Abstract: Apparatus and method for detecting occurrence of a non-pulse interval of predetermined length between a beginning pulse and an ending pulse as included within a sequence comprising a unique timing mark within a stream of pulses supplied from a source with a state machine. The method of the state machine includes the the steps of:advancing through a plurality of nominal states marking in time the non-pulse interval,branching to one of a plurality of available spurious pulse states from one of the nominal states upon detection of a first unexpected pulse,returning to advance through predetermined remaining ones of the plurality of nominal states following detection of the first unexpected pulse,reaching a completion state following the nominal states if only the first unexpected pulse is detected, andreaching an error state following a branch to another one of the plurality of available spurious pulse states upon detection of a second unexpected pulse within the non-pulse interval.