[sdiy] Digital accumulator VCO core?

Brian Willoughby brianw at audiobanshee.com
Sat Feb 13 19:23:51 CET 2021

On Feb 13, 2021, at 03:09, Tom Wiltshire wrote:
> On 13 Feb 2021, at 10:53, Mike Bryant wrote:
>>> One important reason to use MinBLEP is to produce hard sync in a digital synthesizer. See that paper that I mentioned earlier today.
>> I know I've been left-field for decades (my last analogue synth was got rid of in 1983), but this is where I'm getting confused.  I can understand why hard sync saves costs in an analogue synth but digital oscillators are virtually free.  Any step change in a digital system is going to cause aliases so the obvious thing is avoid them rather than introduce them and then try to correct the error.  If you want the 'richer' sound hard sync produces, just add in one or more digital oscillators producing different band-limited waveforms at a different frequency.   In the end any sound can be reproduced by a sum of sinewaves, but I accept this is difficult to program.  But surely adding together several band-limited signals will produce a better sound as there is no aliasing ?  And if you actually find some of the aliasing is part of the sound you are after, use yet another digital oscillator so that it's controllable.  I think having to alter one's playing style to suit BLEP, as someone else suggested may be necessary, is surely the worst of all worlds ?
> Creating a static hard sync sound would be possible like this, but creating the typical “sync bend” sound by an additive method would require complex amplitude envelopes for each individual harmonic, since they each follow a separate evolution. Basic graph for the first ten harmonics here:
> https://electricdruid.net/wp-content/uploads/2016/08/OscSyncHarmonics.jpg
> (From https://electricdruid.net/timbral-evolution-harmonic-analysis-of-classic-synth-sounds/)
> Obviously for a analog synth’s ramp waves, there are going to be a lot more than ten harmonics to deal with, especially if this is bass sound we’re talking about.
> All this just to get BEEEEOOOWWWWMM!! Adding BLEPs into a naive version starts to seem pretty simple by comparision.
> Tom

Mike, hard sync is not part of analog synth design to save costs. You might be thinking of the DCO, where a crystal-based digital square wave clock is used to reset an analog VCO core, but even that's not to save costs: it's to improve tuning accuracy.

Hard sync produces waveforms that are complex, and way more interesting than the standard subtractive waveforms. Even for additive synthesis, it's difficult to find sounds in nature that are similar to hard sync, and even if there are it would be difficult to come up with the proper additive parameters to recreate the sound.

I suppose it's a matter of taste, but I do not own any analog synths that are not capable of hard sync synthesis. Other synths are just too boring, with their vastly limited palette in comparison, to justify spending money on a single-oscillator design or a dual-oscillator design without hard sync.


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