[sdiy] a different kind of slide

[mark verbos]

Hi,


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 problem is that the time it takes for a slide from C1 to C2 is half 
as long as a slide from C1 to C3. That is that the length of the slide 
is dependent on the difference between the last voltage and the current 
voltage. 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 solution I have come up with is a hodge podge of other people's 
working designs. It uses PWM between the current voltage and a sampled 
and held version of the last voltage. The concept is taken from the 
Buchla 248 Multiple Arbitrary Function Generator. The use of this idea 
allows the use of a sequencer to blur the distinction between a 
sequencer and a multi-stage envelope, seamlessly switching between the 
two. The high frequency sawtooth oscillator is shamelessly stolen from 
Grant Richter's linear tracking generator. a comparator receives this 
and a linear ramp. The linear ramp is triggered at the beginning of each 
note that activates the PWM that creates a pseudo crossfade.

There is another comparator that is tripped by the ramp reaching it's 
destination. By patching this output back to the trigger input, an 
oscillator is created that can then be used to step along a sequencer 
that will be creating the voltages to slide between. There is an analog 
switch to enable/disable the slide function so that a row a switches on 
the sequencer can select which steps slide. There is an output of the 
linear ramp that will act as an AD envelope generator with a length the 
relates to the slide length. The ramp length is voltage controllable. 
Allowing the length of individual slide be remotely controlled as well 
as the rate of the VCO that is created when the output pulse is plugged 
to the trigger in.


So far, the circuit is untested. I have posted it here for evaluation at 
www.simple-answer.com/fixed-length-slide.gif
I invite any other ideas how to make it a simpler beast or criticisms of 
why it won't work. It is so far just a discussion piece. I know that a 
portamento built from 8 IC's is too much for some tastes, but I don't 
know any other way to accomplish the goal.


-- 
--
mark verbos

"if you want something done right, build it yourself."