{HEAD ~ A video signal} and a horizontal synchronizing signal provided from a work station are delayed by a plurality of delay circuits having different delay amounts, and phase differences between a sampling clock signal and {TAIL ~ the delayed signals} are detected 
{ HEAD ~ A video signal } and a horizontal synchronizing signal provided from a work station are delayed by a plurality of delay circuits having different delay amounts , and phase differences between a sampling clock signal and { TAIL ~ the delayed signals } are detected
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