Gameboy Genius

Introduction

Voilà Korg monotribe, the successor to its little brother, the monotron. The Monotron is a cheap, squeeky, and easily moddable toy synth with an MS-10/20-like filter topology. It has a ribbon keyboard an audio oscillator, an LFO, a lowpass filter, and not much more. The monotron was an instant hit, and Korg released schematics for it to aid modders further.

When I heard about the monotribe, I had my doubts. Mostly that there’s only one pattern, which is 8 steps long. Well, there are 8 extra steps for the drums, as well as a “flux mode” which records your movements on the ribbon continuously. In that sense, it is limited, and is an instrument made to be played with your hands, rather than be programmed. But as it turned out, this was a design choice, and not a technical limitation. I can easily imagine why. They wanted it to seem as analog and playful as possible.

Same thing with MIDI. Officially, the monotribe doesn’t support MIDI. It does however offer a sync pulse output and input. This allows it to be synced to other monotribes, modular synthesizers or even Korg’s own virtual iMS-20/iElectribe, using a special sync app on a second iPhone/Pod/Pad. However, the lack of MIDI is still a slight limitation.

TOC

Opening it up

Hackability and MIDI

MIDI, the electrical side of things

MIDI, the mechanical side of things

What can you do with MIDI on the monotribe?

Videos

Where do we go from here?/Final thoughts

Opening it up

Before I get to the part everyone has been waiting for, let me remark on something I noticed when I opened up the monotribe. The whole bottom side of the main board is covered in some form of residue, maybe flux or one of the liquids used in the manufacturing process.

This is even nastier. In the top two pictures, there’s some black stuff that I’m not sure what it is. Could be corrosion of some kind, or parts of a chip that was crushed by the pick and place machine. Or something. In the bottom picture, notice how IC17 (an LM324 opamp) as well as the Q2 transistor, are covered in this gunk. Also notice that there’s too much solder on some of the SMD resistors. (Original hi-res photo, 4320 x 3240) None of that is an immediate risk to the function of the device, but depending on what that residue actually is, there’s a very real risk of an increased failure rate over a decade or three, due to corrosion. I’ve seen better soldering, and I’ve seen significantly worse soldering, but all-in-all I expect better from a factory-made product. Maybe mine was a dud, but it makes you wonder.

Hackability and MIDI

I doubt Korg is going to release a schematic for the monotribe anytime soon, like they did for the monotron, but the monotribe board does have a number of useful points for modifying parameters and separating outputs, marked on the board. See the Monomodder blog for inspiration.

However, monotribe is quite a different beast compared to monotron. Even though the sounds are generated with analog circuitry, the heart of the monotribe is a microcontroller. The microcontroller is used as sequencer and, I think, LFO. The microcontroller is a Toshiba TMPM332FWUG. It has an ARM Cortex-M3 core, 128 kB of flash memory and 8 kB of internal RAM. It is clocked at 10 MHz. It has, like most microcontrollers these days, support for UART (serial communication) which means that software running on the microcontroller could theoretically easily implement MIDI, which is really just a quirky variation of UART. As I, and air_variable (Japanese) before me have discovered, there is something going on there, and it turned out to be actual MIDI. All you need to do is hook a few points on the board.

MIDI, the electrical side of things

Upper schematic: MIDI out. Lower schematic: MIDI in. NC means “no connection”.

Note that this schematic is looking at the back of a panel mount style MIDI jack, similar to the one seen in my other images.

The schematic is drawn so that you’re looking at the chip from above. Place the chip so that the little notch is pointing up. Pins 2 and 3 are on the left side of the chip. Ground is the bottom right pin. +5V/+3.3V is the upper right pin on the chip. The things inside the dotted rectangle is what’s inside the chip.

The unspecified diode on the left of the chip is a small signal diode, not a LED. 1n4148 or 1n914 will work, for example.

MIDI is designed in such a way that individual bits are transferred as a current that lights up a LED in an optocoupler, on the receiving side. This LED, in turn turns a phototransistor on and off. In a MIDI circuit, the LED in the optocoupler should be connected between the +5V line and the signal line. Normally, the signal line is high, which means there is no difference between it and the +5V line. To communicate, the signal line will go low, which in turn will light the LED on the receiving side.

A MIDI circuit should have a number of resistors both on the transmitting and receiving side. This design gives MIDI better noise immunity, and protects the gear in case of a number of things that can go wrong, such as a short-circuit between any individual pins, connecting an output to an output or reversing the pinout of the plug.

Let’s first look at how to do MIDI out with the monotribe. The microcontroller is using a 3.3 V power supply. MIDI is normally using 5 V, so if we wanted to be correct, we would convert the signal from 3.3 V to 5 V. However, we don’t strictly need to do this. As long as we can light up the LED on the other side, we’re fine. (You might need a real +5 V supply if you want to connect a MIDI hub that is powered by the MIDI host, however.)

To be guaranteed to light up the LED from 3.3 V, we’ll need to decrease the resistor values slightly. I found that replacing R₁ and R₂ in the schematic above with 150 ohms each works fine with my MIDI interface, but my calculations are suggesting that around 60 ohms may be needed, depending on the optocoupler. Bigger value resistor is safer, so try 150 ohms first.

There are two ways to do MIDI in, either with an optocoupler or without one. If you want to be cheap, you can simply connect the signal pin (the lowest one) from the MIDI jack and connect it through a 820 ohm resistor to the RX pin (see below) of the microcontroller. The resistor is needed to protect the microcontroller chip from the signal which is using a +5V signal level, which is outside of the chip’s specified limits. If you do it this way, you also need to connect ground from the MIDI jack (the “NC” pin) to ground on the board. Do not connect +5 V pin (the upper pin) anywhere.

If you want to do it properly, you can use for example a 6n136 or a 6n138. If you choose to use a 6n136, connect it as pictured above. (Connect +3.3 V, the red wire in the picture, instead of +5V in the schematic.) If you’ll be using a 6n138, you can use the connection below, according to Wooster audio. The part of the circuit that is to the left of the diode is the same as in the 6n136 circuit.

The picture above shows how I connected MIDI in with an optocoupler. The blue wire is ground. The red wire is +3.3V. The yellow wire is where the signal comes back (MIDI in.)

The MIDI output is situated between the points with yellow and the red cables, i.e. 3rd from the top.

MIDI, the mechanical side of things

You drill a hole for the jack, simple enough. However, I’ll share these images as a cautionary tale that you need to pay attention where you drill the hole. Here I made two mistakes:
1) The hole was too high up, so the jack collided with the PCB. (You’re looking into the monotribe from the bottom, so up=down…) I solved this by trimming away a part of the PCB which was just a big ground plane.
2) The less obvious problem is that the hole was too far back, so one of the screw holes ended up behind the standoff. This is a problem because it makes it difficult to insert a nut behind the screw.
So, plan before you drill!

If you’re using an optocoupler you need to add a small PCB for the optocoupler and the other components. (2-3 resistors and a diode.) I found that one suitable place for attaching this board is on the interface board. There’s a hole drilled where you can attach a board with a screw, as pictured below. Once again, however, I slightly misjudged the measurements. The board stuck out too long and collided with the speaker.

What can you do with MIDI on the monotribe?

Simple answer: You can sync the internal sequencer, play notes, and control all parameters the EG type as well as all parameters relating to the LFO section. It also transmits those very same things, so it could be used as a MIDI sync source, sequencer or limited control surface.

Details

• Sync: It will both send and accept MIDI sync, start and stop messages, meaning the sequencer can be reset over MIDI, which can’t be done remotely if you’re only using the regular sync input.

  Quirks/problems: When you’re using the miditribe as a MIDI slave and remove the sync clock, it will keep waiting for a MIDI clock indefinitely. You can fix this by either turning it off and on again, or plugging something in and out of the sync in jack, which will restore it to the internal sync clock.

• Notes: It will send and receive notes within the limits of the sequencer. It’s using channel 1 for notes. Notes outside these limits will be ignored. It does not react to varying velocities. It does however react to pitch bend MIDI messages and is able to record and play back pitch bend in flux mode! I don’t have MIDI out hooked up at the moment, so I can’t confirm this right now, but I’m assuming it also transmits MIDI pitch bend messages when it plays notes in flux mode.

  Quirks/problems: It has a stuck note issue when it doesn’t receive a note off message for a note. Let’s say you send a note on message for D4 and then a note on message for E4, and lastly a note off message for E4. This can happen in Renoise, for example. In this situation, D4 gets stuck, and a new D4 note is played as soon as you release all other keys. Sometimes multiple keys can get stuck, and you need to go through each key to depress each one.

• Drums: It’s using the standard General MIDI specification for drums, which means the following notes on MIDI channel 10.
  Name Note number Note name GM drum name BD 36 C1 Bass drum 1 SN 40 E1 Snare dum 2/Electric snare HH 42 F#1 Closed hi-hat
• Control change messages (CC): It can send and receive CC for all the knobs and switches marked green above. The parameters are mapped as follows:
  Name CC number Values GM parameter name LFO rate 16 – General purpose controller 1 LFO int. 1 – Modulation wheel EG shape 80 32=decay
  64=sustain
  96=attack General purpose controller 5 LFO target 81 32=VCO
  64=VCO+VCF
  96=VCF General purpose controller 6 LFO mode 82 32=Fast
  64=Slow
  96=1shot General purpose controller 7 LFO wave 83 32=Saw
  64=Triangle
  96=Square General purpose controller 8

  Quirks/problems: It’s practically useless as a MIDI control surface because it is really slow at sending CC messages. If you wiggle one of the knobs a few times really fast for a few seconds, it will continue to transmit CC commands for several seconds.

  I’ve noticed that when controlling the LFO rate/int. knobs remotely, the value will sometimes jump back to the value that the knob is at. I’m guessing that this happens because there’s a small fluctuation in the voltage that the microcontroller will pick up as a change in the value. If you only want to control these parameters over MIDI, you can minimize this effect by turning these knobs fully counter-clockwise.

Videos

These are two videos showing me play around with MIDI in and MIDI out.

MIDI in

MIDI out

Video advertising this post, basically

Music sequenced in Renoise, played back on Korg monotribe and fed back into Renoise’s effects, some distortion and some delay. The drums are not played by the monotribe, but are samples in Renoise.

Where do we go from here?/Final thoughts

Some people have been wondering why Korg didn’t just include MIDI support out of the box. First off, to be honest, I think the MIDI support is a bit buggy, and probably wasn’t ready to be released anyway. It might be something they implemented mostly for internal testing. Another possibility, which I don’t believe much in, is that they’re planning to release a MIDI-enabled “monotribe pro” later. I just don’t think that fits into Korg’s profile of how people will use the monotribe, so I don’t think there will be a pro version with MIDI.

But, phew! Hopefully this should be enough information to get some of you up and running and get MIDI working on your ‘tribes. So what can we use this for anyway? Hooking up the monotribe to a computer and just use it as an analogue synth, honestly doesn’t provide any real advantages over using, say, a good softsynth, in my opinion. The monotribe’s charm is its hands-on usage, so I think the modification will be most useful for syncing it with gear where it’s more convenient to use MIDI sync than the sync options at the back of the monotribe.

Another possibility for LSDj users, is to use Arduinoboy+LSDj MIDI out. You’ll be able to control the monotribe nicely and which will give you a nice analogue companion to LSDj’s digital sound.

Other than that, the fact that monotribe can be used with MIDI also opens up the possibility of an internal mod. For the price it’s sold for, it’s a fun toy but not an essential tool. If the thing could be extended to be more TB-303-ish (not going to hide my dreams) it would instantly become more appealing to a lot of people. My original idea was to replace the firmware on the unit’s own microcontroller, but that’s difficult and tedious.

My new idea idea is to add another microcontroller (’duino anyone?) which replaces the sequencer, and also gives the possibility to do things like slide (emulated with pitch bend) or accent (emulated by adjusting the LFO rate/intensity). You could also add a second LFO and all sorts of things. With some extra software and wires, this thing could become truly useful as a standalone unit…

Useful links:
Board photos and more on Flickr
My YouTube channel
Discussion on Muff Wiggler
Monomodder blog