Martin Czech martin.czech at
Mon Jun 23 16:12:05 CEST 1997

> I'd like to see the ASCIImatic.  What type of waveshaping could one expect
> from the circuit?  Ideally one would go from sine to tri to saw to square,
> but that seems like a lot to ask.  One of the Serge VCOs has a pot that
> does that, but I don't know if it's CVable.  Buchla had some CVable
> waveshaping on his system 200 VCOs, but I think it was simpler, like sine
> to saw (or tri?) and tri to square.

Here you are:
e1      R2   |                    |
  ---/\/\/\--|_____|/|________    |
e2       R3  |     |\|       |    |
   ---\/\/\--|  --\   D2     | D1 |
             |  |  \         |    |    R4        R5
             ---| - \---------|/|-+---/\/\/\-----/\/\/\--
                |   /         |\| |          |          |
             ---| +/              |          |          |
             |  | / OP1           /          | --\      |
             |  --             P1 \---       | |  \     |    R6
             |                    /  |       --|-  \-------/\/\/\---SUM
            ---                   \   -------- |+  /
                                  |            |  / OP2
                                 ---           --/


Nothing new, as I said. OP1 acts as "precision" rectifier, with D1 and D2.
The voltage at pot P1 will remain at 0, as long as the sum of input e1 and
input e2 is not negative. e1 could be the signal to be shaped, e2 could be a
controll-voltage (or pot) to controll, when the circuit is "on".
E.G. if e2 is 5V, e1 has to be lower than -5V in order to have an effect.
(If diodes are reversed, the circuit will work with positive input sum).
The voltage at P1 is either inverted or not inverted with OP2, acting
as "bipolar coefficient" from -1 to 1. This depends on the setting of P1.
(This coefficient could also be voltage controlled using an ota, like in the
Harris 3080 app. note. Look at Harris web site).
Now, if a number N of such circuits (positive and negative diode polarity) with
the same input signal e1 but different voltages e2i are summed (not shown),
you get a pice-wise-linear-funktion. The voltages e2i control, what branches
will turn on at what voltages, one after another,
and the regulation of ALL active coefficients with P1i
will determine the actual slope. Thus the funktion is always steady, no jumps.

The fact, that all active elements determine the slope may be confusing for
the user. So another idea could be to replace the rectifiers with cmos switches,
controlled by a chain of comparators, so that every coefficient will have it's own
input voltage window. Some kind of offset pot needs to be aplied for each window, in
order to get steady functions.


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