Wavetable synthesis

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Wavetable synthesis is a technique used in certain digital music synthesizers to produce natural tone-like sounds. The sound of an existing instrument (a single note) is sampled and parsed into a circular sequence of samples or wavetables, each having one period or cycle per wave; a set of wavetables with user specified harmonic content can also be generated mathematically. Upon playback, these wavetables are used to fetch samples (table-lookup) in the same manner as in a Numerically-controlled oscillator to produce a waveform. However, in wavetable synthesis, the output waveform is not normally static and evolves slowly in time as one wavetable is mixed with another, creating a changing waveform. Looping occurs when the wavetable evolution is halted, slowed, or reversed in time.

Put more simply, a wavetable synthesiser will store two parts of an instrument's sound. A sample of the the attack section (e.g. the sound of the hammer hitting a piano string) and a small segment of the sustain portion of the instrument's sound. When triggered, the 'attack' sample is played once immediately followed by a loop of the sustain segment. The endlessly looping segment is then enveloped to create a natural sounding decay (dying away of the sound).

Special effects are created by selecting a wavetable at random or in a special pattern from the table on a tempo-tick from a sequencer. Because of the low processing power required early wavetable synthesizers imitated dynamic filters and other expensive synthesis methods by rapidly playing successive wavetables in sequence. If each waveform is a little duller (or brighter) than the previous, a moving filter effect can be imitated.

This method differs from simple sample playback in that the output waveform is always generated in real time as the CPU processes the wave sequences; the effect is comparable to other early digital synthesis methods such as frequency modulation synthesis. Some early wavetable synthesizers used the term "wavetable" to refer to wave sequences, and not the actual waveform data.

Some later variations on the technique are Roland's "Linear Arithmetic" synthesizers such as Roland D-50 and Roland MT-32/LAPC-I, which combined complex sampled attack phases with less complex sustain/decay phases (basically a wavetable synthesizer with a 2-entry wave sequence table), and vector synthesis, used in the Sequential Circuits Prophet-VS and Korg Wavestation, which can move through wavetables and sequences arranged on a 2-dimensional grid.

Contents

1 Palm's wavetable systems
2 Confusion with
3 External links
4 See also

Palm's wavetable systems

The German synthesizer designer Wolfgang Palm began experimenting with wavetable synthesis in the late 1970s, with his research realized in PPG's line of synthesizers such as the Wavecomputer, Waveterm, and Wave. Palm's implementation of wavetable synthesis employed an array containing 64 pointers to individual (symmetrical) single-cycle waves stored within the instrument. Usually, only a few pointers to these waves were actually used, spread throughout the breadth of the wavetable. The distinguishing feature of the PPG Wave series was that it would interpolate the remaining waves in between the defined pointers, so that changing the position within the table would result in a smooth, unique "morphing" effect between the waves. It should be stressed that Palm's wavetable scheme's strength was in its generation of harsh digital sounds and bell-like timbres, not the emulation of acoustic instruments. It was possible to sample a complex sound into a wavetable by way of the Waveterm device, but the results were invariably artificial, and usually not as interesting as the powerful and bizarre sounds resulting from competent exploitation of the synthesizer's capabilities.

After the demise of the PPG company, Waldorf Music adapted Palm's wavetable oscillator design into their wildly successful Microwave synthesizer module (1988). This design was extrapolated into the Waldorf Wave in the early 1990s, a very large and expensive instrument offering facilities for resynthesis and user-friendly wave and wavetable construction. An all-digital revision of the Microwave soon followed, with a complete line of wavetable synthesizers remaining in production until 2003. Aside from a few improvements designed to eliminate audible aliasing and quantization (signal processing) errors, this wavetable oscillator scheme had not significantly changed since the first days of the PPG wave series.

Confusion with
Palm's systems used wavetable to mean the list of waveforms being processed as part of a patch or program; in more general technical use, however wavetable means the sample data used to generate a sound. This was noticed in the early 1990s by several makers of PC soundcards; starting around 1993, with the introduction of Creative Labs' Sound Blaster AWE-32 and Gravis's Ultrasound cards, the term "wavetable" started to be applied to any card that had a better General MIDI subsystem than the then-common OPL2 and OPL3 FM synthesizers. These subsystems were generally based on low-end sampler ICs, so using anything more involved than sampled instruments with them was difficult. To add to the confusion, there were also low-cost "wavetable" engines that had no expansion capabilities and were limited to playing sounds from ROM; these were usually found on cheap ISA sound cards starting in the late 1990s. This usage of the term has been downplayed in recent years with the drop in popularity of General MIDI-based music in games, in favor of custom sound engines (which typically call for a more modern DSP-based sound card) and pre-recorded music.
The AWE32 and the Ultrasound are somewhat more powerful than their specification sheets say. They allow direct access to the DCOs (they lack a MIDI interpreter) and can be reprogrammed to support wave sequences. To attempt this requires low-level driver support.
The description of wavetable synthesis in previous sections is the most original definition of the term and it should be noted (shown in the reference below) that 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).

Wavetable synthesis

Palm's wavetable systems

External links

See also