Re: [AH] Synthex oscillators

From Brian Willoughby
Sent Sat, Sep 1st 2018, 22:57

On Sep 1, 2018, at 11:21 AM, Andre Majorel <xxx-xxxxx@xxxxxx.xx> wrote:
> On 2018-08-31 19:26 -0700, Brian Willoughby wrote:
>> 8-bit counter makes it digital, even though the DAC is cheap.
>=20
> Depends what makes a VCO digital. If it is having a DAC in the
> signal path, yes. If it is having its frequency controlled
> digitally, not necessarily.
>=20
>> A pure analog ramp oscillator still has a cap charging and
>> getting reset, so that?s not exclusive to DCO. The difference
>> between a DCO and VCO is that the DCO resets based on a
>> digital crystal oscillator that has been divided down, while a
>> VCO resets based on an analog voltage crossing an analog
>> threshold (think: comparator).
>=20
> Agreed.
>=20
>> One step further away from a VCO than a DCO would be an
>> accurate characterization.
>=20
> Unless the clock that increments the counter is derived from a
> VCO, in which case it's arguably the opposite.

Good point. There are two dimensions here.

The voltage can be continuous (VCO, DCO) or discrete (Sampler, Synthex).

The time dimension can also be continuous (VCO) or discrete. I would say =
that there are two kinds of discrete time: continuously variable =
frequency discrete time, or the limited sets of frequencies that you get =
by a digital divide-down circuit that can only manage rational divisors. =
The latter is doubly discrete because there are time samples and the =
frequency of those samples can only come from a finite list of discrete =
divisors.

Effectively, the time domain has three options: Continuous time, =
discrete time with continuous frequency, and discrete time with discrete =
frequency. I=92ve not seen many texts distinguish between the latter =
two, but they=92re quite relevant to consider for synthesis.

Brian Willoughby