Using Arduino and Pd for musical live performance

Connecting an Arduino microcontroller to Pd is no big problem: You can just put Firmata on your Arduino and use the Pduino library for communication on the Pd side.

But sometimes Firmata is not the answer to every problem: You may want to turn on the built-in pull-up resistors for digital input pins, you may want to do some calculation on the Arduino before sending a message, or you may want to send a message from Arduino to Pd only on change.

In fact, I wanted to do all of the above, but not use Arduino for output. This led to the following electronics and code.

Connecting Inputs to Arduino

I have connected several push buttons to the Arduino, and one variable resistor. All push buttons are open an close on push. All connected devices:

  • Three foot switches in a box.
  • Three push buttons and a force-sensitive resistor mounted on a glove, worn on the right hand.
  • A keypad consisting of 12 push buttons with a resistor matrix similar to this setup (in German). The main difference in my setup is the usage of values ten times as in the example, as the thin cables have a non-negligable resistance. This is mounted on my left forearm


circuit diagram

The Arduino Shield

The shield alone The shield with the PCB for keypad added Fully mounted

The Glove

Push buttons on the glove Push buttons on the glove Force sensitive resistor

The Stompbox

Stompbox Stompbox

The Keypad

Keypad Keypad

Message Passing between Arduino And Pd

I wanted to minimize the data sent from Arduino over USB to the host computer. I came up with the following "protocol", that uses 1 byte for a digital value and 2 bytes for analog values.

Digital message
Analog message
11pppvvv 0vvvvvvv

p denotes bits used for encoding the pin number, v bits used for values.

As the Arduino Duemilanove - the one that I use - has 14 digital pins, 4 bits (0-15) are necessary to encode the values, 6 analog ins can be encoded in 3 bits (0-7). Digital pins only read 0 or 1, so only one bit is needed for encoding the value, analog pins read values 0-1023, which corresponds to 10 bits.

If a byte starts with a value of 11, then this byte and the following one starting with 0 together are an analog message. If a byte starts with 100, then the message is a digital message.

On Pd's side, messages are parsed and then output in the same way as [pduino] does it: (a|d) [pin number] [value]. That design allows you use it as a drop in replacement.

I first tried to parse messages with bit shifting and [expr], but that was too CPU intensitive, so I have created my own external in C. That external does the bit shifting and then outputs messages as lists.

Pd Patch

I am using the setup mostly for live performances, and the demo patch included in the source code is the actual patch that I use. It also makes heavy use of my abstractions.

A demo video will follow soon.