Analog television

Analog (or analogue) television encodes television picture and sound information and transmits it as an analog signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal. All systems preceding digital television, such as NTSC, PAL or SECAM are analog television systems.

Broadcasters using analog television systems encode their signal using NTSC, PAL or SECAM analog encoding and then modulate this signal onto a VHF or UHF carrier. An analog television picture is "drawn" on the screen an entire frame each time, in the manner of a motion picture (cinematograph) film, irrespective of the picture content.

Analog television technology

Analog television [EE 498 Course Notes, University of Washington [http://web.archive.org/web/20070204024644/http://www.ee.washington.edu/conselec/CE/kuhn/ntsc/95x4.htm] ] , like all other motion picture systems, exploits the properties of the human eye to create the illusion of moving images. The human eye retains an image for a fraction of a second, which is called "persistence of vision". Due to the persistence of vision effect, a rapid sequence of images will be perceived as an integrated moving image. If the rate of frames is too low, the sequence of images is not intuitively linked by the brain, causing the illusion of animation to be lost. The common frame rate of 24 frames per second, (which superseded more experimental frame rates during the sound revolution of the late 1920's) is used for motion pictures to create a smooth moving image. When the American television standard were developed, 30 Hz was chosen at the frame rate, modified to 29.97 Hz when color was introduced. Systems used in Europe have a frame rate of 25 frames per second.

When on screen images are bright, the persistence of vision effect does not last as long, which meant that more frames had to be projected per second. Motion picture projectors resolve this problem by using shutters. Since shutters cannot be used for televisions, television engineers increased the repetition rate to two "flashes" per frame by interlacing and scanning a single frame two times. These interlacing repeated frames do come at a cost, though; in some cases, the repeated frames cause aberrations such as serrations on the edge of moving objects, misalignment, interline flicker, or a shimmering effect.

In black and white television based on a cathode ray tube (CRT), a single electron beam scans a phosphor screen from left to right and then returns to the top. The electron beam is brightness-modulated to create intensity changes which cause the different shades of grey. Analog television equipment has been manufactured using alternative forms of display, such as LCD, but the picture display is still updated a frame at a time in the same manner as the flying-spot CRT.

To support color signals contained in the broadcast, a color synchronization signal called a "color burst" is added to the basic black and white information. When color television was introduced, engineers ensured that black and white televisions would still be able to display signals that were broadcast in color. To do this, the original monochrome information is still transmitted in the color signal, and then the color difference information is added on top.

Analog broadcast television systems comes in a variety of frame rates and resolutions. Further differences exist in the frequency and modulation of the audio carrier. The monochrome combinations still existing in the 1950s are standardized by the ITU as capital letters A through N. When color television was introduced, the hue and saturation information was added to the monochrome signals in a way that black & white televisions ignore. This way backwards compatibility was achieved. That concept is true for all analog television standards.

However there are three standards for the way the additional color information can be encoded and transmitted. The first was the American NTSC (National Television Systems Committee) color television system. The European PAL (Phase Alternation Line rate) and the French-Former Soviet Union SECAM (Séquentiel Couleur Avec Mémoire) standard were developed later and attempt to cure certain defects of the NTSC system. PAL's color encoding is similar to the NTSC systems. SECAM, though, uses a different modulation approach than PAL or NTSC.

In principle all three color encoding systems can be combined with any of the scan line/frame rate combinations. Therefore, in order to describe a given signal completely, it's necessary to quote the color system and the broadcast standard as capital letter. For example the United States uses NTSC-M, the UK uses PAL-I, France uses SECAM-L, much of Western Europe uses PAL-B/G, most of Eastern Europe uses PAL-D/K or SECAM-D/K and so on.

However not all of these possible combinations actually exist. NTSC is currently "only" used with system M, even though there were experiments with NTSC-A (405 line) and NTSC-I (625 line) in the UK. PAL is used with a variety of 625-line standards (B,G,D,K,I) but also with the North American 525-line standard, accordingly named PAL-M. Likewise, SECAM is used with a variety of 625-line standards.

For this reason many people refer to any 625/25 type signal as "PAL" and to any 525/30 signal as "NTSC", even when referring to digital signals, e.g. on DVD-Video which don't contain any analog color encoding, thus no PAL or NTSC signals at all. Even though this usage is common, it's misleading as that is not the original meaning of the terms PAL/SECAM/NTSC.

Many countries have, or have decided to, cease analog transmissions to switch to digital broadcasting - this is generally referred to as Digital switchover. In the United States, analog tranmission will end on February 17, 2009. In Canada, it will cease on August 31, 2011. [Ottawa Citizen, August 18, 2007 [http://www.friends.ca/News/Friends_News/archives/articles08180701.asp] .]

Common analog television systems

* NTSC
* PAL
* SECAM
* Slow-scan television
* Narrow-bandwidth television

ee also

* Broadcast television system
* Terrestrial television
* Analog transmission

References