A DSL filter or microfilter is an analog low-pass filter installed between analog devices (such as telephones or analog modems) and a plain old telephone service (POTS) line, in order to prevent interference between such devices and a digital subscriber line (DSL) service operating on the same line.he usual symptom of a failed filter is frequent DSL disconnects or slow internet speed, and the usual procedure to test for failed filters is to remove all filters from the telephone line leaving only the router and checking to see whether the DSL speed increased.