[sdiy] SAW core VCO flyback time

[mskala at ansuz.sooke.bc.ca]

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?"

-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before principles.
http://ansuz.sooke.bc.ca/