[sdiy] a different kind of slide

[media.nai at rcn.com]

>> 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.

An MC-202 is an example of exponential portamento using an RC filter -- a
rheostat sets the time from 0 to 2 seconds.  The slide on the TB-303 is
altogether different. (Although you can somewhat mimic the 303 with a 202
by programming "premonition" notes.)

>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.

Which is not be the case with the TB-303.  On almost every other synth, if
play quickly with a long portamento you never reach the note.  This is the
case with the MC-202, but not the case with the TB-303.  On the 303, slides
always seem to arrive just a hair short of their destination regardless of
tempo.

>(In theory, it never actually reaches, but in practical terms, a certain
>percentage of the distance is close enough).

Right.

>To create this type of glide you need a resistor and a capcitor.

As you know, the 303 is a bit more complicated.  The CV is the output of a
resistor network going into an op-amp.  There are two mux switches
connected to capacitor shunted to ground.  One switch connects to the input
of the op-amp, the other connects to the output of the op-amp.  When slide
is off, only the output is connected.  Since op-amps have such a low output
impedance, there is no portamento effect.  When the slide is on, only the
input is connected.  Since the resistor network has a significantly high
output impedance, the cap charges slowly from the voltage value of the
previously held note.  However, that still is not enough of an explanation.

Now, the resistor network that generates the CV is controlled by a hex
flip-flop, and its clock input is connected to the same uP output that
controls the slide.  So something else is going on, but I haven't quite
figured out what.  Any ideas??

>Linear portamento is more complicated - you need a constant current source
>to charge/discharge the cap.

Yes, that's obviously not the case with the TB-303.

>The reason the 303 had constant time glide is probably because it was
>cheaper.

It seems everything about the 303 was because it was cheaper :)