Under adequate sampling rate and time range, the convolution can be approximated by discrete convolution and the spectrum by Fourier Transform can be approximated by Fast Fourier Transform (FFT).

• 适当之离散取样密度及时域范围，则以离散迴旋积取代迴旋积，可获得近似输出讯号之波形，以快速富氏转换取代富氏转换，获可得输入及输出讯号之近似频谱及系统之近似频率响应。