vlampreia / midi2cv Goto Github PK

Another MIDI to CV project!
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This project arose out of the desire to control antiquated drum machines which
respond to analogue pulses with MIDI enabled devices. MIDI to CV solutions exist
in abundance, nonetheless I saw this as an opportunity to involve myself in more
embedded software development and as an introduction to hardware development.

This project does not consider modular synthesiser CV in any way whatsoever. The
end devices this converter is targeted to drive respond to simple voltage-level
triggers, and so it lacks output voltage scaling and MIDI processing for
anything other than channel, note, and velocity parameters at this time.


SYSTEM OVERVIEW AND REQUIREMENTS
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At a top level, the system processes MIDI and outputs up to 8 analogue pulses
simultaneously. It must be capable of mapping channel, note, and velocity data
from MIDI packets to their respective analogue values and CV DACs as programmed
by the user, in real time and with imperceptible jitter.

MIDI channel and note to CV channel mappings are user configurable using the
front panel, which provides a 3 character display and a set of momentary
switches. User settings are stored in the MCU flash memory and persist after a
loss of power.


INTERFACE
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The system is abstracted into programmable CV channels. Each CV channel has a
MIDI channel and note assigned to it. Channels may also be configured to respond
to all MIDI channels and or all note values.

The user interface consists of a display showing the current mode and value, and
three buttons, one to cycle mode (CV channel, MIDI channel, and MIDI note, in
that order), and two to increment and decrement the value. The omni value can be
selected by holding down both value buttons.


FIRMWARE
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The MCU firmware is written in C and is listed under ./firmware.

This is currently a work in progress.

A makefile is present to facilitate building and flashing the firmware. It
targets the ATMega328P-PU using a USBASP programmer.


HARDWARE
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The hardware platform is designed using KiCAD and is listed under ./hardware.

This is currently a work in progress. A full BOM will be rendered once the
hardware has been finalised.

The platform consists of two boards interconnected via standard pin headers:
- The main board hosting the MCU, power and MIDI input stages, as well as the CV
  output stage.
- The HUI board hosting the display and navigation buttons.

The MCU is an ATMega328, selected for it's simplicity and active community base,
it is a fairly capable package boasting a 20MHz clock, 2KB RAM, USART and a SPI.
The 328 lacks any digital to analogue conversion, for this the 8bit MCP4802 was
selected. Each IC packages 2 DACs, addressable via SPI by the MCU.

The MIDI input stage follows the standards set out by the MIDI spec. A H11L1 was
selected for the optoisolator, which is identical to the PC900 suggested in the
spec.

Control voltage outputs are buffered internally by the DACs. Output voltage can
vary from 0v to about 4v on max gain.

Supply input is regulated and can scale from 8v to 35v.


KNOWN LIMITATIONS AND FUTURE IMPROVEMENTS
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- The output voltage scale is an unknown and can change from target device to
  device. The hardware currently lacks support for programmable output voltage
  scaling.