On parasitic oscillations in digital filters under looped conditions

If a filter is used in multiplex telephone equipment, it actually operates in a loop due to the presence of two-wire/four-wire terminating equipment. Due to this, the power fed back into the filter input can, under adverse conditions, nearly equal the power leaving the output. This can be a cause for parasitic oscillations in case of digital filters. If properly designed, however, wave digital filters and nonrecursive digital filters remain stable.