What characterizes a limiter in audio processing?

A limiter in audio processing is characterized by its primary function of controlling the dynamics of a signal by reducing the gain when the audio signal exceeds a predetermined threshold level. This action is often performed at high ratios, typically 10:1 or greater. When the signal exceeds the threshold, the limiter effectively "limits" the output level to prevent distortion or clipping, ensuring that the audio remains within a specified amplitude range. This feature is crucial in live sound engineering and recording situations, as it helps to maintain sound quality and protect audio equipment from damage due to excessive levels.

Other options do not accurately capture the essence of a limiter's functionality. For instance, the first option describes the general operation of a device that controls amplitude but lacks the specificity regarding the high ratio of reduction that characterizes a limiter. The second option suggests that an audio signal processor increases volume without any limitations, which contradicts the purpose of a limiter. Lastly, the fourth option refers to a device that adds harmonics to an audio signal, which is unrelated to the limiting process.