Video Signals And Timing: Difference between revisions

When you increase the resolution, you will need to send more pixels to the display. If you would keep sending pixels at the same rate, the time to transmit one frame will go up, and consequently the amount of frames in a certain timespan will go down. Since a CRT displayed pixel only gives light for a short time before running out of energy, it needs to be repeatedly refreshed. If this is done fast enough (at about 60Hz, 60 times a second) the screen appears almost constant to the human eye. This improves further when the refresh rate goes up to a point where it doesn't matter to the human eye. However when it drops too much, the screen starts appearing flashing, causing headaches to the user. Hence, we need to keep the frequency at at least 60Hz for user's sanity, and below some other rate dictated by the monitor's capabilities. To make a full frame of pixels fit within one sixtieth of a second, we will have to adjust the speed at which these pixels are transmitted. This speed is called the pixel clock, or dot clock. For example, a VGA's dot clock is either 25MHz or 28MHz, corresponding to 25 million pixels per second or 28 million pixels per second, the latter one being only just enough to display a resolution of 720x480 at 60Hz (recall that the active display is only a part of the frame). Most higher resolution video can therefore use a wide range of dot clocks, well above 25Hz, with the current range allowing enough bandwith to easily exceed 1600x1200 at a 100Hz.