This is my attempt to implement a MIDI-CV-Converter using a Atmel ATMEGA 8 microprocessor in combination with a DAC8568 as 16 Bit DAC. For the DAC8568 can only produce voltages between 0-5V I am using some OpAmps to double that voltage to for a full range of 0-10V out of my MIDI-CV-Converter.
I started this project as I was in need of a MIDI to CV Converter for my crOwBX 4 Voice Synthesizer (see www.cs80.com/crowbx/). I did not want to buy one and therefor naturally am now building and programming my own. It is not yet finished - no warranty at all what so ever :-) .
- 4 Voice CV + 4 Gate
- switchable polyphonic/unison mode
- 4 additional CV outputs assignable
- either velocity per voice...
- ... 4 CC outputs (MIDI learn)...
- ... or 2 soft LFO (switchable) + 2 clock divided trigger outputs
- syncable to MIDI Clock or free running
- adjustable LFO/clock trigger rate
- 4 different waveshapes for the LFOs (triangle, pulse, sawtooth, reverse sawtooth)
- accurate octave tuning (~500 steps per semitone)
- each C note can be tuned seperately to even out non-linear behavior
- MIDI learn for assigning CC controls
Two modes explained
As the firmware grows more and more there are now two different modes: NORMAL_MODE and CONTROL_MODE
In NORMAL_MODE the unit works as a MIDI-CV converter to be played polyphonically or in unison. 4 Voices just as explained above - nothing too special here.
In CONTROL_MODE different parameters of the firmware can be set up to liking and saved to the EEPROM (persistent throughout reboot). This includes
- Octave tuning
- CC assignments
- Velocity/CC mode flag
To reach CONTROL_MODE the button connected to PC0 must be held down for at least 2 seconds (LED flashes fast while pressing and then changes to slower flashing when ready for CONTROL_MODE). If the button is not pressed until the flashing light flashes slow the unit switches back to NORMAL_MODE. To exit CONTROL_MODE saving the adjustments the button connected to PC0 must be held down again for at least 2 seconds (LED flashes fast while pressing and then changes to constant light when back to NORMAL_MODE). If the Button is not presset until the flashing light changes to constant light the adjustments are not saved to EEPROM and thus the editing in CONTROL_MODE is aborted. In CONTROL_MODE the MIDI-CV converter switches to unison. It will however switch back to whatever has been chosen on the UI on exit of CONTROL_MODE.
In CONTROL_MODE the octave tuning can be achieved as followes:
- pressing any note C of an octave (except lowest C (MIDI Note 0) which cannot be adjusted) selects this octave to be tuned
- while still pressing note C of the octave to be tuned press
- note D to coarse tune down the C
- note E to finer tune down the C
- note F to make final adjustments downwards
- note G to make final adjustments upwards
- note A to finer tune up the C
- note B to coarse tune up the C
- releasing and repeatedly pressing the same note C of an octave switches through the voice cards
- repeat the steps for all octaves as necessary
CC assignemnt (MIDI learn)
In CONTROL_MODE CC assignment (MIDI learn) is achieved by holding down the
- lowest C (MIDI Note 0) for CC-output 1
- lowest C# (MIDI Note 1) for CC-output 2
- lowest D (MIDI Note 2) for CC-output 3
- lowest D# (MIDI Note 3) for CC-output 4
The standard CC assignments are
- CC 16 - output 1
- CC 17 - output 2
- CC 18 - output 3
- CC 19 - output 4
switching Velocity or CC output
In CONTROL_MODE MIDI Note 4 (lowest E) toggles between using note velocity (polyphonic or unison depending on the selected mode) or CC values as source for the CV-conversion.
The code has been tested on the hardware prototype PCB designed for it and runs as expected. I have a growing testing program to test whether or not the data structures and algorithms/functions work as expected (on my Linux-PC).
Tested in hardware so far:
- DAC output voltages (polyphonic mode, unison mode, lfo and velocity outputs)
- doubling OpAmp-circuit
- syncing LFO to MIDI Clock in different rates
- rate adjustment in synced and free running LFO mode via potentiometers
- shift registers input for multiple switches
- MIDI channel switching via dip switch
- cmos logic for switching the lfo wave forms to get 4 switch positions down to 2 input bits
- MIDI THRU
Test not completed for:
- Clock divided trigger outputs
- update the PCB layout to remove the known bugs from the prototype PCB
- test remaining untested parts :) little
- add a bootloader