Oakley midiDAC User Manual - page 9
Two forms of setting the initial pitch CV are provided. One is the TUNE pot mounted on the
front of the panel. The other is INIT, which is a multiturn trimmer that will allow precise
setting of the initial pitch CV, and thus aid centralising the TUNE pot’s range. In this version
of the midiDAC, the pot and trimmer take their end voltages from the +5V and -5V reference
voltages. This should lead to greater stability of the CV output even if the power supplies
change slightly.
There are 12 notes in one octave, and a jump of 1V must represent one octave when applied
to a VCO. Thus, 1/12 = 0.083333V or 83.33mV per semitone step with a perfect DAC. There
are 127 notes in the midi scale, so the highest voltage must be 127 x 83.3mV = 10.58V.
With a -5V reference the smallest step our DAC will increment is only 78.7mV, so we need to
amplify up the pitch CV by around 1.06 to get the desired 83.3mV stepping. This is done in
the first summing circuit. The V/OCT trimmer allows to fine tune this gain to match your
midiDAC to your VCO’s sensitivity.
The PIC generates the gate signal directly from on if its outputs. It actually an inverted gate to
allow the main gate output to be generated by a single NPN transistor, Q2. The collector may
be wired to either the +5V or the +15V rail depending on the size of the gate signal required.
The GATE LED is connected to a 2 pin header on the PCB, labelled ‘Gate’. Its driven by a
spare gate within the inverter chip, U6. Note that if you use the suggested layout, the LED
may be mounted straight into the board for a tidy appearance.
U11 provides the regulated +5V supply for the PIC and DAC. R54 and C16 provide power
supply decoupling from the higher +15V rail. The four ferrites on the board, L1 to 4, act as
high frequency suppression to remove any digital noise from the power supply. The level
shifting chip, U15 and the demultiplexor, U14 also have their own decoupled and filtered
supply. This is to prevent the fast switching pulses generated by U15 from affecting the power
supplies in the modular. Any digital noise would be heard as audible whining from any audio
output.
PIC Firmware Data Version 2.2
The PIC could in theory generate 8 output control ‘voltages’ when used with a single DAC
and 8-channel demultiplexer. However, we only use six of them in this version of the
midiDAC and tbDAC firmware:
Output
CV generated
1
Pitch CV
2
Modulation Wheel
3
Note on velocity
4
Pitch bender
5
Aftertouch
6
CC 100
7
Reserved
8
Reserved
9