Apparatus for detecting frequency of input signal with a digital

Abstract

A frequency detector is used with a digital scanning converter that enables a monitor receiver having a particular horizontal deflection frequency to monitor various video signals having various horizontal frequencies. The frequency detector has a counter for counting a reference clock during a predetermined time interval derived from an input signal to be measured and a computing circuit for adding or subtracting a first constant to or from a setting cycle in response to a difference between a count value of the counter and the setting cycle. When the difference between the count value and the setting cycle exceeds a second constant during a predetermined number of such time intervals, the setting cycle is adjusted by an output from the computing circuit, and an approximate value of a cycle of the signal to be measured is computed, thereby detecting the approximate frequency. This process goes through as many iterations as necessary to bring the difference between the count value of the counter and the setting cycle to a value less than the second constant. Even when the frequency of a signal to be measured has a value selected from a number of values that differ greatly from one another, the frequency detector thus homes-in on the period of the signal to be measured and enables detection of the frequency with high accuracy. Also, even when pulse dropouts or time base fluctuations occur in the signal to be measured, the frequency detector can obtain a stable detected value.