A television or recorded video image is basically made up of scan lines. Unlike film, in which the whole image is projected on a screen at once, a video image is composed of rapidly scanning lines across a screen starting at the top of the screen and moving to bottom. These lines can be displayed in two ways. The first way is to split the lines into two fields in which all of the odd numbered lines are displayed first and then all of the even numbered lines are displayed next, in essence, producing a complete frame. This process is called interlacing or interlaced scan.
The second method, used in digital video recording, digital TVs, and computer monitors, is referred to as progressive scan. Instead of displaying the lines in two alternate fields, progressive scan allows the lines to displayed sequentially. This means that both the odd and even numbered lines are displayed in numerical sequence.
Analog Video: NTSC/PAL/SECAM
The number of vertical scan lines dictates the capability to produce a detailed image, but there is more. It is obvious at this point that the larger the number of vertical scan lines, the more detailed the image. However, within the current arena of video, the number of vertical scan lines is fixed within a system. The current analog video systems are NTSC, PAL, and SECAM.
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. There is a down side to PAL however, since there are fewer frames (25) displayed per second, sometimes you can notice a slight flicker in the image, much like the flicker seen on projected film. Countries on the PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East.
SECAM is the "outlaw" of analog video standards. Like PAL, it is a 625 line, 50 field/25 frame per second interlaced system, but the color component is implemented differently than in either PAL or NTSC. Countries on the SECAM system include France, Russia, Eastern Europe, and some parts of the Middle East.
The number of scan lines, or vertical resolution, of NTSC/PAL/SECAM are constant in that all analog video recording and display equipment conforms to the above standards. However, in addition to vertical scan lines, the amount of dots displayed within each line on the screen contributes to a factor known as horizontal resolution which can vary depending on both the ability of a video recording/playback device to record the dots and the ability of a video monitor to display dots on a screen.
Continue on to Page 2 - Analog Video - Horizontal Resolution...