AVRMIDI-CV

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 :-) .

implemented Features

• 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

NORMAL_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.

CONTROL_MODE

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.

Octave tuning

In CONTROL_MODE the octave tuning can be achieved as followes:

1. pressing any note C of an octave (except lowest C (MIDI Note 0) which cannot be adjusted) selects this octave to be tuned
2. 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

3. releasing and repeatedly pressing the same note C of an octave switches through the voice cards
4. 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.

Teststatus

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:

• MIDI-IN
• 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

TODO

• update the PCB layout to remove the known bugs from the prototype PCB
• test remaining untested parts :) little
• add a bootloader