A scanline is a line of vertical resolution on a CRT computer monitor or SDTV screen, created by an electronic gun illuminating each pixel one line at a time. Each line is referred to as a scanline, and the process is too fast for human perception. There are two types of scanning: interlaced and progressive. The number of scan lines is used to indicate the resolution of the display.
A scanline typically refers to a line of vertical resolution, running horizontally across a screen, on a cathode ray tube (CRT) computer monitor or standard definition television (SDTV) screen. Monitors and television screens that used a CRT tube have a screen made up of rows arranged horizontally across the screen, which were counted vertically from top to bottom, and were therefore referred to as vertical resolution. The images displayed on this type of screen are created by an “electronic gun” that causes images to appear on each pixel of the screen one line at a time. Each line displayed was referred to as a scanline, although the process was usually too fast for human perception.
The basic way a CRT computer monitor works is through a tube that includes numerous pixels arranged in a series of horizontal lines across the screen. These lines are counted to determine the vertical resolution of that screen; an SDTV usually has about 525 with 480 visible lines. Each of these lines is referred to as a scan line. The image on a screen is created by the pixels in each line being correctly illuminated by electrons moving from a filament, called an “electron gun”, on the back of the monitor or television towards the screen.
A scanline refers to each line of pixels lit from top to bottom and usually left to right. This process is so fast that the human eye and brain typically do not recognize the series of scans as individual processes, since each full scan of the screen on a progressive scan TV or monitor usually occurs between 30 and 60 times per second . There are two different ways that each scanline on a screen can be illuminated or scanned, referred to as interlaced and progressive scanning.
Interlaced scanning occurs when every other scanline on a screen is displayed together, alternately. This is typically done with the odd lines displayed and then the even lines. The human brain and eye do not notice this as the usual 30-60 frames per second are split in two so that two sets of frames occur 30 to 60 times per second. Persistence of Vision effectively combines the two sets of scanned images into one full screen. In contrast, progressive scan displays scans each line of an image each time the screen is scanned.
Interlaced screens are indicated with a small “i” while progressive screens are indicated with a small “p”. The number that usually accompanies this letter indicates the scan line count for a particular monitor or screen. An SDTV usually has 480 lines and would be labeled as 480i or 480p. Although a high-definition television (HDTV) doesn’t use a scanline system in the same way a CRT display does, the number of lines of pixels is still used to indicate the resolution of the display, such as 720i or 1080p.
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