A filter system (and associated filtering method) is disclosed, the filter
system being connected between a large-amplitude, high-frequency noise
source and a frequency-division multiplexed communications channel. The
large-amplitude, high-frequency noise commonly being a POTS (plain old
telephone service) ring signal generated by a CO (central office)
telephone switch. The frequency-division multiplexed communications
channel generally used to carry digital signals at frequencies above the
POTS baseband in order to implement a digital subscriber line (DSL).
Large-amplitude, high-frequency signal components from the POTS ring
signal impair the performance of frequency-division multiplexed channels
on a transmission line such as a subscriber loop. The filter system of the
present invention utilizes amplitude and also may utilize frequency to
selectively suppress the transients and harmonics on the transmission line
caused by a telephone ring signal. In addition, the present invention uses
bidirectional current limiting filters to implement some of the
amplitude-dependent filtering behavior.
