Hi everyone,

I'm interested in understanding the principle behind the kind of wavetable synthesis as implemented in such synths as the PPG wave, SCI Prophet VS and the likes where the oscillators would read single cycle waveforms from lookup tables with the ability to "scan" through them, which as far as my understanding goes means crossfading between them one after the other (manually, with an lfo or an envelope or whatever).

I wonder how this could be implemented in Pd. Suppose I have a bank of n single-cycle waveforms, I'm not sure how to go about it, whether having n separate tables or join them together consecutively and have the master phasor read two tabread4~, crossfading back and forth between them and offsetting the read point.

Has anyone tried to implement this? Any insights or ideas would be very helpful.

thanks in advance.

Here is my attempt with a single table. The crossfading is a bit involved but at least it works and doesn't click. I'm still not entirely sure if "morphing" the shape of two waveforms is the same as crossfading their volumes. Any ideas?

wavetable.pd

(There is some awful aliasing but I think it's because there's way too many partials in the waveforms).

Turns out I didn't really understand what a "wavetable" in those kind of synths is. I can't find a lot of information on this but from what I understood, a "wavetable" is not a collection of unrelated waveforms one after the other (like in my previous patch), but rather the collection of "intermediate" steps from waveform A to waveform B, therefore scanning the wavetable (aka, crossfading between one step and the next) has the effect of morphing between the shape of waveform A and waveform B. The morphing is already in the wavetable.

Here is how I've implemented it in a polyphonic context. You load a wavetable and then scan the various intermediate positions with the crossfade slider.

wavetable.synth.zip
(Requires bsaylor external for the envelope)

In the utilities folder there is a patch where you load waveform A and waveform B (both must be single-cycle waveforms of the exact same length) and it generates a wavetable, which can be saved to a .wav file.

That's right. Crossfading between 2 waveforms is what you get in synths like Yamaha TG33, Prophet VS, and the Seiko ones. PPG stuff and Ensoniq stuff is more complicated. As far as I know, the PPG ones didn't crossfade at all, they just switched waves at the beginning/end of the next cycle (when the waveform is at zero so there's no click).

There are lots of ways to get from A to B as far as changing between two waveforms go. Consider the change between a sawtooth wave through a resonant filter. As you open and close the filter, the sound changes. Crossfading between samples of "fully open" and "fully closed" would not get you even close! There are other interesting "morphs" between sounds that you can do with wavefolders with additive techniques, or with acoustic instruments, that cannot be emulated by simple corssfading, so that is where larger wavetables come in.

So it just depends what you want to acheive!

@nuromantix Thanks for the info. So we could say that a simple crossfade between two waveforms (like the TG33 and Prophet VS) would be equivalent to a mix between two oscillators. On the other hand, Stepping through a wavetable (aka a bank of pre-computed waveforms) like the PPG would be equivalent to re-doing the transition that was recorded in the wavetable. In your filter example, crossfading between a filter-open and a filter-closed waveforms is not the same as opening or closing the filter, but if a high number of snapshots of the filter sweep have been saved to a wavetable as consecutive, single-cycle waveforms, stepping through the wavetable achieves a much similar result. (In my last example the patch is also crossfading between adjacent waveforms in the wavetable, which might not be necessary).

My idea was to be able to morph between two arbitrary waveforms, but I think that creating a wavetable of the "morph" between wave A and wave B is pretty much the same as crossfading. Nevertheless, a wavetable is useful if the transition from A to B involves some other processing that the crossfade can't reproduce.

Yes I think you will soon discover that the sounds made by crossfading between 2 arbitrary waveforms are not all that exciting. You can add interest by having a few layers of different waveforms crossfading at different rates but it's always quite bland sounding.

@nuromantix Yes, I think I already have It was a misunderstanding of how those wavetable synths work. Perhaps finding some of those original wavetables would be nice, or making new ones in a way that is not by crossfading. Thanks