A controller is any device that can be used for controlling music software. The most common type of controller is a keyboard controller. These resemble a musical keyboard and can also contain actuators for pitch bending or modulation. The keyboard itself produces no sound on its own and is used only to drive other MIDI devices. Dedicated control surfaces resemble an array of switches, sliders and dials and are common place in any studio.
Keyboard controllers connect via USB and/or Midi cables and allow you to play your softsynths and samplers in real time. With most keyboards having extra hands on dials and slider controls they option up more options allowing for expressive performances that can be easily recorded and later edited.
Accessories to go with your studio midi controllers, with covers for keyboards and stands for your equipment
Control surfaces come in many size and shapes with smaller units offering basic playback control functionality and in the other extreme the larger units will replicate a complete mixing desk. Having a physical and tactile controller will allow quick and easy control over your software package of choice helping to improve your workflow in the studio.