SCAN LINES

What is a Scan Line?

A scan line is one vertical resolution line, which goes across the screen horizontally, on a Cathode Ray Tube (CRT) monitor.

Scan lines are stacked vertically to produce one image. The number of horizontal lines across the screen are counted vertically from top to bottom and this number is referred to as vertical resolution.

The Illumination Process of Scan Lines

A scan line refers to a line which is illuminated from left to right and travels on the screen from top to bottom. This illumination process is very fast, even human eyes cannot distinguish between a series of scans as individual processes.

Once a scan line has traveled across the screen vertically, it represents a field or in other words one complete cycle of a scan line from top to bottom is called a field. These fields are generated usually 50 to 60 times in a second.



Types of Scans

1. Interlace Scan: To display an interlaced video on modern monitor (LCD, LED) a process called deinterlacing is required. As deinterlacing is a software / hardware induced, it may cause certain artifacts in the display.




2. Progressive Scan: The image is displayed by producing all the vertical lines sequentially, producing full image in one frame.




Benefits of Interlace Scan (in comparison with Progressive scan)
1. Uses half the bandwidth for transmission
2. Double the refresh rate
3. Produces higher resolution

Downside of Interlace Scan
1. Causes interlacing effect on modern monitors (LCD, LED)
2. Causes artifacts in a video (in some cases)

Video Formats

PAL: Phase Alternating Line (PAL) is the most widely used broadcast video format. PAL plays 50 interlaced scans/second, which means 25 alternating fields or 25 frames in one second.

PAL Standard uses 576i that is 720×576 split into two interlaced fields of 288 lines.

NTSC: National Television System Committee (NTSC) is a display format which displays 60 interlaced scans/second, which means 30 alternating fields or 30 frames in one second.

NTSC standard uses an analog system of 486i split into two interlaced fields of 243 lines.