说明：  1理想过滤函数名：idealfilter 2函数：将待滤波信号的FFT变换结果（即待滤波信号的频谱）作为输入参数发送给理想滤波函数，该函数对输入的FFT变换结果进行理想低通或带通滤波，输出滤波后的频谱。
(1. Ideal filter function name: idealfilter 2. Function: the FFT transform result of the signal to be filtered (i.e. the spectrum of the signal to be filtered) is sent to idealfilter function as the input parameter, and the function performs ideal low-pass or band-pass filtering on the input FFT transform result, and outputs the filtered spectrum.)