Many pieces of electronic music equipment communicate via MIDI, a serial protocol. The signal, according to MIDI 1.0, must be a signal at 31,250 bits per second, 8-N-1 format: A start bit (must be 0), eight data bits, and one stop bit (must be 1). MIDI messages are usually a few bytes, so it takes about a millisecond to transmit a simple piece of information.
My MIDI tools
Traktor (see DJ_mixers) accepts MIDI commands. This page http://blancodisco.com/2009/02/01/itm-dj-iphone-app-works-perfect-with-traktor-3-pro-review/ shows how to use iTouch MIDI (see below) to control Traktor.
iTouchMIDI sends MIDI commands from an iTouch or iPhone or iPad and inserts them into an Mac's internal MIDI bus. There are different control surfaces on the iOS screen, such as a keyboard, a matrix of buttons, or a movable target. The company offers a special version that has a control surface useful for controlling MIDI-controllable mixers, such as Traktor: http://itouchmidi.com/?q=node/192
A Yamaha QY10 can be used as a battery-powered MIDI keyboard.
I have built a PICAXE controller to send data from a potentiometer to TRAKTOR.
A nice page on MIDI and micro-controllers, with circuit diagrams and notes: http://www.tigoe.net/pcomp/code/communication/midi