Butterworth, Chebyshev, Bessel and other three filters introduced above all have the same feature, that is, the zero point of the transfer function is at infinite frequency. In other words, these filters can only give infinite attenuation at infinite frequency, which is the so-called all-pole filter.

Non-all pole filter

For example, the similarity between elliptic function filter and anti-Chebyshev filter is that the transfer function contains both zeros and poles, so that infinite attenuation can be obtained at the frequency limit. The above two filters are non-pole filters. The above filters are all non-all pole filters.

The knowledge of all-pass filter is easy to find on Baidu, so I won't go into details here.