Wavetable Synthesis

Wavetable Synthesis History

Before the wavetable synthesis's birth, analog oscillators could mostly offer 4 basic oscillator waveforms. However, in the second part of the 1970s, Wolfgang Palm, today's famous Waldorf music company based in Germany, introduced a new form of audio synthesis. In his PPG Wave synthesizer, he used this new form of digital oscillator called a wavetable oscillator. This has opened up fresh opportunities in terms of audio synthesis. While standard analog oscillators had only basic waveform types, wave-table oscillators could have up to 64. Furthermore, the PPG wave synthesizer had 32 wave-tables containing 64 waveforms each.

How Does Wavetable Synthesis Work?

There are 2 essential aspects in terms of how wavetable synthesis works. Firstly, the waveform lookup table contains samples for not just a single period of a sine function but for a single period of a more general wave shape. Secondly, a mechanism exists for dynamically changing the wave shape as the musical note evolves, thus generating a quasi-periodic function in time. The advantage of this is that it is possible to use LFOs, envelopes, or velocity to step through these waveforms.  

Wavetable Synthesis Techniques

This method of synthesis was a leap forward in terms of saving memory. We must remember that memory was expensive in the last 3 decades of the 20th century. Here are the techniques used in the ave-table synthesis method to reduce the required memory.  

Looping

One of the primary techniques used to preserve memory is the looping of the sampled sound segments. Usually, a sound can consist of two main sections - attack and sustain. The attack is the initial sound part where amplitude and spectral characteristics can change vapidly. On the other hand, the sustain section is where sounds change less dynamically. A great deal of memory can be saved in wave-table synthesis systems by storing only a short segment of the sustain section of the waveform and then looping this segment during playback.  

Pitch Shifting

Another way to reduce the memory burden is to store only a few selected notes from the instrument. If you have to play a note that is not one of the selected notes, you can shift the pitch of the nearest note. This is done to achieve the desired pitch. The farther you shift the pitch, the more unnatural it will sound. Shifting by a few semitones to an octave up or down usually sounds OK.  

Re-Sampling

A recording of a low note on an acoustic bass does not have many high-frequency partials. So, it is possible to re-sample the wavetable for that note to a lower sample rate without significantly affecting the timbre.  

Aliasing Noise

The re-sampling techniques used to shift the pitch of a stored sound sample can also result in aliasing noise. Furthermore, aliasing noise can also limit the amount of pitch shifting. Additionally, sounds rich in upper harmonic content will generally have more problems with aliasing noise. Low-pass filtering after interpolation can help eliminate the undesirable effect of aliasing noise. What is more, the use of oversampling also helps eliminate aliasing noise.