Horizontal synchronizing signal frequency measuring instrument for multi-synchronism type display unit

Claim 1

A horizontal synchronizing signal frequency measuring instrument for a multi-synchronism display unit which receives a horizontal synchronizing signal and a vertical synchronizing signal from the outside to produce another horizontal synchronizing signal and another vertical synchronizing signal, comprising: an inputting section for receiving the vertical synchronizing signal and the horizontal synchronizing signal from the outside; a reference clock signal generation section for generating a reference clock signal having a frequency sufficiently higher than that of the horizontal synchronizing signal received by said inputting section; a first counter for receiving and counting the reference clock signal from said reference clock signal generation section; a second counter for receiving and counting the horizontal synchronizing signal received by said inputting section, said second counter being of the preset type wherein a count number to be counted can be set as a preset value; and a control section operable to detect one of a rising edge and a falling edge of the vertical synchronizing signal received by said inputting section, set a first preset value to said second counter, reset and start said second counter, detect that a count value of pulses by said second counter reaches the first preset value, renders a count control signal thereof active, set a second preset value to said second counter, reset and start said first counter and said second counter, detect that the count value of pulses by said second counter reaches the second preset value, render the count control signal inactive, read out a count value of said first counter then, and calculate a frequency of the horizontal synchronizing signal from the count value of said first counter, the second preset value set to said second counter and a frequency of the reference clock signal; said first counter counting the reference clock signal within a counting enabled period thereof which is controlled by the count control signal from said control section.