of correlation between the two signals. An example pair of curves that keep power equal across the mix are and (where mix ranges from 0 to 1). Equal power shapes typically have the sum of their curves (in the middle of the mix range) exceeding the nominal maximum amplitude (1.0), which may produce clipping in some contexts. If that is a concern, then "equal gain" (or "constant gain") shapes should be used (which may be linear or curved) that are designed so the two curves always sum to 1. In the digital signal processing realm, the term "power curve" is often used to designate crossfade shapes, particularly for equal power shapes. Fader A fader is any device used for fading, especially when it is a knob or button that slides along a track or slot. It is principally a variable resistance or potentiometer also called a ‘pot’. A contact can move from one end to another. As this movement takes place the resistance of the circuit can either increase or decrease. At one end the resistance of the scale is at 0 and at the other side it is infinite. A. Nisbett explains the fader law as follows in his book called The Sound studio:"The ‘law’ of the fader is near-logarithmic over much of its range, which means that a scale of decibels can be made linear (or close to it) over a working range of perhaps 60 dB. If the resistance were to increase according to the same law beyond this, it would be twice as long before reaching a point where the signal is negligible. But the range below -50 dB is of little practical use, so here the rate of fade increases rapidly to the final cut-off". A knob which rotates is usually not considered a fader, although it is electrically and functionally equivalent. Some small mixers use knobs rather than faders, as do a small number of DJ mixers designed for club DJs who are creating seamless mixes of songs. 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). Analogue faders are found on mixing consoles. A fader can also be used as a control for a voltage controlled amplifier, which has the same effect on the sound as any other fader, but the audio signal does not