Signal processing device, and image output device

In a case of changing the resolution of the synthesized image signal obtained by synthesizing the first image signal and a second image signal, identification information for identifying whether or not the character signal is contained is added to the synthesized image signal based on presence/absence of the character signal, by a character signal creating unit and a character signal synthesizing unit. Then, based on the identification information, a resolution changing unit separates the synthesized image signal into a third image signal containing a component of the character signal and a fourth image signal containing no component of the character signal, changes the resolutions of the third image signal and the fourth image signal, and synthesizes the third image signal and the fourth image signal with the resolutions changed, based on the identification information.