[sdiy] SAW core VCO flyback time

[rsdio at audiobanshee.com]

On Aug 30, 2016, at 9:42 PM, mskala at ansuz.sooke.bc.ca wrote:
> On Tue, 30 Aug 2016, Donald Tillman wrote:
>> The high C on a traditional piano is 4186 Hz.
>> 
>> A 2uSec reset delay would pull that down to 4152 Hz, for a 35 Hz beat.
> 
> Maybe we're just looking at a difference in terminology - I thought the
> discussion was about how long it took to discharge the capacitor, which
> shows up in the fall time of a signal that is still at the right
> frequency; but you're saying reset "delay" and assuming it is added onto
> the correct period of the signal, necessarily giving a signal of the wrong
> frequency by some amount.
> 
> A 2usec reset *time* only pulls down the frequency as described if the
> rising slope is 2usec too long in context, and the reset pulses are 2usec
> too far apart.  The whole wave ought to be 1/4186 of a second; if it
> isn't, yes, that's a problem, but I think it's a problem with tracking,
> not necessarily with how one drains the capacitor.
> 
> The time taken to reset the capacitor is only related to the tracking at
> all in some core designs - we just had an interesting discussion of other
> ways to build cores where it wouldn't be - and when it is an issue, there
> are ways to deal with it that may or may not work well, so that the more
> interesting question is "How good is your tracking?", not "What's your
> reset time?"

Good points.

In the (evil) DCO, the frequency of the reset pulse's leading edge is crystal controlled, so the time that it takes to drain the capacitor is irrelevant to frequency. The spectrum will be affected, but not the pitch.

Brian Willoughby