MIDI-to-CV converters

[WeAreAs1 at aol.com]

In a message dated 5/12/00 10:30:28 AM, you wrote:

<<I've been messing around with modifications to the SIEL DK-600 (splitting
the controls for LFOs I & II, adding external audio inputs, etc), and
noticed that, like other early MIDI synths (the SCI Six-Trak comes to
mind), this one converts MIDI information -- either incoming through the
port, or from the internal CPU board -- into analog control voltages;  the
DK-600 provides six perfectly useful pitch cvs, dynamics cvs and gates that
can drive external analog instruments.  The only problem with this
particular synth is the rotating note assignment, which makes it difficult
to use when driving something like a Pro-One.

My question is:  has anyone out there ever tried something like this with
the Six-Trak or some other early MIDI synth that employed MONO mode?  (sold
all my Six-Traks some time ago;  never got a chance to try this out).>>

Have you actually tried driving a VCO with those filter CV's?  If you did try 
it, you probably noticed that the pitch tracking was not exactly perfectly in 
tune, nor was it stable over time.  These pitch CV circuits found in 
DCO-based synths are usually not that useful for driving oscillators, unless 
you don't care about tuning accuracy.  These CV's are not really designed for 
accurate tuning, since they don't have to drive oscillators, per se.  Also, 
it's typical that the filter pitch CV signal is pre-summed (in software) with 
other CV's that are intended for the filter, such as cutoff freq., velocity 
CV, and envelope CV.  (this may not be the case with the DK600, since it has 
hardware envelope generators, but it's certainly true of most of the others, 
such as the Oberheim Matrix 6/1000, Roland Juno 106, JX3P, JX8P, JX10, etc.)

In the case of the Sequential Six-Trak (and its derivatives such as the Max 
and the Split-Eight), the VCO pitch CV that they are using in there is not 1 
volt per octave - it's something like around 0.5 volts per octave.  This, 
presumably, was done so that they could keep all their CV's within a 0 to 5 
volt range, but still have around ten octaves of pitch range.  So, you'd have 
to figure out some way to scale that signal for your external synths.  
Scaling that CV to a larger factor wouldn't be all that difficult, but a 
larger problem would soon become apparent:  These synths, which have real 
VCO's (Curtis all-in-one VCO/VCF/VCA chips), also have an auto-tune system 
that actually measures the output of each VCO at various pitch intervals 
(about three intervals per octave), and then it creates a table of pitch CV 
offset corrections for EACH VCO that corresponds to the measured inaccuracies 
of each VCO at all those various intervals.  It then applies these offsets to 
the final CV for each VCO.  This means that if you tried to use one of those 
CV's to drive an external oscillator, the autotune system would be applying 
whatever weird offsets the internal oscillators were requiring at that moment 
to the external VCO.  The likelihood that the external VCO would play in tune 
with those offsets would be very, very slim.

On possible solution to this dilemma would be to have the autotune measure 
the pitch output of the external VCO's, rather that the internal ones.  This 
could be done by disconnecting the internal VCO's from the point in the 
circuit where the CPU looks at their current pitches, and connecting the 
square wave outputs of your external VCO's there instead.  This way, the 
autotune system would create offset tables that would match your external 
VCO's (and if you're using more than one VCO per voice, you'd better make 
sure the scaling of the VCO's in each voice is PERFECTLY matched, or the 
autotune will throw the voice out of tune with itself).

You'd also need to make some kind of muting circuit for your external synth, 
so that it wouldn't make all that godawful racket while the autotuning 
operation is being done (a few seconds of weird pitch bleeps, sweeping 
several times from the bottom to the top of the pitch ranges of each VCO - at 
full volume, of course!).  There would be some other pesky housekeeping 
details as well, to make such a system work - I won't go into all of them now.

Furthermore, those Sequential instruments also pre-sum several various 
parameter CV's into the one final analog CV that is sent to the VCO (and one 
CV for the VCF, etc.).  This summed CV could include pitch bend, LFO, octave 
range, envelope output, velocity output, etc..  These additional CV signals 
may or may not be useful for your external synth (especially the ones for the 
filter).

Also, since the Six-Trak's envelopes are created in software, the instrument 
has no actual gate signal anywhere inside it that you could tap into to drive 
your external synth.  The only solution would be to set program the 
Six-Trak's VCA envelope to a "square" organ-type envelope, and use the VCA CV 
as your gate signal.  (this might make it difficult to utilize any velocity 
CV that the Six-Trak might be able to provide, though)

The one advantage the Sequential instruments would have, if you tried to use 
them as MIDI to CV converters, is that they can be put into MIDI Mono Mode, 
which would allow you to have them receive on six separate MIDI channels for 
six separate external voices, eliminating the rotating voice allocation 
problem that the DK600 would have.

Nevertheless, unless you already have an old broken-down Six-Trak sitting 
around, and you're really itching to take on this sort of challenge, it would 
probably be more cost effective and reliable to go out and spend the $600.00 
on one of the cuurently available top-of-the-line polyphonic MIDI-to-CV 
converters, such as the Kenton or Encore Electronics units.  (Six-Traks are 
going for about $300 on Ebay these days, not that they are worth it...)

Michael Bacich