[sdiy] Synthex Oscillator

[rsdio at audiobanshee.com]

Sounds like a fun challenge. Thanks for putting together the annotated schematic and posing the questions to the list. I'll be sure to take a look at this tomorrow. I may have a lot more to say.

I wanted to point out some errors in your message, though, assuming that I understand you correctly.

On Jul 8, 2017, at 1:16 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> There's lots I agree about - it's a variable sample rate system based off an analog clock, so there's no aliasing to worry about. Worse case would be a ramp wave with steps in, but (I thought) there are measures in place to limit that too. But now the designer seems to be telling me that's wrong.

You seem to be thinking that a variable sample rate based on an analog clock has no aliasing to worry about, but that's totally false on all counts.

The fact that the rate is variable does not remove aliasing. A variable sample rate simply has a variable Nyquist frequency, such that aliasing moves around as the rate varies.

The fact that there is an analog clock does not remove aliasing. Any kind of sampling is susceptible to aliasing if filtering isn't handled correctly. An analog clock merely means that the rate is continuously variable, rather than varied by steps, but that is completely independent of aliasing concerns.

Even without digital quantization, a circuit can introduce aliasing, even if it's pure analog sampling. For example, a Sample-and-Hold circuit can introduce aliasing. Apart from the switch in the S/H circuit, there's nothing digital about it, yet aliasing is still a phenomenon based on the rate of switching between the Sample and Hold modes. Nyquist did his work in 1928, and I'm pretty sure that digital sampling didn't exist at the time, so the origins are entirely analog (although sampling makes time a discrete variable, which is sort of digital in the sense that it's not continuous).

Anyway, off to rest before diving in to the schematic.

Brian