[sdiy] VCO Jitter, Slop ...

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jun 11 00:13:13 CEST 2013


On 06/11/2013 12:00 AM, Richard Wentk wrote:
> I've experimented with most of these features using digital synthesis. It's actually easy to add individual frequency drifts to individual overtones with a controllable mod rate/depth using (say) SuperCollider.
>
> I haven't found any reliable way to make a digital oscillator sound fat yet. I can make some awesome stacked oscillator sounds, especially if each note has a different detuning with an exponential distribution around the nominal pitch. But 'fat' - not so much.
>
> There are a few things I haven't tried, but I'm fairly sure that bandpass filtered noise modulating the frequency won't transform a digital sound into an analog-like one.
>
> My suspicion is that in addition to the tanh distortion generated in a filter, analog op-amps added a lot of colour with intermodulation distortion and limited bandwidth. A lot of early analog circuits use poor (by modern standards) op amps, and the sound isn't at all clean or simple. It's also not very bright - often there's not much happening at the top end.
>
> Simulating these effects requires some fairly scary Volterra expansions, so I haven't gotten very far with modelling it yet.
>
> It's also worth remembering that analog waveshapes are often non-textbook. A cubic sine approximation has a lot more character than a perfect clean function generator sine. The sawtooth in the early Moog 901s was nothing like a good saw. The rounded shape varied with frequency and added extra emphasis on the low partials, which made the sound bigger and warmer.
>
> And so on. Basically perfect VCOs sound boring. And a lot of early designs were well short of perfect - in interesting ways.

Agree. It is annoying and amusing at the same time to realize that we 
have not "cracked it" yet.

So, there is still some research to be done. It also means building 
stuff, and that is fun too. :)

Cheers,
Magnus



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