Analog television

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Analog television (or analogue television) encodes television and transports the picture and sound information as an analog signal, that is, by varying the amplitude and/or frequencies of the broadcast 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.

Contents 1 Analog television technology 2 Comparison of analog and digital television 3 Digital switchover 3.1 United States 3.2 United Kingdom 3.3 Australia 3.4 Finland 3.5 Canada 4 Common analog television systems 5 References 6 External links 7 See also

Analog television technology

Analog television, 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, such as 16 frames per second, the moving images will seem jumpy and jerky. Frame rates of 24 frames per second were used for motion pictures to create a smooth moving image. When NTSC television standards were developed, 30 Hz was chosen at the frame rate, and then later 29.97 Hz. ^[1]

When onscreen 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.^[2]

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. Analogue 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. ^[3]

There major standards for television are American NTSC (National Television Systems Committee) color television system, the European PAL (Phase Alternation Line rate) and the French-Former Soviet Union SECAM (Sequential Couleur avec Memoire) standard. The three systems have different numbers of vertical lines. NTSC uses 525 lines (interlaced). In contrast, PAL and SECAM use 625 lines. NTSC displays more frames per second than PAL and SECAM. PAL's color encoding is similar to the NTSC system's. SECAM, though, uses a different modulation approach than PAL or NTSC. ^[4]

Comparison of analog and digital television

An analog television picture is re-drawn on the screen an entire frame each time, as occurs with a motion picture (cinematograph) film. This re-drawing of each frame is irrespective of the picture content. As there is no image compression of any kind in analog TV, there is a common belief there are signals in an analog broadcast considered as carrying redundant information when the signal bandwidth is compared to that of a digital version of the same broadcast. However, this is partly incorrect, when viewing an analog transmission, subject movement appears always to be 'natural and flowing' at all times, with a total absence of the 'jerky' unnatural movement effects which are seen with present-day digital TV technology such as MPEG2. Each frame is refreshed in its entirety from the original source, as in the case of a motion picture cine film and has a more 'real-life' look to it, as opposed to digital TV which takes on a distinct 'computer-graphics' appearance.

The picture quality of analogue tv is (early 2007) of a generally 'snowy' appearance when compared to a decade ago, as most components in the UHF signal chain from the transmitter to the viewer, especially rooftop antennas are no longer being maintained.

The vertical and horizontal color resolution of analogue tv is half that of the monochrome picture resolution, because of PAL or NTSC processing. This effect goes unnoticed, as the eye is considerably more tolerant of a reduction of color detail to that of monochrome detail. Digital TV has equal color and monochrome resolutions because of the lack of PAL/NTSC signal processing.

Note that distributing the broadcast in the native uncompressed digital form (such as is present on a Digital Audio Compact Disc 'CD') would require an approx 250 G bit/s data rate, which is many times in excess of the bandwidth needed by a standard analog UHF channel.

Analog television does not suffer from certain picture defects seen on present-day digital television such as the Paint-By-Numbers color effect, in which light obliquely falling onto a surface should appear smoothly-graduated, but instead appears as a series of bands, this is Quantisation Distortion due to reduced sample depth. Another problem that may occur with digital television is poor lip synchronization. Because analog TV receives and outputs the video and sound simultaneously in real time, with no decompression and error correction, synchronization problems do not occur.

Additionally, digital television may also have a complete loss of reception, known as the 'stonewall curve'; in contrast, analog TV quality will degrade in proportion to the reduction in signal strength in a predictable manner, however digital TV allows a significant drop in the signal strength before the picture is affected, whereupon it will unexpectedly freeze up or 'block out'). In some cases, digital television may have delayed reception of live broadcast events, such as Big Ben chimes in London for the New Year. Signal processing in analog TV is instantaneous and does not involve constructing a picture while unpacking compressed audio, which may take place over several seconds. Lastly, digital images have 'jerky' unnatural movement effects when slowly-moving subjects are depicted.

Digital switchover

United States

US television stations are scheduled to switch to digital output by February 2009. ^[5] In 1996, the US Congress had declared December 2006 to be the switchover date, but the date was extended to allow more consumers to purchase televisions with digital tuners. As of March 2007, all newly produced televisions of all sizes and all newly produced recording devices sold in the US were required to have digital reception capability if marketed as "televisions." At that point tunerless sets will be marketed as "monitors." It is expected that legacy analog televisions will be accommodated by inexpensive converter devices available by the time analog is shut off in 2009.

Although 85% of US residents get their television service from cable or satellite companies, the remaining 15% still use over-the-air television. In 2007 the United States federal government announced a subsidy program for purchasing digital tv converters. Under the rules, all U.S. households will be able to apply for up to two $40 coupons to defray the cost of a basic digital-to-analog converter box during the program's initial phase, in which up to 22.5 million coupons are expected to be available. Beginning January 1, 2008, households will be able to make such requests through a toll-free phone number, a Web site, fax or postal mail. March 31, 2009 is the last day to make the requests. Boxes are expected to cost between $50 and $70 apiece. [1]

United Kingdom

UK analogue TV is to be switched off between 2008 and 2012.^[6] See main article.

Australia

In Australia, Analogue will continue until at least 2008.^[7]

Finland

Finnish analog television broadcasts are to be switched off on August 31, 2007.

Canada

In Canada, no official switchover date has been set yet, because the broadcast regulator, the CRTC wishes to let the market set the pace for the changeover.

Common analog television systems

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

References

Wikipedia information about Analog television This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Analog television". It may not have been reviewed by professional editors (see full disclaimer). Help contribute to Americola and edit this article.