A method of operating a display device includes: receiving image data at an input frame frequency; generating a modulated clock signal by modulating an input clock signal according to a modulation frequency; randomly selecting an output frame frequency within a data frequency selection range, the input frame frequency being within the data frequency selection range; determining an output start timing of the image data based on the output frame frequency; initiating, at the output start timing, output of the image data in synchronization with the modulated clock signal; and displaying an image based on the outputted image data.