lowerfilter

代码说明：

  ex5_11 频率采样技术：低通，朴素法 ex5_12 频率采样技术：低通, 最优法T1 & T2 分别用朴素法和最优法，并且在时域和频域进行比较。(ex5_11 sampling frequency: low-pass, a simple method ex5_12 sampling frequency: low-pass, optimal method T1 & T2, respectively, and the optimal use of a simple law, and in time domain and frequency domain comparison.)