[sdiy] a different kind of slide

[Colin f]

----- Original Message -----
From: "mark verbos" <a0284520 at addcom.de>
To: "synth DIY" <synth-diy at dropmix.xs4all.nl>
Sent: Monday, March 18, 2002 9:55 AM
Subject: [sdiy] a different kind of slide


> Normally, when the subject of slide or portamento comes up the 2
> variations on the theme that are discussed are linear and exponential
> slide. You of course have control over the speed of the slope.  Since my
> modular is of the Buchla aesthetic, I favor the linear slope variety.
> All controllers should be linear and all VCO, VCF and VCF inputs
> exponential in response to them.

The linear/exponential slopes also differentiate constant rate/constant time
portamento.

> A Roland TB-303 uses a microprocessor based sequencer that
> outputs a slide between notes that is the same length no matter how far
> it slides. For 10 years I have been interested in this type of slide,
> but have never been able to imagine a module that would obtain it.

The 303 doesn't do anything fancy - it just uses exponential i.e. constant
time portamento.
The further from the destination the start note is, the faster the initial
rate of glide.
As the cv approaches it's destination it slows down.
The effect is that no matter how far apart your start and end points are,
the glide takes the same time.
(In theory, it never actually reaches, but in practical terms, a certain
percentage of the distance is close enough).
To create this type of glide you need a resistor and a capcitor.
Linear portamento is more complicated - you need a constant current source
to charge/discharge the cap.
The reason the 303 had constant time glide is probably because it was
cheaper.

Colin f