Wavetable Synthesis history
Before the birth of the wavetable synthesis, analog oscillators were mostly able to offer 4 basic oscillator wave-forms. However, in the second part of the 1970`s, Wolfgang Palm of 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 oscillators called wave-table oscillators. This has opened up fresh opportunities in terms of audio synthesis. While common analog oscillator had only basic waveform types, wave-table oscillator could have up to 64. Furthermore, the PPG wave synthesizer had 32 wave-tables, which were containing 64 wave-forms each.
How does Wavetable Synthesis work?
There are 2 basic 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 LFO`s, envelopes or velocity to step thru these wave-forms.
Wavetable Synthesis techniques
This method of synthesis was a leap forward in terms of saving memory. We need to remember that memory was quite expensive in the last 3 decades of the 20th century. Here are few techniques used in wave-table synthesis method in order to reduce the amount of required memory.
Looping
One of the primary techniques used for the purpose of preserving memory is looping of the sampled sound segments. Usually, a sound can consist of two main sections – attack and sustain. The attack is the initial part of the sound where amplitude and spectral characteristics can change very rapidly. On the other hand, sustain section is where sounds change in a less dynamic way. 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 only store a few selected notes from the instrument. If you have to play a note that is not one of the selected notes then you can shift the pitch of the nearest note. This is done to achieve the desired pitch. The farther you shift the pitch then the more unnatural it will sound. Shifting by a few semitones to an octave up or down usually sound 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 wave-table for that note to a lower sample rate without greatly 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 which are rich in upper harmonic content will generally have more problems with aliasing noise. Low-pass filtering applied after interpolation can help eliminate the undesirable effect of aliasing noise. What is more, the use of oversampling also helps eliminate aliasing noise.
Additional Resource & Source Texts
http://www.musicdsp.org/files/Wavetable-101.pdf
