DIY MIDI Breath Controller

Breath controllers are a difficult subject to bring up.  They have a bit of a bad reputation...perhaps because of synthesized saxophone sounds.  Breath controllers often induce people to roll their eyes dismissively.  It's kinda like how many people respond to keytars.  Are they awesome?  Or are they cheesy?  How can you know, if you've never played one?  Well, a buddy of mine started sending me links about DIY breath controllers (see Mixtela's MK1 and MK2).  He knows I'm a sucker for DIY.  So, with this as motiviation, I pulled together my own plan and made my own DIY MIDI breath controller.

Breath Controller?  The idea of a breath controller is that you use your pressure from your breath to control some aspect of your synthesizer.  The harder you blow, you make your synth louder, or you open its filter more, or you bend its pitch.  Some breath controllers are shaped like wind instruments and include both breath sensitivity and buttons (keys) to change notes.  Other breath controllers (such as mine) are simpler and just have the breath-sensitive part.  With only the breath-sensitive part, you control sound parameters on our synth, like having a breath-controlled mod wheel.

My Approach.  I'm choosing to have my breath controller use MIDI messages to control my synth.  Therefore, I'll need to (1) choose a pressure sensor, (2) connect it to some sort of electronics to read the signal, and (3) write some software to send MIDI messages to my synth.  Let's do it!

The Shopping List.   First, let's go shopping.  There are lots of ways to build this device.  Below are the components that I used:

• Pressure Sensor: Honeywell NSCDRRN0001NDUNV.  This is a differential pressure sensor whose output voltage changes as pressure (positive or negative!) is applied.  The pressure range of this sensor is +/- 1 inch of water (about +/- 250 Pa).  It can be bought it from Mouser, but it's not cheap.  Cheaper sensors will probably work, too. 
• Flexible Tubing: I used Tygon clear tubing.  To fit onto the sensor's snout, you need a short length of 1/16" ID tubing (here), but most of your length should be 1/8" ID (here).  You want that larger diameter so that you can push enough air through it to be comfortable.  Bigger tubing might be even better.
• Electronics:  I used an Arduino Uno because I had one around.  You can probably use any hobbyist microcontroller board (including the Redboard or Metro) but you want one that'll mate to your MIDI Interface.
• MIDI Interface:  I used the Sparkfun MIDI Shield.  I like that it has built-in pots so that I can adjust things while playing.  Note that it does not come with the header pins, so be sure to buy some...you should definitely buy stackable headers.
• Prototyping Supplies:  I did my prototyping on a solderless breadboard.  I also used some little jumper wires that are typical in the Arduino world.   
• Power:  During development, I powered the system from my computer's USB port.  But, when I was done, I wanted to be free from the computer.  So, I've been using USB power wall-wart like this one along with a standard USB cable.

Wiring It Up.  After assembling the MIDI shield and plugging it on top of the Arduino, I needed to figure out how to wire up the pressure sensor. After digging through the datasheet for the pressure sensor, I found the information I needed (copied it below).  It has three types of connections: power pin, ground pin, and two output voltage pins (and you can ignore one of them, if you want).  Easy.

Using the solderless breadboard and my jumper wires, I connected the sensor to the Arduino via the MIDI Shield's stackable headers:

• Sensor Pin 1 to Arduino 5V
• Sensor Pin 2 to Arduino analog input A2
• Sensor Pin 3 to Arduino analog input A3
• Sensor Pin 4 to Arduino ground

After adding the flexible tubing to the snout of the pressure sensor, the hardware is ready.

Arduino Software.  Once it is assembled, you need to write a software to command the Arduino to read the sensor and to issue MIDI commands.  There are lots of choices here, but at it's core, it is three steps:

• An "analogRead" to measure the output of the pressure sensor
• Arithmetic to convert the measurment to a valid MIDI value (usually 0-127)
• A few "Serial.write" commands to send the MIDI message to the synth

In practice, things are modestly more complicated.  Really, though, the primary question is what MIDI message you want to send.  Everything depends upon that. 

Choose Your MIDI Command!  There are many ways to command your synth via MIDI.  You need to find out what MIDI commands your synth can respond to and what is easy for you to configure (three good choices are shown below).  On my synth, aftertouch is very easy to set up without any menu diving.  So, I chose to have my breath controller send aftertouch MIDI messages.  Later, I made it more complicated, but aftertouch is still what I use most of the time.  My code is available on my GitHub here.

From midi.org

Initial Disappointment.  Once I got my system working, I had to learn how to play a breath controller with my synth.  I quickly become bored because all I could make it do were relative slow filter sweeps or unusable pitch bends.  There was no way to give it articulation because it was simply about lung pressure...and my chest does not have much dexterity for quick motions.  Disappointment.

The Key is Airflow.   After some messing around, I had the idea to poke a hole in my tubing so that some air could flow through the tube and out the hole.  Once I did that, everything changed.  I quickly found that I could use very quick motions like tongue-stops on the tip of the tube (in my mouth).  Suddenly, it could respond quickly, like any other wind instrument.  Without the hole, tongue-stops had no effect because the sealed tube kept the pressure high.  With the hole, however, the air vents out when the tongue stops its end.  Therefore, the sensor sees a quick change in pressure and articulation is achieved.  It's so much more varied and expressive this way.  The vent hole in the tube is the key.

It's Fun.  With this modification, I found myself having fun with the breath controller.  Yes, I could still do slow swells on sustained pads.  But, now I could also articulate fast riffs, giving them a new kind of bounce.  Using the breath controller on repetitive arps, I could literally "breathe" life into them via breath-modulating the filter.  And, for single note work, the breath controller is great for contouring one's phrases in a way that is very difficult to do any other way.  So, while I wouldn't use a breath controller in every situation, it is a good time.  I recommend trying it.  And maybe even making it yourself!

DIY MIDI Ribbon Controller

In a previous age of the world (2011), I built myself a ribbon controller.  I used it to drive my Korg Mono/Poly via CV.  And it was good.  But, that was before "synthhacker" first awoke under the stars on the shores of Cuivienen.  Has its glory been forever lost?  No!  The ribbon lives again!  And it has been updated!  Behold:

The Overall Setup:  This ribbon is just a controller -- it controls a synthesizer.  It doesn't make noise itself.  Those black-and-white keys that are built into the synthesizer are also just a controller.  They tell the brain of the synth to make noise at fixed pitches.  My ribbon controller uses the synth's MIDI interface to tell the synth to make noise across a continuous spectrum of pitches.  More bendy.  More swoopy.  Fun!  My controller has two parts: the ribbon itself, and some electronics (Arduino +  MIDI Shield) to do the ribbon-to-MIDI conversion.

The Ribbon Parts:  The ribbon itself, I built a long time ago.  But, as stores on the internet never forget what you've ordered, I easily went back and looked it up.  The core of the ribbon controller is a 500mm-long "soft pot" (Sparkfun).  This is the part that senses where you touch.  Since it is soft and floppy, however, I also bought a 2-ft long strip of plastic (McMaster) to act as a backbone.  For the electrical connection to the ribbon, I used a headphone jack (Sparkfun) and a small perfboard (Sparkfun).  The only real innovation with my ribbon controller (there are lots of DIY ribbons out there) is that I also added a magnetic strip along its back (McMaster).  Since most of my synths  have a metal front panel, the magnetic strip is a great way of keeping the ribbon in place!

Components of the Ribbon Controller

The soft pot is on the top.  The magnetic strip (partly unrolled) is in my hand.

Assembling the Ribbon:  I started with the plastic strip.  Its edges were surprisingly shart, so I got out some sandpaper to smooth the edges.  Now it's much nicer to touch.  After cleaning off the sandy bits, I spray-painted the whole thing black.  When fully dry, I attached the magnetic strip and the soft pot, both via their adhesive backing.  Then, I carefully soldered the ribbon's terminals (don't melt the soft pot!) to the perfboard and then to the audio connector.  I wired the connector so that the "top" of the pot was to the tip, "bottom" of the pot was to the sleeve, and the "wiper" of the pot was to the ring.  Once it was all soldered together, I epoxied the perfboard to the plastic strip to make it all nice and solid.

Headphone jack is connected to the perfboard, which is connected to the soft pot.  The whole thing is epoxied to the plastic strip, for strength.

Arduino-MIDI Electronics:  The ribbon is just a potentiometer -- it needs electronics to interface to a MIDI synth.  After my previous project making an MIDI-to-trigger converter, I realized that I could re-use that exact same hardware for this ribbon-to-MIDI converter.  Repeating the recipe list from that project, my parts include an Arduino Uno, a MIDI Shield* from Sparkfun, some stackable headers, and a 3.5 mm stereo headphone jack (or maybe one like this).

(* Note: I used the old Sparkfun MIDI Shield but, presumably, the new model linked above works well, too).

Components for the "Brain" of the Ribbon Controller

Assembling the Electronics:  I soldered together the MIDI shield, including soldering on the stackable headers.  I then soldered some wires to the audio jack and soldered those wires (being sneaky) to the header pins sticking down from the MIDI shield (detailed soldering pics here).  Once it was all assembled, I connected the ribbon to the MIDI shield using a standard 3.5 mm audio cable.  I also connected a USB cable to my laptop so that I could power it and program it.  The photo below also shows the MIDI cable connected, but we're not quite ready for that, yet.

Connecting my ribbon controller to the MIDI Shield (the Arduino is under the MIDI shield).

The Ribbon is a Potentiometer:  Touching the ribbon (the "soft pot") is like turning the knob on a potentiometer to a particular spot.  If I touch it near the low end, it'll show a low resistance.  If I touch it near the high end, it'll have a high resistance.  The job of the Arduino, therefore, is to sense the resistance of the ribbon so that it knows what note to play.  In theory, pretty easy.  But, to get some details on how this works, I hooked used my setup to collect some actual data.

The Ribbon's Response via Arduino:  To measure the response of the ribbon with an Arduino, I used the setup described above, which has the wiper of the pot connected to one of the Arduino's analog input pins (A3).  The bottom of the pot is tied to ground.  I activated the input pin's pullup resistor (via pinMode(A3, INPUT_PULLUP)) and started reading ribbon values via AnalogRead(A3).  I printed the values to the serial port so that I could see them on the P and log them to a file.  Sliding my finger to different octave points on the ribbon, I made the plot below,

The AnalogRead() Values Generated by the Arduino when Touching the Ribbon at Different Spots.

Values When Not Touching the Ribbon:  The first thing to notice is what happens when *not* touching the ribbon: the ribbon shows a very high value.  The high value results from the fact that the wiper is floating when you're not touching the ribbon.  As a result, the pull-up resistor on the analog input pin is able to pull the value all the way to the maximum value (which is 1023 on the Arduino Uno).  So, it's easy to know when you're touching the ribbon versus non touching the ribbon -- just look to see if the value is near 1023.

Values When Touching the Ribbon:  The middle of the graph shows the values when I touch the ribbon between its bottom (C1) and its top (C4), Looking at the detailed values, the lowest value seen in my ribbon (C1) is about 12 whereas the highest value (C4) is about 339.  So, I know that I have to program the Arduino to map values between 13-339 to musical notes between C1-C4.

Touching the Ribbon To Record the Arduino's AnalogRead() Value at Different Locations.

Software:  After seeing how the ribbon responds, I wrote my Arduino software.  It's basically a big loop that reads the ribbon value, converts it to a MIDI note number, and sends those MIDI commands to my synth.  While it seems conceptually simple to do an AnalogRead() for the ribbon followed by a Serial.write() to send the corresponding MIDI message, it does actually take some thought to get it to respond the way that you'd like.  Also, to make the ribbon pitches change continuously (and not be quantized like with the piano keys), you need to compute and transmit all the pitch bend commands.  It takes some work to get it right.  If you're interested, my Arduino code is available on my GitHub here.

Linearizing the Response:  One tricky spot is if you want the pitch to track linearly with your finger position on the ribbon.  In my case, I found that the notes were too closely-spaced at the top of the ribbon and too widely-spaced at the bottom.  I didn't like that.  The key to linearizing the response is to recognize that the ribbon and the Arduino's pull-up resistor actually form a voltage divider.  Once you do the math on that voltage divider, you program the Arduino to better deduce the note you want given the apparent voltage at the soft pot's wiper.  It's not perfect, but it's much better than before.

I was lucky at how well the ribbon fits on the face of the synth.  I took the extra effort to make the software fit well, too.

Bend Range:  The ribbon is fun because you can use your finger to sweep through all the pitches in between the normal notes.  Since the MIDI messages assume a piano-like keyboard, you do these "in between" pitches by commanding a note to one of the standard pitches followed by commanding a pitch bend to get the in-between pitch that you actually want.  Ideally, you could set the synth so that its allowable amount of bend would span the whole ribbon.  Unfortunately, on the Prophet 6, the max bend amount is +/- 1 octave.  As a result, with my setup, I get a brief note discontinuity when I try to continuously slide across more than 1 octave.  This is a limitation of the synth, not my ribbon controller. (If the synth's software were open source, I could fix that!)

Wobbling Pitch (ie, Pitch "Noise"):  Another subtlety that I uncovered is related to the fact that one's ear is very sensitive to changes in pitch.  The issue is that any noise in the measurement of the ribbon value will directly translate into wobble (instability) in the pitch commands sent to the synth.  When I was powering my system from the laptop and when the laptop was running on battery, my system sounded great.  But, when I plugged the computer into AC power, the pitch wobbled a lot.  It was quite unpleasant.  My solution was to add a 33 nF capacitor between the ribbon's analog input pin and ground and to add some filtering in software (revised code is here).

Future Work:  It's pretty fun to have my ribbon controller working with my MIDI synth.  Sure, I don't like that there is still a note discontinuity that happens when I try to exceed the 12-step max bend allowed by the Prophet 6, but it's still quite fun.  (Maybe I can sweet talk someone at Dave Smith Instruments to help me out on that one remaining issue :).  Looking for other ways to improve the ribbon, I'm considering adding a DAC so that the the Arduino (or Teensy!) can directly create its own audio without needing to command an external synthesizer.  That'd be fun.  How about a ribbon wavetable synth!  Or, how about a ribbon FM synth?  Fun!

Follow-Up:  I also made a version of this ribbon that outputs CV signals.  Now it can drive my old pre-MIDI synths like my Korg Mono/Poly.  Check it out here!