Apparatus for passing digital pulses from a bilevel digital signal source having widths wider than a given minimum and narrower than a given maximum is described. A first duration sensitive signal gate is coupled to the signal source and passes only signals having widths wider than the given minimum duration. A second duration sensitive signal gate is also coupled to the signal source and passes only signals having widths equal to or wider than the given maximum duration. A signal selection means is coupled to the first and second duration sensitive signal gates, and passes pulses which have been passed by the first but not the second duration sensitive signal gate.