[sdiy] "Timer Controlled VCO" (TCVCO) ?

[rsdio at audiobanshee.com]

My quick read of that section of the interview leaves me with the impression that he's trying to be more precise about the terminology.

Some people seem to use the term DCO for a fully digital synthesized waveform, i.e. samples, and I agree that the term is misleading. Digital Control implies that the rest of the Oscillator is not digital, and also implies that the Control is separate from the Oscillator. Sample-based oscillators should simply be called Digital Oscillators to remove doubt, but I guess marketing will never allow that. The Schmidt is not based on a Digital Oscillator.

As for TCVCO, Stefan seems fairly clear that his oscillators are like certain vintage DCO synths. I may be slightly off in my assessment, but I like to think of these as a VCO with hard sync to a Digital Square Wave. The oscillators are Analog circuits that make Analog waveforms in the Analog domain, but there is a Reset signal coming from a Digital circuit that forces the frequency to have the accuracy of the crystal-based digital timing. I've not experimented with these DCO circuits enough to know whether they perform like a true Hard Sync setup. I think you'd need to alter the firmware so that the VCO frequency could be set completely independently of the Timer Control frequency. I assume that if the controls were allowed to diverge, then you'd get the kind of interesting Analog Oscillator waveforms that Hard Sync provides. Instead, I assume that the Schmidt firmware keeps the Timer Control frequency close to the Voltage Control, but I can't be sure whether there's an actual Voltage Control in there. I suppose the Analog circuitry would be potentially less expensive if some or all of the Voltage Control components were left out, leaving the basic Analog Oscillator with a Digital Reset Control input. If I weren't headed to work right now, I'd grab the schematics for the Juno 6/60, Matrix 1000 or the Matrix 6 and see what's common. It may be that the TCVCO doesn't really have any Control input other than the Timer, in which case the *VCO part of the name is misleading again. Maybe it should be a TCO? Timer Controlled Oscillator? DTCO : Digital Timer Controlled Oscillator?

I agree with your question about why the term TCVCO would be appropriate unless there is Control from both the Timer and a Voltage. It's at least theoretically possible to design a circuit that does have both Control inputs, but as I described above it would act like Hard Sync.

Brian Willoughby
Sound Consulting


On Sep 23, 2014, at 8:19 AM, Michael Zacherl wrote:
> recently I stumbled upon this in an interview [1] and wanted to find out how this would be implemented in detail.
> But apparently it's a term Stefan Schmidt just coined back then? Searching the Net isn't very helpful, alas.
> It's a DCO variant, he stated … but what? 
> The term itself suggests some 'dualism' (timer controlled - voltage controlled … ),
> so it's probably not too precisely formulated anyway?
> 
> Michael.
> 
> [1] http://www.noiz.gr/index.php?topic=197390.0