stable true when output 522 is set the output of and gate 509 is high when high the 2 to 1 multiplexer 508 copies signal 521 on its output therefore dff 510 loads an image of the input value u201c pha u201d on signal 525 otherwise dff 510 and multiplexer 508 re circulates the data 525 both cells multiplexer 509 and dff 510 constitute a sample and hold function as a consequence if u201c pha u201d is not stable for more than 4 clock cycles u201c pha u201d is not copied on output 525 the filter circuitry described in fig5 delays the output by 4 clock cycles compared to the input pha fig6 illustrates the waveforms of the circuitry of fig5 the circuitry shown in fig5 can be found on many existing integrated circuits because it prevents false detection of direction changes in rotation a spurious pulse locally looks like a direction change refer to fig4 if this circuitry is efficiently filtering spurious pulses introduced by dusts located on the disk the circuitry cannot efficiently filter the noise introduced by large dust particles or scratches because a scratch may cover several reflective bars if the filtering capability e g the u201c max u201d value of the circuitry of fig5 is increased then correct pulses from the reflective bars may be filtered resulting in no rotation detection so another technique and circuitry is needed for large dust particles and scratches such circuitry can be located downstream compared to the previously described filtering circuitry shown in fig5 fig7.