Interlaced scanning

Interlaced scan-based images use techniques developed for CRT (Cathode Ray Tube) TV monitor displays, made up of 576 visible horizontal lines across a standard TV screen. Interlacing divides these into odd and even lines and then alternately refreshes them at 30 frames per second. The slight delay between odd and even line refreshes creates some distortion or 'jaggedness'. This is because only half the lines keep up with the moving image while the other half waits to be refreshed.

Interlaced scanning has served the analog camera, television and VHS video world very well for many years, and is still the most suitable for certain applications. However, now that display technology is changing with the advent of Liquid Crystal Display (LCD), Thin Film Transistor (TFT)-based monitors, DVDs and digital cameras, an alternative method of bringing the image to the screen, known as progressive scanning, has been created.

There are two dominant interlaced scan systems used in the world today: NTSC and PAL.

NTSC is based on a 525-line, 60 fields/30 frames-per-second at 60Hz system for transmission and display of video images. This is an interlaced system in which each frame is scanned in two fields of 262 lines, which is then combined to display a frame of video with 525 scan lines. NTSC is the official analog video standard in the U.S., Canada, Mexico, some parts of Central and South America, Japan, Taiwan, and Korea.

PAL is the dominant format in the World for analog television broadcasting and video display and is based on a 625 line, 50 field/25 frames a second, 50HZ system. The signal is interlaced, like NTSC, into two fields, composed of 312 lines each. Several distinguishing features are one: A better overall picture than NTSC because of the increased amount of scan lines. Two: Since color was part of the standard from the beginning, color consistency between stations and TVs are much better. In addition, PAL has a frame rate closer to that of film. PAL has 25 frames per second rate, while film has a frame rate of 24 frames per second. Countries on the PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East.

Progressive scanning

Progressive scan differs from interlaced scan in that the image is displayed on a screen by scanning each line (or row of pixels) in a sequential order rather than an alternate order, as is done with interlaced scan. In other words, in progressive scan, the image lines (or pixel rows) are scanned in numerical order (1,2,3) down the screen from top to bottom, instead of in an alternate order (lines or rows 1,3,5, etc... followed by lines or rows 2,4,6). By progressively scanning the image onto a screen every 60th of a second rather than "interlacing" alternate lines every 30th of a second, a smoother, more detailed, image can be produced on the screen that is perfectly suited for viewing fine details, such as text, and is also less susceptible to interlace flicker. The primary intent of progressive scan is to refresh the screen more often.

Conversion of interlaced to progressive scan:

Converting interlaced to progressive scan requires different processing compared with line doubling just to increase the number of scan lines per frame for large screens. To convert to progressive scan and preserve both detail and fullness, material from two consecutive fields should be combined for "stationary" subject matter. For "moving" subject matter a needed portion of an even line should be a blend of the odd lines immediately before and after, and vice versa

A bit of history:

Up until the late 1980's, flicker on computer screens was very noticeable since single scan line details made up a much larger portion of screen content. Also with memory as a limiting factor, consumer PC's only had about 240 scan lines of picture information which incidentally hid most of the flicker. The "regular VGA" standard was based on NTSC, exactly twice the scan rate using the same 525 scan lines per frame and progressive scan, with 480 scan lines holding the picture and with up to 640 details on a scan line.

As larger TV screens were developed, more viewers started noticing the flicker due to the fading phosphors when the electron beam visited any given spot on the screen only once every 1/30'th of a second. When you "see the scan lines" you are really seeing the even gaps between the odd scan lines or vice versa, as the phosphors fade between refreshes. On small screens in the early days of TV, the electron beam was thicker than 1/480'th the screen height so these gaps were not as noticeable.