synthesis to modify the sound. Also, some wavetable synthesizers (such as the PPG Wave 2.3 with Waveterm) can reset the loop point on the phase accumulator to a period longer than a single cycle, making a PCM mode possible with minimal hardware changes.

Comparison with other digital synthesis techniques

Wavetable synthesis has similar capabilities to other synthesizers in the real-time additive synthesis family, as well as to digital frequency modulation synthesis systems such as the Yamaha DX and OPx series; however, wavetable synthesizers require less hardware to produce a usable system. The entire oscillator can be implemented using a few 7400 series TTL ICs and small-capacity static RAM ICs, something that was important in the late 1970s and early 1980s (when memory prices were still relatively high, and high-powered CPUs such as the Motorola 68000 were uncommon and expensive); most other digital synthesizers of the time either implemented each partial separately, making assembly more complex (this is also how most electronic organs are built), or used custom ICs to bring the chip count down.

Later wavetable synthesizers have antialiasing capabilities (where the transitions between waves are mediated by the CPU instead of simply switching the starting address of the loop) as well as subtractive-style filters (since the moving filter effect that a wavetable patch provides is somewhat "harsh" and FM-like without antialiasing, and filters were easier to implement at the time).

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 had a better General MIDI subsystem than the then-common OPL2 and OPL3 FM synthesizers. This was based on a misunderstanding between the technical definition of a wavetable (which is the actual sample data used to generate an arbitrary wave), and PPG's usage of the term (which referred specifically to their implementation of additive synthesis, as described above). The AWE32 was not an additive synthesizer, but a high-end sampler and subtractive synthesis system based on technology from the E-mu Emulator family.

The description of wavetable synthesis in previous sections is the most original definition of the term and (as shown in the reference below) wavetable synthesis is equivalent to additive synthesis in the case that all partials or overtones are harmonic (that is all overtones are at frequencies that are an integer multiple of a fundamental frequency of the tone).

External links

* [http://www.musicdsp.org/files/Wavetable-101.pdf Wavetable Synthesis 101, A Fundamental Perspective]
* [http://crca.ucsd.edu/~msp/techniques/v0.03/book-html/node22.html Wavetables and samplers]
* [http://www.mcrow.net/Orpheus.htm Orpheus and the New Sample-Loop Synthesis method]

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