[sdiy] CEM3340 question -- interpolating scanners (again)

David G Dixon dixon at mail.ubc.ca
Thu Dec 3 19:19:20 CET 2020

I have both 4-channel and 8-channel interpolating scanners that work
perfectly and are not terribly parts-intensive.  You can see YouTube
demonstrations of them here:

My "simple" 8-channel scanner, is shown here:


There is also a slightly less parts intensive 4-channel version.  This
module works by controlling a single linear crossfader to process two
adjacent channels in a group of 8 (or 4) channels.  The scanning is
completely seamless and can be done at audio rates with no glitching.

My older 4-channel scanner design with variable width and spacing of the
control trapezoids, is shown here:


The beauty of this older design (which I've just redesigned and built again
in Euro format for a local synth enthusiast here in Vancouver) is that it
can be built just as a trapezoid generator, and used to drive any quad-VCA
module (such as the Intellijel Quad VCA, or two Intellijel uVCAs).  This
option involves very little circuitry.  I've redesigned this circuit to put
not only the fader, but also the trapezoid width, under voltage control, and
I use an onboard linear VCA to keep the trapezoids perfectly within the
fader field regardless of the width control.  This scanner can be adjusted
to go from complete overlap of all channels to significant separation (with
dead-space in between) of the channels.  Regardless of the width setting,
Channel 1 will come on from the bottom of the fader, and Channel 4 will just
disappear at the top of the fader.

I will happily share schematics for these circuits, or make PCBs for a very
reasonable price (through-hole Doc Sketchy specials -- not terribly small,
but a real pleasure to stuff and solder).  For a bit more moolah I will
happily stuff and test the boards, and for a bit more, I will install them
behind 5U panels.  If you are a Eurorack user, I can do that too, but the
modules are pretty deep.  Here's a couple pictures of my eurorack 4-channel
variable width scanner buddy (for external VCAs) that I just delivered two
days ago:


The top board (the one you can see) is the trapezoid generator -- four
identical circuits generating -0.6V to +5.0V trapezoids at different
locations on the main CV ramp to turn on a linear VCA (assuming the VCA is
at unity gain when the CV is 5V).  The bottom board (which you can't see) is
the CV control board, processing the panel controls and CV inputs to
generate the voltages to control the position and width of the trapezoids.
These boards are 2.1" wide and 3.5" long, and they plug into each other.  I
made this circuit on two boards to accommodate Eurorack.  I could rearrange
the various inputs and outputs so that it would actually plug into the panel
PCB in true Eurorack style, but I was in a hurry to get this done.
So, if you watch the second video, you will notice that the width control
(the red knob) alters the width of the fader field such that the CV must be
adjusted to put all the voices in the field symmetrically.  I've solved that
problem with my new circuit.  To demonstrate, the following plots show the
position and width of the control trapezoids across the entire fader range
as a function of the Width voltage (which I called "Range" in the video):

Notice how Channels 1 and 4 just appear and disappear at the bottom and top
of the fader range, respectively, no matter the Width voltage.  This
involved a bit of algebra.
Anyway, back to your previously scheduled programming....

-----Original Message-----
From: Synth-diy [ <mailto:synth-diy-bounces at synth-diy.org>
mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Joel B
Sent: Thursday, December 03, 2020 1:30 AM
To: Synth-diy at synth-diy.org
Subject: [sdiy] CEM3340 question

[CAUTION: Non-UBC Email]

The Behringer Neutron uses two 3340s but it has a feature that I'm not
seeing in the CEM3340 data sheet - voltage controlled wave shape - you can
sweep between four different shapes. It's a little glitchy unless you
modulate at a fairly low frequency. Yet when I look at what is available
otherwise, especially in Eurorack format using this chip, nothing has that

How did Behringer implement that?


Sent from my iPhone

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