Progressive and Interlaced Scanning

Pictures displayed on television screens, whether it be an old black & white set from the 40’s or one of the new Plasma HDTV sets sold today, are made up of a series of horizontal lines. These lines are comprised of tiny dots, called pixels, of black or white for old black and white sets or red, blue, and green for color televisions. The number of these lines and dots determine the “resolution” of the picture we see. The higher the resolution, the sharper the picture. The format of the picture you are viewing determines how these lines of dots are formatted and displayed on your TV screen.

Television pictures are created by a sequence of lines of resolution scanned and displayed in one of two different ways. One is called progressive scanning and the other is interlaced scanning. Progressive scanning creates an image by scanning horizontal lines made up of tiny dots. These lines are scanned starting with the first line then the second, third, fourth, fifth, and so on until the entire frame is scanned. Thirty of these frames are scanned in one second. These frames, like the frames of a motion picture, when played back one after the other create the look of motion.

Our current analog interlaced picture is achieved by scanning 525 horizontal lines. The total number of lines scanned to produce an image is the resolution of the system, i.e. 525i meaning, 525 lines scanned using the interlaced method. A single frame of an interlaced picture is made up by scanning the 525 lines, but not at the same time. First, the 262.5 odd numbered lines are scanned. We call that the first field of the frame. Then the 262.5 even numbered lines are scanned. This makes up the second field of the image. These two fields make up one complete frame of the image. But not all of those 525 lines actually make up the picture we see. Lines 1-20 in field 1 and lines 264 – 283 in field 2 are reserved for signal information pertaining to picture synchronization. Therefore, only 480 of the total scan lines (240 per field) actually make up the picture.

It takes 30 frames to make up one second of television image. Most television sets and video monitors in the U.S.A. still display an image based on this 60-year-old standard set by the NTSC, (National Television Standards Committee) in 1941.

Even though VHS tape players and DVD’s are relative newcomers to the television arena, they playback even lower resolution than the analog television standard. DVD players only output 450 lines; while VHS players are only capable of outputting a low resolution of only 240 lines, one half the capable resolution of analog televisions.

(NOTE: The reference to 525 lines of resolution refers to vertical resolution. When we say that a DVD is capable of 450 lines and a VHS s capable of 240 lines, we are actually talking about their ability to reproduce a certain number of distinct dots along the horizontal plane. They both continue to produce 525 actual lines from top to bottom. In other words, vertical resolution is always locked at 525 lines in our old NTSC format. Horizontal resolution is dependent on the equipment’s ability to respond rapidly to change. VHS can only switch on an off about 240 times as the horizontal scan occurs so it is said to have 240 lines of resolution.)