Patent ID: 8294820

Claim:
A video signal synchronization signal generating apparatus for making a display reference synchronization signal that serves as a reference of video display and has a first frequency and an input synchronization signal that constitutes images and has a second frequency synchronized with each other and outputting a resulting signal as a display synchronization signal, the apparatus comprising: a frequency ratio generating section configured to divide a frequency that is double the first frequency by the second frequency to calculate a frequency ratio; and a synchronization signal generating section configured to generate a signal having pulses that are inserted by equally dividing one period of the input synchronization signal by the frequency ratio and remove the alternate pulses of the signal to generate a synchronization signal of a same phase as the phase of the input synchronization signal; wherein the frequency ratio generating section calculates the frequency ratio using a counter value of a first counter which is reset according to the display synchronization signal and a counter value of a second counter which is reset according to the input synchronization signal; a display synchronization selecting section configured to output the synchronization signal as the display synchronization signal if the synchronizability determining section determines that the synchronization signal is within the frequency limit range, and output a synchronization signal having the displayable lowest frequency as the display synchronization signal if the synchronizability determining section determines that the synchronization signal is not within the frequency limit range.