Patent ID: 8164689

Claim:
A synchronizing signal control circuit comprising: a phase detecting section configured, when among a plurality of asynchronous video signals, a video display state based on a video signal synchronized with a first synchronizing signal is switched to a video display state based on a video signal synchronized with a second synchronizing signal, to detect a first phase difference between a vertical synchronizing signal of a display synchronizing signal and a vertical synchronizing signal of the second synchronizing signal for each vertical synchronization of the display synchronizing signal; an adding section configured to add a set value for setting a synchronization compensation period and the first phase difference detected in the phase detecting section to output the addition result; a first synchronization phase correcting section configured to correct the phase of the vertical synchronizing signal of the second synchronizing signal on the basis of the output signal of the adding section to obtain a third synchronizing signal; a gate signal generating section configured to generate a gate signal representing a synchronization compensation period from the output signal of the adding section on the basis of the vertical synchronizing signal of the display synchronizing signal; a synchronization determining section configured, when receiving the gate signal, to determine whether or not the synchronization can be effected, by detecting whether or not the vertical synchronizing signal of the second synchronizing signal exists within the synchronization compensation period; and a selecting section configured to perform switching from the first synchronizing signal to the third synchronizing signal on the basis of the determination result of the synchronization determining section.