butterworth

代码说明：

  Most practical filters exhibit steady roll-off characteristic, in contrast to the sharp transition from the pass and stop bands. The important families of filters are: Butterworth, Chebyshev and Elliptic. As part of this lab, we study and design Butterworth filters. Butterworth filters are simple to design and have a maximally flat pass band response. In contrast the Chebyshev and Elliptic filters are difficult to design and have ripples in pass/stop band but have sharper roll off characteristics than Butterworth filters.