对信号进行分析和处理时，常遇到有用信号被噪声污染的问题。因此，从信号中消除或减弱噪声，成为信号传输与处理中十分重要的问题。根据有用信号与噪声的不同特性，消除或减弱噪声，提取有用信号的过程称为滤波。实现滤波功能的系统或装置称滤波器。
经典的滤波器是具有选颁特性的电路，当噪声与有用的信号具有不同的频带时，它们通过滤波器后，噪声将被衰减乃至消除，有用的信号得以保留。
当噪声与有用信号的频带重叠时，上述滤波器(或称为经典滤波器)就无法实现既消除噪声，又保留信号的功能。(When analyzing and processing signals, we often encounter the problem that useful signals are polluted by noise. Therefore, eliminating or reducing noise from signals has become a very important problem in signal transmission and processing. According to the different characteristics of useful signal and noise, the process of eliminating or weakening noise and extracting useful signal is called filtering. The system or device that realizes filtering function is called filter.
Classical filters are circuits with selective characteristics. When noise and useful signals have different frequency bands, they will be attenuated or even eliminated after passing through the filter, and useful signals can be retained.
When noise overlaps with the frequency band of useful signals, the above filters (or classical filters) can not achieve the function of both eliminating noise and retaining signals.)