New(?) ideas for the new year

Rene Schmitz uzs159 at uni-bonn.de
Sat Jan 3 20:44:57 CET 1998


Hi all, and a happy and successful new year!

During the holidays I had some interesting ideas:

A waveshaper for a VCO, that uses an integrator(!) to convert squarewaves
to triangles. Well using a standard opamp buffered integrator
will result in unwanted amplitude drop at high frequencies.
But when one uses an OTA integrator controlled by the exponential converter,
the frequency drop does not occur. 
(This is similar to low pass filtering with the right cutoff freq.)
The first advantage is that one can integrate a pulse width modulated square
and 
gets interesting waveshapes. And the second advantage is that it avoids
rectifiers,
with all the imperfections of the diodes. And the problems with limited 
slew rate of the sawtooth. I wonder why I never found a synth that uses this
method.

The other thing is: I calculated a method of temperature compensation with
cheap negative tempco resistors. (Those who look similar to ceramic caps)
Most of them have a negative exponential behaviour, with a tempco of -4% / deg .
By adding a resistor eleven times the value of the NTC one gets a resulting 
coefficient of about -3333ppm/deg . Of course one would have to place it
into the 
upper leg of a voltage divider. (For example replace the 56K in the ASM-1 with
a 51K resistor + 4.7K NTC in series.) I did some simulations about it. It
seems to 
work as good as the Q81, and improves the costs: they are about 25cents in this 
country and easy to find.

What do you think?

Bye Rene
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