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代码说明：

说明：  主要指令：butter（产生巴特沃斯滤波器）；freqz（绘制幅频、相频曲线）；zplane（绘制零、极点图）；ss2zp（状态方程变零、极点）；zp2tf（零、极点变分式多项式）。(Main instruction: butter (produced bart wo filter) Freqz (draw amplitude frequency, phase frequency curve) Zplane (draw zero, poles chart) Ss2zp (state equation is zero, poles) Zp2tf (zero, pole change fraction polynomial). )