Discussion of the frame rate of a system has been delayed until this point, where frames, fields, field rate, refresh rate and interlacing have been discussed.
The frame rate of a graphics or video system is the rate at which frames are displayed. Since a frame is a complete image, another way of stating this is the frame rate is the rate at which scanlines are updated. For a graphics system using progressive scanning, this is simply its refresh rate. However, for a video system (which uses interlaced scanning), this is half the field rate, since a field contains half the scanlines of the display. For example, NTSC systems have a field rate (and hence refresh rate) of 60Hz11, so they have a frame rate of 30Hz.
Since human persistance of vision is approximately 1/25th second, frame rates should ideally be 25Hz or greater to avoid noticeable flicker12. All the major video formats, that is, PAL, NTSC and HDTV have frame rates greater than or equal to 25Hz.