A pixel clock is an oscillator, timing circuit or external signal that divides incoming video into pixels. The speed of the pixel clock refers to the capability of a monitor, television or computer graphics card to process pixels per second. This is expressed in megahertz or million pixels per second.
Video Graphics Array, or VGA, was first introduced by IBM in 1987. A VGA's pixel clock operates at 25 or 28 megahertz. VGA is recognized as the analog computer display standard. Digital Visual Interface, or DVI, offers a much faster pixel clock. Speeds can range from 165 megahertz for a single link to 348 megahertz for a dual link.