MOTM

So, I tried some experiments to understand linear vs. exponential
lag/portamento/slew limiting and found some very wide ranging results.
This wasn't very scientific (YMMV) and I'm still not sure I understand all
the ins and outs and terminology but here's what I found:

Encore Expressionist:
This MIDI/CV converter has "fixed" and "variable" portamento.  "Fixed" is
an extremely linear rate of glide.  "Variable" changes the rate to make all
the slides the same amount of time, regardless of the key distance.  [In
some sense, these terms are backwards in terms of glide time, IMHO.]

Korg Mono/Poly:
This was very linear (with different rates for the different oscillators)!

Oberheim Xpander:
The lag generators in the expander appear to be linear with *very* long
time values.  I'm guessing that you could combine a lag generator with
tracking generator and make something more exponential.

Juno-106:
Very linear with a long range.

Minimoog:
This wasn't really "linear" in that the rate tended to slow a bit at the
very end of the slide.  However, it was more linear than a Moog modular
keyboard.  This slight exponential behavior might be part of the key that
makes the Minimoog portamento very "playable"

Moog Modular:
The keyboard I used was so old that it didn't have a model number and the
voltage will slowly increase when you let up on a key because of the very
simple design (a real bummer when trying to do drones...).  The portamento
on this keyboard is *very* long and *very* exponential.  It will slide up
the keyboard very fast and take its own sweet time settling into the final
note.  Again, this seems very *playable* to my ears (on lower settings,
obviously).

Doepfer A-170 Dual Slew Limiter:
Adding this in really screwed up my keyboard tracking so it was hard to
tell what was really happening since the intervals I was playing didn't
match what I was hearing.  One of the slew limiters has variable rise and
fall times so you can set the portamento going up the keyboard to one rate
and going down to another rate (something Paul sounds like he is planning
to do).  This tended to me more linear in the low time values and became
more exponential in the higher values.  Because of the keyboard tracking
problems, I couldn't really determine the "playability" of the portamento.
[Note to Paul: Please don't let this happen with the MOTM version!]

Conclusions:
Exponential curves are certainly more playable to my ear.  The more
"modern" synths with digital keyboard control or MIDI tended to use very
linear portamento which is quite playable at lower values where the lin/exp
differences are small.  As the rates increase the linear becomes less
playable but more "predictable".  So, for really long sweeps, the linear
might work better.

Anyway, I'd like to see switchable lin/exp control with variable rise and
fall times (and I could also use it as a ADR EG or "Pulser" if it can be
made to self-retrigger).