[sdiy] Glide question

[David G. Dixon]

> There are two ways of doing glide/portamento:
> - "constant time" in which sliding between C3 and C5 takes as much
> time as sliding between C3 and C4.
> - "constant rate" in which sliding between C3 and C5 takes twice as
> much time as sliding between C3 and C4.
> 
> I might be wrong about this, but a RC filter on the CV will make it
> constant time. For voltage control, any VCF design will do - glide is
> just a filter/slew limiting of the pitch CV signal. Actually, a filter
> with resonance / ripples (which will translate into overshoot in the
> time domain) might make for a cool "natural" (?) portamento effect
> simulating a singer or instrumentist moving up and down zeroing on the
> target pitch.

An RC circuit imposes an exponential decay on the voltage.  Hence, the
"take-off" is always faster than the "landing".  This could be linearized by
using an integrator, and feedback on a pot could even allow the response to
be varied from linear to log (I think).

> Another interesting variation is to have distinct RC constants for the
> up and down segments, like on this thing which looks like a nice
> portamento factory:
> http://www.synthtech.com/motm820.html

I believe you could still use both halves of an LM13700 for that.  You're
essentially talking about an AR envelope generator with voltage-controlled
time constants.