Installing patch points

[WeAreAs1 at aol.com]

On Fri, 26 Mar 1999, wils0450 at tc.umn.edu wrote:

<< I was staring blankly at my Prophet 600 trying to come up with a good
layout for the rackmounting/modularisation.I started thinking about the
patch points and where to put them and decided that replacing the curremt
modulation switches with patch points made sense (does it?). 
My question:What does placing a patch point in a circuit require/how are
they usually wired up?

>>

It sounds like you haven't examined the Prophet 600 schematics yet, or maybe
just not closely enough.

The Prophet 600 would not be a very good candidate for patchable
modularization, since most of its modulation signals are created digitally,
and are already summed by the time they are converted into voltages.

In other words, the LFO signal, pitch wheel data, envelope modulation, coarse
and fine tuning, as well as simple note data are all summed together into one
control voltage *in the software*.  There is no way that you could, for
instance, unplug the LFO, and plug an external control signal in its place.
The same is true for the filter control signals.  All summed in software.

It's also important to note that the Prophet 600 does not necessarily have 1
volt per octave CV response.  This is because Sequential uses a clever
software system (as part of its auto-tuning routine) to "scale" each VCO and
VCF in software.  The P600 does have VCO scaling trimmers on each VCO, but
they are there only for rough approximations of 1v/octave scaling.  The
software does the actual fine scaling adjustments by analyzing each VCO's
tuning over the entire range, then adding minute CV offsets to make that
oscillator play in tune in any range of the keyboard.  Each VCO has a new,
separate offset for every few semitone steps of its CV range, and this large
table of offsets is stored in a temporary buffer and updated every time you
press auto-tune.

What this means is that even though the VCO's play in perfect tune with each
other, if you tried to play them from an external CV, they would surely be out
of tune.  The only way to make them work would be to add legitimate, complete
VCO scaling calibration trim circuitry to each VCO, along with a way to
disconnect it from all of the P600's auto-tune and CV-generating circuitry,
and drive them with an external CV (for initial calibration).  This is how
they used to do it back in the early days of polyphonic analog synths - for
example, on the OBx, the Jupiter 8, and on Rev 2 Prophet Fives.  Yes, these
units did have auto-tune, but they just added a simple, *single* fine tuning
offset to each VCO that affected its entire range, not correcting for specific
ranges up and down its range (so if the VCO's scaling was off, auto-tuning
wouldn't help).  It's also one reason why most of those units are much more
difficult to calibrate, are generally less reliable, and for the most part,
sound less in tune than later units that use the software scaling system.

I do agree however, that a P600 in a rackmount package would be nice, with or
without modularization.  Its keyboard stinks, and those membrane switches...
someone should be severely punished for those!

Michael Bacich