A video picture is usually drawn as a series of horizontal lines. An electron gun at the back of the picture tube draws lines of the video picture back and forth, much like the printer head on your printer moves back and forth as it prints words on a page. All three broadcast video standards - NTSC, PAL, and SECAM - are interlaced, meaning the horizontal lines are drawn in two passes rather than one. Every other line is drawn on each consecutive pass, and each of these passes is called a field. So on a PAL display, which shows 25 fps, there are actually 50 fields per second.
Noninterlaced displays are becoming more common. Modern computer monitors, for example, are all noninterlaced - all the lines are drawn in a single pass. Some HDTV (high-definition television) formats are noninterlaced; others are interlaced.
Most camcorders record interlaced video, but some high-end camcorders also offer a progressive-scan mode. Progressive scan - a fancy way of saying noninterlaced - is a worthwhile feature if you can afford it, especially if you shoot a lot of video of fast-moving subjects. With an interlaced camera, a problem that is sometimes called interlacing jaggies can show up on video with a lot of movement. Figure 1, for example, shows video of a fast-moving subject that was shot using an interlaced camera. The stick seems to have some horizontal slices taken out of it, an unfortunate illusion created because the subject was moving so fast that the stick was in different places when each field of the video frame was captured. If the entire frame had been captured in one pass (as would have happened with a progressive-scan camcorder), the horizontal anomalies would not appear.
Figure 1: Interlaced cameras can cause interlacing jaggies on fast-moving subjects.
