special direct current filters are used to eliminate high frequency interference. such filters are required if the transmission line will use power-line communication techniques for communication and control, or if the overhead line will run through populated areas. these filters can be passive lc filters or active filters, consisting of an amplifier coupled through transformers and protection capacitors, which gives a signal out of phase to the interference signal on the line, thereby cancelling it. such a system was used on the baltic cable hvdc project.