Horizontal synchronization detection device

Abstract

A horizontal synchronization detection device includes a pulse detection portion that detects a pulse in a horizontal synchronization signal contained in a video signal and acquires a pulse width of the detected pulse, a synchronization pulse decision portion that determines the pulse satisfying a condition that a difference between its pulse width and a reference pulse width is within a first predetermined range as a synchronization pulse, a mean pulse width acquisition portion that averages out a pulse width of the synchronization pulse for each field and obtains a mean pulse width thereof, and a reference pulse width correction portion that determines, for each synchronization pulse in a current field, whether a difference between its pulse width and the mean pulse width in a previous field is within a second predetermined range, and corrects the reference pulse width and/or the first predetermined range based on the determination result.

1 . A horizontal synchronization detection device comprising:
 a pulse detection portion that detects a pulse in a horizontal synchronization Signal contained in a video signal and acquires a pulse width of the detected pulse;   a synchronization pulse decision portion that determines the pulse satisfying a condition that a difference between its pulse width and a reference pulse width is within a first predetermined range as a synchronization pulse;   a mean pulse width acquisition portion that averages out a pulse width of the synchronization pulse for each field and thereby obtains a mean pulse width of that field; and   a reference pulse width correction portion that determines, for each of the synchronization pulse in a current field, whether or not a difference between its pulse width and the mean pulse width in a previous field obtained by the pulse width acquisition portion is within a second predetermined range, and corrects either one or both of the reference pulse width and the first predetermined range based on the determination result.   
     
     
         2 . The horizontal synchronization detection device according to  claim 1 , wherein when a number of occurrence of the synchronization pulse for which the difference between the pulse width and the mean pulse width in the previous field exceeds the second predetermined range exceeds a threshold in the current field, the reference pulse width correction portion carries out either one or both of correcting the reference pulse width so as to get closer to the mean pulse width in the current field and correcting the first predetermined range so as to become narrower. 
     
     
         3 . The horizontal synchronization detection device according to  claim 2 , wherein the threshold is 1/m (m>1) of a number of occurrence of the synchronization pulse for which the difference between the pulse width and the mean pulse width in the previous field is within the second predetermined range. 
     
     
         4 . The horizontal synchronization detection device according to  claim 1 , wherein the second predetermined range is ±1/n (n>1) of the mean pulse width in the current field.