Disclosed is a signal input system to input an image signal from source equipment to sink equipment through a High Definition Multimedia Interface, wherein the source equipment comprises a transmission section to transmit a format signal to the sink equipment through a predetermined first line in the High Definition Multimedia Interface, the format signal having a voltage value corresponding to a format of the image signal and the sink equipment comprises: a reception section to receive the format signal transmitted through the first line by the transmission section; a judging section to judge the format of the image signal based on the voltage value of the format signal which is received by the reception section; and a setting section to set an image output based on a judgment result of the judging section.