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Eurorack MIDI to 16x16 CV Gate Module

This is a big one. I have been pursuing an idea to make all the modules in a full Eurorack signal chain and to be able to do polyphonic voices in it. The first thing in the that chain is a midi module that can take midi from the computer or keyboard and turn it into signals for other modules. 

Early on I needed a module like that so I bought a Befaco Midi Thing kit from Thonk and started using it, but I wanted to have the ability to drive even more outputs.

So this module has 16 CV outputs and 16 Gates. It consumes 18HP. It has MIDI input and output. the brains of the operation is an Arduino Pro Mini which I've written firmware for to handle polyphonic input (more on that later). There's a single button on the front which for now lets me set a MIDI channel. This thing doesn't have nearly as much configurability as the Midi Thing but I don't have to open a manual to change the main thing I need to configure.

Design Process

This design is much more in my comfort zone than most of the analog modules. The 'hard' part of this one is the digital design and code, but that's my day job so it's a place I can flex.

Like most of my designs this one starts on the breadboard.

Obviously there aren't 16 outputs on that breadboard. There's an Arduino and a breakout board for a MCP4922 DAC and some LEDs. I used this breadboard to sort out the driver code for the DAC.

When I went from breadboard to PCB it got complicated. One issue is component shortages, I couldn't find the MCP4922 or any DAC good enough for this at the time in a surface mount package, so I went with the DIP through hole package instead. Driving 8 of those (they're dual output DACs) and 16 gates presents challenges for the Arduino Pro Mini because it doesn't have enough pins. So the PCB also has two 74HC595 Serial to Parallel shift registers for that stuff.

Every available pin on the Arduino is used in the v1 PCB for this design, but two of them (A6 and A7) are not actually usable the way I intended. They can't be outputs so the LEDs I wanted to drive with them can't work.

In the end there were 28 ICs, 34 jacks and more than 200 resistors on this PCB. Even splitting it into two PCBs and using surface mount components for most things it was a challenge to layout and route. Here's a screenshot from in the middle of the design (many components changed along the way).

With each module I get a bit better at the layouts. Now I start with the front panel components to get them in the best locations and then the through hole and finally the surface mount. That general order makes it a lot less likely to need to redo a whole bunch of layout and routing. I probably rerouted portions of this PCB at least 3 times to get it to work though.

There are simulations in the schematic for the output stages, but they're pretty simple, the CVs and gates are amplified to my desired CV range of 0-10V (almost). There's a flaw in the CV output circuit though, more on that later.

Code

The code for this is functional for 16 voice polyphonic output. I won't go into too much detail on it here, the code is likely to change over time. I'll cover the existing features though.
  • The MIDI channel can be configured. Press and hold the button for 5 seconds and the current channel selection will blink on an LED. Press the button to change to another channel. Press and hold the button to exit and save the selected channel. The channel config is persistent through power cycles.
  • The incoming MIDI traffic is passed to the output ('Thru' Midi).
  • Incoming note on/off messages are the only messages processed. Each simultaneous note goes to the next available output until all outputs are used up.
  • There are some test programs available in the repo for checking new builds.
  • Pressing the button in normal operation stops all notes. This is handy for some midi interfaces that don't send note off commands reliably.
Future features...
This thing is a beast and could be reconfigured for a number of tasks. In no particular order these are some ideas I've had for it that I'd need to write code for.
  • A 16 output sequencer mode, I'd have to figure out how to program new sequences.
  • A 16 step sequencer mode.
  • A multi-channel mode listening to 4 midi channels and output in 4 independent polyphonic setups.
  • Handle pitch bend commands
  • A drum rack mode (like polyphonic midi, but notes fixed to specific output pairs). Maybe use CV for velocity or simple envelope in this mode, or just more gates.

Testing

This is still ongoing. 

I've solved several bugs in the code (mostly weird edge cases handling fast polyphonic MIDI stuff).

Early in the testing I was pretty confused about the LEDs not working as expected, it wasn't where I thought I'd have issues... as it turns out the A6 & A7 pins can't be used as outputs. Those might be repurposed in the future for something else.

In the beginning of my tests I was observing some jitter (note length error) in the gate outputs that I thought might be down to slow code; but, after some triage, it turns out that the jitter is almost entirely on the MIDI connection. I was testing with Abelton via a Scarlett 18i20 and the MIDI jitter is proportional to the buffer sizes used in the interface. I watched the MIDI traffic and the Gate output with a scope and measured the jitter. Setup was 16th notes at 120bpm from Abelton.

Testing different buffers as the manual suggests using smallest possible buffer.
  • buffer at 1024, 22ms jitter
  • buffer at 512, ~12ms jitter
  • buffer at 256, 6-8ms jitter
  • buffer at 128, ~5ms jitter
Calibration of the output CVs is still a bit of a pain. As part of v2 I may design a test jig to do this more quickly and more accurately. So far I measured 4 of the CVs at increasing voltage outputs and plotted that:


Very linear until the top end which is great. After that I computed the average at each measurement point and looked at the deviation from the average for each CV output.


This is a lot more illuminating. The CVs are varying ±20mV from the average over the whole range. That's a bit more than I would like. Part of it is probably due to amplification of the DAC outputs and variance in the amplifier circuit components. I think with a rig to measure the CVs and compute a calibration curve for each I could tighten this up considerably but that's going to be a lot more effort. For now I used this data to configure a scale value to get pretty close to the desired 1V/Octave output on the CV.

At this point I started testing a few of these in my skiff. In the middle of testing my VCO design I discovered that this v1 design is very sensitive to the impedance of whatever's plugged into the CV. This is a design flaw. I compared my CV output to the Befaco design and quickly saw the issue. On each CV output, just before the jack, I have a 1k resistor to limit the current the op-amp has to source. That 1k should be in the feedback loop of the amplifier. As it is the voltage across that 1k is different with any difference in impedance on the output. So the calibration above only works out if the calibration rig has the same impedance as whatever the unit is plugged into which is unlikely. I need to make a v2 to fix that because of where the resistors ended up in the layout, a rework on 16 outputs would be too difficult. I can kind of compensate for this with calibration tweaking but I'd rather not, the v1 modules are still fine for other uses.

Changes for v2:
  • Some LEDs are misaligned, not on the 14mm grid.
  • A6 and A7 on the Arduino can't be used as digital outputs, so LED1 and LED2 don't work. 
    • A6 and A7 could be used on V2 for a sample and hold function and or a clock input function or for calibration.
  • LED1 and LED2 probably need to be removed, unless some of the pins tied to control signals for the DAC or 74HC595s can be used instead.
12V Current: 87mA
-12V Current: 12mA
5V Current: None

BOM & Test Data: Google Docs

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