A fader is any device used for fading, especially when it is a knob or button that "slides" along a track or slot. A knob which "rotates" is usually not considered a fader, although it is electrically and functionally equivalent. A fader can be either analogue, directly controlling the resistance or impedance to the source (e.g. a Potentiometer); or digital, numerically controlling a digital signal processor (DSP). Digital faders are also referred to as "virtual" faders, since they can be viewed on the screen of a digital audio workstation. Modern high-end digital mixers often have piezo-electric actuators attached to the faders such that they can be multi-use and will jump to the correct position for the selected function and/or saved setting.