what is analog television
Analog television service is the traditional method of transmitting television signals. Analog transmission has been the standard broadcast technology since the inception of television. Analog is not as efficient as digital television: it uses up much more of the valuable spectrum than digital, and TV stations can only transmit one channel of programming with an analog signal versus up to four or more programs with a digital signal in the same amount of spectrum. Analog is also susceptible to interference and “snow” that make a picture less clear.
analog and digital television
Digital television (DTV) is a telecommunication system for broadcasting and receiving moving pictures and sound by means of digital signals, in contrast to analog signals used by analog (traditional) TV. DTV uses digital modulation data, which is digitally compressed and requires decoding by a specially designed television set, or a standard receiver with a set-top box, or a PC fitted with a television card. Introduced in the late 1990s, this technology appealed to the television broadcasting business and consumer electronics industries as offering new financial opportunities.
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[1], 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 The common frame rate of 24 frames per second, (which superseded more experimental frame rates during the sound revolution of the late 1920’s) is used for motion pictures to create a smooth moving image.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.