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).

That's to Eric for this nice comparison!

The MOTM VC Lag has

a) Independent Rise/Fall times
b) no offset issues al la Doepfer because we use the correct parts
c) Lin to EXP and rates in-between with a spin of a knob

Paul S.

Some thoughts of my own on exp vs. lin...

I used to use an Emu mono series keyboard (4000) before I used their 4060
poly kbd. It had a pot to sweep between linear and exp. glide shape. I think
JH has documented just the glide portion of this circuit and it's floating
around somewhere on Synthfool DIY.

It is a very musical and versatile setup. Exp provides equal glide time no
matter what interval you play. It also sounds more natural to your ear, with
that rate slowdown as you approach the final voltage. Lin is also good for
certain playing effects.

The Encore Expressionist does not have exponential glide as noted below, but
just glides linearly at different rate for different intervals to make the
total time come out the same. This does not sound nearly as good as gliding
along an exponential curve. I've asked Tony to consider adding interpolation
tables to allow true exponential glide, and he's considering it for his next
software update. Of course, I've also bombarded him with about 10 other
requests, so I don't know what he'll manage to get in. For us modular users,
it is maybe easier to just use a true analog lag circuit to get what we
want.

Another good use for exponential lag: input a gate signal, get an AR
envelope signal out.

Dave

>
> From: "Eric S. Crawley" <esc@...>
>
> 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.]
>

<snip other good stuff>

Dave,

>I used to use an Emu mono series keyboard (4000) before I used their 4060
>poly kbd. It had a pot to sweep between linear and exp. glide shape. I think
>JH has documented just the glide portion of this circuit and it's floating
>around somewhere on Synthfool DIY.
>
Sounds very cool! 

>It is a very musical and versatile setup. Exp provides equal glide time no
>matter what interval you play. It also sounds more natural to your ear, with
>that rate slowdown as you approach the final voltage. Lin is also good for
>certain playing effects.
>
This confuses me a bit.  I think there are 3 types of glide:

-Linear: linear rate in volts/sec

- Exp: exp rate so the pitch "slows" a bit as it reaches the end point

- "Variable": This is what the Expressionist does.  The rate changes to
make the glide *time* constant.  It is still a linear glide but with a
slope that varies depending on the interval so that the glide time is
constant.  I *think* that is what I said in my original message.  It
certainly isn't exponential.  I think it is useful but it still isn't quite
as musical as exponential glide.

Am I missing something here?  Paul says he is going to allow continuous
control from lin to exp which is really cool.

I had the first ELP album on the CD player this morning and it is really
easy to hear the exponential glide on "Lucky Man".  The glide takes its own
sweet time getting to the final pitch.  I think the reason this is more
appealing is that singers tend to use "exponential glide" when sliding into
the correct pitch.

>I've asked Tony to consider adding interpolation
>tables to allow true exponential glide, and he's considering it for his next
>software update. Of course, I've also bombarded him with about 10 other
>requests, so I don't know what he'll manage to get in. For us modular users,
>it is maybe easier to just use a true analog lag circuit to get what we
>want.
>
Indeed, I think I've resigned myself to needing a lag generator instead of
relying on the MIDI/CV to do it.

>Another good use for exponential lag: input a gate signal, get an AR
>envelope signal out.
>
And if you can make it retrigger itself, it can become an LFO or a Pulser
in Buchla terms.

	Eric

Dave,

>I used to use an Emu mono series keyboard (4000) before I used their 4060
>poly kbd. It had a pot to sweep between linear and exp. glide shape. I think
>JH has documented just the glide portion of this circuit and it's floating
>around somewhere on Synthfool DIY.
>
Sounds very cool! 

>It is a very musical and versatile setup. Exp provides equal glide time no
>matter what interval you play. It also sounds more natural to your ear, with
>that rate slowdown as you approach the final voltage. Lin is also good for
>certain playing effects.
>
This confuses me a bit.  I think there are 3 types of glide:

-Linear: linear rate in volts/sec

- Exp: exp rate so the pitch "slows" a bit as it reaches the end point

- "Variable": This is what the Expressionist does.  The rate changes to
make the glide *time* constant.  It is still a linear glide but with a
slope that varies depending on the interval so that the glide time is
constant.  I *think* that is what I said in my original message.  It
certainly isn't exponential.  I think it is useful but it still isn't quite
as musical as exponential glide.

Am I missing something here?  Paul says he is going to allow continuous
control from lin to exp which is really cool.

I had the first ELP album on the CD player this morning and it is really
easy to hear the exponential glide on "Lucky Man".  The glide takes its own
sweet time getting to the final pitch.  I think the reason this is more
appealing is that singers tend to use "exponential glide" when sliding into
the correct pitch.

>I've asked Tony to consider adding interpolation
>tables to allow true exponential glide, and he's considering it for his next
>software update. Of course, I've also bombarded him with about 10 other
>requests, so I don't know what he'll manage to get in. For us modular users,
>it is maybe easier to just use a true analog lag circuit to get what we
>want.
>
Indeed, I think I've resigned myself to needing a lag generator instead of
relying on the MIDI/CV to do it.

>Another good use for exponential lag: input a gate signal, get an AR
>envelope signal out.
>
And if you can make it retrigger itself, it can become an LFO or a Pulser
in Buchla terms.

	Eric

> From: "Eric S. Crawley" <esc@...>
>
> This confuses me a bit.  I think there are 3 types of glide:
>
> -Linear: linear rate in volts/sec
>
> - Exp: exp rate so the pitch "slows" a bit as it reaches the end point
>
> - "Variable": This is what the Expressionist does.  The rate changes to
> make the glide *time* constant.  It is still a linear glide but with a
> slope that varies depending on the interval so that the glide time is
> constant.  I *think* that is what I said in my original message.  It
> certainly isn't exponential.  I think it is useful but it still
> isn't quite
> as musical as exponential glide.
>
> Am I missing something here?  Paul says he is going to allow continuous
> control from lin to exp which is really cool.

I agree with what you said, I think we're just nitpicking on semantics here.
If you play 1 note and glide to the next on the Expressionist, it glides
linearly, and so sounds just like normal linear glide to the ear (in the
context of just those 2 notes). How about a new term: "variable slope
linear", or as JH likes to call it in his interpolating scanner notes,
"piecewise linear".

> I had the first ELP album on the CD player this morning and it is really
> easy to hear the exponential glide on "Lucky Man".  The glide
> takes its own
> sweet time getting to the final pitch.  I think the reason this is more
> appealing is that singers tend to use "exponential glide" when
> sliding into
> the correct pitch.

Exactly - that's my theory, too.

Dave Bradley
Principal Software Engineer
Engineering Animation, Inc.
daveb@...

At 03:16 PM 11/29/99 -0600, Dave Bradley wrote:
>From: "Dave Bradley" <daveb@...>
>context of just those 2 notes). How about a new term: "variable slope
>linear", or as JH likes to call it in his interpolating scanner notes,
>"piecewise linear".

I like "Variable Slope Linear" or "Linear w/ Variable Slope"!