What is it?
Whatever you want it to be! MIDI is just a protocol that was created to simplify the method of machines communicating and synchronising with each other. As such, anyone is able to create their own compliant device and have it work perfectly with any MIDI-capable software.
This may sound like a daunting task, but there are many dedicated How-to and enthusiast sites on the internet such as Instructables. Best of all, you get to build something that has exactly the controls you want, and you might save a buck or two along the way.
If all this sounds a bit hardcore, you could always start off by making one of the many kits available, such as those from DJ Tech Tools or Livid's Builder series.
What can I do with it?
How long (or useful) is a piece of string? With just the limits of your own imagination you can be as conservative or as adventurous as you please.
Perhaps you just want a simple trigger style controller for shooting off samples to complement your more elaborate MIDI surface? Maybe you've thought of the ideal setup that is going to usher in a whole new era of DJ control? Either way, when you are building your own why do something that's been done before when you can tailor something to your specific requirements?
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