Nonlinearity is the behavior of a circuit, particularly an amplifier, in which the output signal strength does not vary in direct proportion to the input signal strength. In a nonlinear device, the output-to-input amplitude ratio (also called the gain) depends on the strength of the input signal. In an amplifier that exhibits nonlinearity, the output-versus-input signal amplitude graph appears as a curved line over part or all of the input amplitude range. Two examples are shown below. The amplifier depicted by the red curve has gain that increases as the input signal strength increases; the amplifier depicted by the blue curve has gain that decreases as the input signal strength increases. Nonlinearity can be tolerated in devices and systems that use digital modulation, and also in frequency modulation (FM) wireless transmitters. These signals are either full-on or full-off; the amplitude wave forms are not analog, so analog distortion cannot occur. In analog devices and systems, however, linearity is important. Nonlinear circuits generally cause distortion in applications such as amplitude-modulation (AM) wireless transmission and hi-fi audio