说明：  本工程实现了将三路合成的DSB信号用椭圆滤波器（低通，带通，高通）滤出； 其中的freqz函数，filter函数，fft函数都自己实现。 每种滤波器都分别用专门的函数分别写出，可以模块化。
( This project will be three-way synthetic a DSB signal elliptic filter (low pass, band-pass, high pass) filter out Freqz function of which, the filter function, fft function. Each filter are, respectively, with a special function, respectively, to write, can be modular.)