Calibration
There are two trimmers, V/OCT and INIT, on the midiDAC PCB. Both of them are designed
to allow you to make the midiDAC respond correctly to the rest of the modular system. If you
already have a perfectly tuned and calibrated system, it is best to trim the midiDAC to suit
your system rather than the other way around.
V/OCT
This controls the scaling of the pitch CV output. That is, how much the VCO pitch will
change with every note increment on the controlling keyboard or sequencer.
If you already have a calibrated set of VCOs with your current system it is best to adjust the
midiDAC’s V/OCT trimmer to match your existing set up. Therefore, connect the midiDAC
to one of your VCOs and adjust V/OCT until you get perfect tuning.
Remember altering V/OCT will affect all notes on the keyboard, so you need to be looking at
getting an octave interval between the notes rather than setting absolute pitch of one note. The
INIT trimmer can be adjusted later for setting absolute frequency.
If you do not have an existing system and the midiDAC is your first module, a perfectly
calibrated midiDAC will then ensure that the rest of the system will be true. Connect the
midiDAC to a keyboard, sequencer, or computer midi interface and power up. Allow the
module to settle by leaving it on for ten minutes or so.
Now play the highest C on your controller keyboard. Use the keyboard's octave select buttons
to ensure your controller is set to produce the highest octave. Your keyboard will have sent
one of two notes. Older midi controllers may actually have sent midi note 127 which is G9. If
this is the case you should adjust V/OCT until pin 1 of U1 gives -10.58V (or -10V.583V if
your voltmeter is that accurate). If your controller sent midi note number 120 which is C9 then
you should adjust V/OCT until pin 1 of U1 gives exactly -10.00V.
If you are unsure which note your controller is sending then don't worry. If it's sending G9
you'll be able to get -10.58V easily enough. If it's sending C9 then you won't be able to trim to
-10.58V but you will be able to trim to -10.00V. In either case you should verify that for every
octave you go up the keyboard the voltage on pin 1, U1 changes by -1.00V. That is if C9 is
-10V, then C8 will be -9V, C7 will be -8V and so on.
If you are using a DAW or other midi software program then send the midiDAC note 120 and
adjust V/OCT until pin 1 of U1 gives -10.000V.
If you have an older issue 4 midiDAC you will need to correctly adjust the Tune control on
the front panel before you measure the output U1 pin 1. You need to centralise the Tune
control and ensure that the tune pot's wiper voltage is exactly 0.000V with respect to module
ground (0V). The easiest way to monitor this voltage is to put your meter's red lead, or scope
probe, on the left hand end of R4.
10
