This page is an overview of two techniques for creating wavetable oscillators.
The Korg DW series use a DDS oscillator scheme. Korg called it ‘DWGS’ for ‘Digital Waveform Generator System’ in the 80’s style of inscrutable acronyms, but in reality it is a single cycle of digitally stored waveform that can be played back repeatedly.
It uses a counter (phase accumulator) to keep track of the waveform phase, and then uses the counter value as a lookup into a wave memory to convert the phase value into an output value. By altering how much the counter is incremented, the samples can be read out of the memory either more or less quickly, producing different frequencies.
In order to avoid aliasing, the system uses a different wavetable for each octave. These wavetables are arranged so that they lose harmonics as you go up the octaves. In this way, no harmonics above the Nyquist limit are ever present.
In the example above you can see the way that the tables are arranged, with long, detailed wavetables for the lowest octaves, going up to shorter tables for the higher octaves. The lowest octaves have a good quality bandlimited ramp wave, but as you go up the octaves, more and more harmonics are removed until the highest octave only has the first and second harmonics. The enlarged image also shows the effect with 3 other DW waveforms.
The PPG Wave uses a different digital oscillator scheme. Instead of having a large wave memory and missing samples out, it uses a smaller wave memory and reads it at different speeds – it uses a variable sample rate. Paul Maddox’s Monowave was based on the PPG and uses a similar scheme, shown below.
Basic pitch selection is done by the variable clock. The overall oscillator octave is determined by a binary counter whose outputs are selected by an analogue switch. The output from this switch feeds another binary counter that generates phase addresses for the wave memory. The high byte of the wave memory address acts as waveform selection. The output from the wave memory is fed to an output DAC.
In his piece Birth of the Prophet VS, Chris Meyer describes how the Sequential design team wondered which approach to use, before picking the PPG as a good model to use. Dave Smith Instruments then copied the Prophet VS oscillators for the recent Evolver.