[sdiy] Moog Rogue question...

[Pete Hartman]

I'm working on a Moog Rogue...  One thing that bothers the owner is
that the Rogue's voltage range is 'rooted' at F -- the lowest key on
the keybed is F and that's 0V -- but most CV sources are 'rooted' at C
(for example both of the MIDI-CV modules I have generate 0/1/2/3/4/5V
at C0, C1, C2, C3, etc on the keyboard).  This can be worked around
with the pitch wheel and tune knob in concert, but then you can't
really use the pitch wheel for anything else if you want things to
stay in tune.

My thought is that it ought to be feasible to add another voltage
source that is switched between 0V and whatever is necessary to offset
things so that the incoming voltage maps to an actual "C" for the
Rogue on the even 1V boundaries.  I expect that something with a
trimmer that matches the pitch wheel's input to the CV summing op amp
ought to be good.  Switch the "input" from 0V for no effect to the
trimmer for the correct offset.  (obviously I'll be testing this
theory with clips before taking an iron to it).

However, there's a detail in that section whose purpose I'm unsure about.

http://elmegil.dynathome.net/~elmegil/rogue-pitch.png

The pitch wheel is a simple +12V / -12V divider that goes through
parallel diodes pointing in opposite directions, then through a
resistor into the summing node.  I don't understand what the diodes
are intended to accomplish, and I'm unsure whether the specific type
is important or not.  It's hard to read in the screenshot, but they
are FDH333--available inexpensively, but with some delay and shipping
expense.

So the questions are:

1) what does this diode configuration accomplish?

2) Given whatever that purpose is, is the type of diode important?
This isn't a common signal diode, the datasheet describes it as "high
contraction, low leakage".  So low leakage is obvious enough, but I'm
not clear on what "high contraction" is.

Thanks for any insight :)

Pete