A: Moving pictures, be them television or film, use a succession of still images or frames played in rapid succession to simulate the perception of motion. Film uses 24 frames per second while video uses 30 frames per second. These frame rates display the images fast enough so that you don’t notice the individual frames. With an interlaced video format, each frame is divided into horizontal lines, or strips, which are then alternately divided into two fields. If you numbered the lines from top to bottom, field one would contain the odd numbered lines and field two would contain the even numbered lines. The two new fields are shown twice as fast (1/60th of a second each) as the original frame, creating a 60 cycle or 60 Hz system. If both fields came from the same frame, your eye will blur the two together and the effective resolution will be as high as if you saw a single frame. This is an easy way to get a signal with full resolution at only half the bandwidth. If, however, the two fields are different (ie there’s motion between the two fields), then the effective resolution can drop as much as in half. When interlaced signals are displayed on high-resolution CRT-based projectors or TVs, the gaps between the scan lines can be significant.

A progressively scanned image displays each horizontal line of information from each frame in sequential order. Line 2 follows line 1. Line 3 follows line 2 and so on. When a TV or video processor de-interlaces video signals each frame is just repeated so that the progressive image retains the same 60-frame cycle. Progressive images provide more information per field than comparable interlaced signals, but require twice as much bandwidth to transmit the signal. Nearly all pixel-based devices (DLP, LCD, LCoS and most plasma TVs) display progressive images. On high resolution CRT-based TVs or projectors, a progressively scan image can fill in much of the black space otherwise visible between interlaced scan lines which, in turn, increases light output.