[sdiy] How DCOs work

[Colin f]

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of
> Tom Wiltshire
> Sent: 10 October 2016 14:48
> Subject: Re: [sdiy] How DCOs work

> In the video, he argues that the ramp core part is
> merely another wave shaper, since it would stop without the counter, and
> that would stop without the master clock, so there's only really one
oscillator
> in the synth - the master clock Which is true in a way, but kind of means
> there's no such thing as a "DCO", since you've just defined them away.
> Instead, you've got an oscillator followed by digitally-controlled analog
wave
> shapers.

Another way to look at it is that the counters are oscillators.
Instead of charging a capacitor with voltage, they charge a 16-bit register
with numeric increments.
Instead of a voltage comparator resetting the timing cap, a numerical
comparator resets the counter register.
To say that the DCOs are not oscillators because they would stop oscillating
if the master clock was turned off is like saying VCOs are not oscillators
because they wouldn't run without the power supply.
In reality you have to consider the circuit as a whole.

> But I'm nitpicking. It's a considered and knowledgable discussion of the
> technology, which is a rare thing with DCOs!!

It misses the fundamental limitation of counter-based DCOs compared to VCOs,
which is that frequency modulation of a VCO affects the shape of the ramp
directly, and the period of the oscillator derives purely from the behaviour
of the ramp.
Frequency modulation of a DCO applies to the period only, as it can only be
changed by re-loading the counter reset value.
They only update the charging rate of the ramp for each cycle, so you get a
straight ramp for every cycle, where a VCO would have a convex or concave
ramp shape dependent on the direction of pitch modulation.
The DCO ramp behaves like an analogue VCO only for a static pitch.
As soon as you apply any kind of modulation (which is pretty much always)
their limitations become apparent.
One fairly recent DCO based synth even had a basic design error where they
didn't update the ramp control CV quickly enough, so that a large drop in
pitch would lead to an audible click, as the counter switched to a long
period but the first cycle of the low frequency rose at the ramp rate for
the high frequency.
Ugh.
Give me VCOs any day :)

Cheers,
Colin f