The number of times per second the screen is updated or "repainted." Depending on the video standard, the actual image is changed only 30 times per second for NTSC signal or 25 times per second for PAL. However, most LED systems use pulse-width modulation to generate the color levels, and if the image were only "painted" once for every change, there would be a noticeable flicker on the display. A refresh rate of greater than 60 times per second will minimize the flicker. In general, LED displays should be refreshed at 120 times per second (120 Hz) or greater.