A
fader is used to control the
intensity of a signal, usually music. It differs from a typical
volume control in that it is
linear in operation, not
rotary, with the travel of a
knurled slider along a slot(usually marked with
calibrations of some kind)controlling the signal.
Faders are used mostly in audio and video control equipment, video editing and audio mixing boards, and equalizers.
The primary advantage of an optical fader over conventional linear potentiometers is that the optical fader is contact-free. That means that the devices have an unlimited lifetime with no signal degradation. In addition, the output is digital, unlike the analog output of a traditional device.
Standard faders work by simple resistance, the drop in voltage when current passes though a length of wire. A basic fader has an electrical contact on the slider that rubs on a rail that runs the length of the slot that is wrapped in wire. The farther along the contact is on the spool, the more wire is in the circuit and the more the signal is cut. Over time, the rubbing causes the contact to wear out and become noisy.
An optical fader works by reflecting infrared light from an LED onto phototransistors via parabolic reflectors and an optical shutter. The light is selectively blocked and allowed to pass thought to the phototransistors by the shutter, with the amount and position of the light determining the location of the slider exactly. A user-adjustable magnetic resistance system provides a variable slide "feel" without affecting the operation.
This fader takes wear and noise out of the mixing board, significantly increasing its reliability and performance to the point where the board can be used as long as the electronics in it are still functional.