[sdiy] Continuously variable waveshaping (was Behringer Neutron)

[Guy McCusker]

I've been tinkering with this sort of thing for a while so here's a
collection of existing circuits that I know about.

As Vladimir says, the obvious way to do this is to use a bunch of VCAs
and control them as many-way crossfaders somehow. Mutable Instruments
Frames does this with 2164s in a very flexible way. Gosh, things are
easy once you have a micro controller at your disposal.

In the analog realm, the first version I know of is Jürgen Haible's
interpolating scanner.
http://jhaible.com/legacy/tonline_stuff/jh_ipscan.html
This uses a chain of ramp-to-triangle converters with different
thresholds to generate the control voltages for the VCAs. I think that
is similar to what the Doepfer A-144 does. David Dixon's recent design
is kind of similar in its idea if not execution, but does ramp to
trapezoid conversion and is more controllable.

Don Tillman came up with a different design for the scanner controller
http://www.till.com/articles/scanner/ which produces nice curved CV
(actually, control current) sweeps for the VCAs (actually CCAs).
Haible used this idea for the second version of the scanner
http://jhaible.com/legacy/interpolating_scanner_and_scanv/jh_interpolating_scanner_and_scanvib.html

The Tillman circuit seems to have something in common with Barrie
Gilbert's scanning controller in this patent
https://patents.google.com/patent/US5077541
Gilbert later came up with a linear version of this:
https://patents.google.com/patent/US5432478A/

I'd love to know whether Don Tillman was aware of the Gilbert design
when he came up with his one. Don... if you're reading...?

These scanners work really well for morphing between the wave shapes
from a single VCO, because the sources are all nicely in phase. When I
built up the Tillman circuit I sat looking at the scope for far too
long, watching a sawtooth fold itself over into a triangle, then
soften into a sine, and back. But like Vladimir, I bet Behringer are
using a microcontroller.



On Sat, Apr 7, 2018 at 7:33 AM, Vladimir Pantelic <vladoman at gmail.com> wrote:
> given that Behringer can buy 2164s cheap, I bet it's a bunch of these, under
> CPU control...
>
>
> On 06/04/18 21:55, Mike Beauchamp wrote:
>>
>> I've seen continuously variable waveshaping on a few different synths
>> (Voyager, Kobol) and now the Neutron. I've always wondered how this was
>> done, but have never found a schematic for this. Any examples available?
>>
>> Mike
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