History

Main article: History of television

Television was developed by the combined efforts of several inventors. The origins of what would become today's television system can be traced back to the discovery of the photoconductivity of the element selenium by Willoughby Smith in 1873. The telectroscope was a hoax based on an article published in The New York Sun of 29 March 1877[1] and popularized by French editor Louis Figuier. Paul Nipkow proposed the first practical television principle based on a scanning disc in 1884, but the Nipkow principle had to wait until suitable amplifiers were developed before it became practical. In 1885 Australian inventor Henry Sutton [4] invented the Telephane. Sutton's invention included Nipkow's scanning disc. Although Sutton intend to view the Melbourne Cup at his Ballarat home he did not complete the manufacture of his invention.[5]

All practical television systems use the fundamental idea of scanning an image to produce a time series signal representation. That representation is then transmitted to a device to reverse the scanning process. The final device, the television (or TV set), relies on the human eye to integrate the result into a coherent image.
A transistor-based portable television, typical of NTSC models of the late 1960s and 1970s

Electromechanical techniques were developed from the 1900s into the 1920s, progressing from the transmission of still photographs, to live still duotone images, to moving duotone or silhouette images, with each step increasing the sensitivity and speed of the scanning photoelectric cell. John Logie Baird gave the world's first public demonstration of a working television system based on the Nipkow principle that transmitted live moving images with tone graduation (grayscale) on 26 January 1926 at his laboratory in London, and built a complete experimental broadcast system around his technology. Baird further demonstrated the world's first color television transmission on 3 July 1928. Other prominent developers of mechanical television included Charles Francis Jenkins, who demonstrated a primitive television system in 1923, Frank Conrad who demonstrated a movie-film-to-television converter at Westinghouse in 1928, and Frank Gray and Herbert E. Ives at Bell Labs who demonstrated wired long-distance television in 1927 and two-way television in 1930. Camarena invented the "Chromoscopic adapter for television equipment", an early color television transmission system. As it is written in the patent: The invention relates to the transmission and reception of colored pictures or images by wire or wireless. Even though the invention was not already adaptable to standard television equipment then in use; the invention was considered easy to adapt to any transmitter or receiver of black and white television equipment. He applied for this patent August 14, 1941 and obtained the patents for color television systems September 25, 1942 (U.S. Patent 2296019), 1960 and 1962.



Color television systems were invented and patented even before black-and-white television was working; see History of television for details.
A 1950s television

Completely electronic television systems relied on the inventions of Philo T. Farnsworth, Vladimir Zworykin (disputed as his product did not produce a satisfying image) and others to produce a system suitable for mass distribution of television programming. Farnsworth gave the world's first public demonstration of an all-electronic television system at the Franklin Institute in Philadelphia on 25 August 1934. All modern television systems derive directly from Farnsworth's model.

Regular broadcast programming occurred in the United States,[2] the United Kingdom,[3] Germany,[4] France,[5] and the Soviet Union[6] before World War II. The first regular electronic television broadcasts began in Germany in 1935, using first an electronic system with 180 lines, followed in 1937 with an improved system with 441 lines. The first regular public (i.e. not cable) television broadcasts with a modern level of definition (240 or more lines) were made in England in 1936 from Alexandra Palace. Baird's mechanical 240-line system alternated with EMI-Marconi's so-called "System A" with 405 lines; as this proved far more reliable, Baird's system was dropped after four months. Regular network broadcasting began in the United States in 1946, and television became common in American homes by the middle 1950s. While North American over-the-air broadcasting was originally free of direct marginal cost to the consumer (i.e., cost in excess of acquisition and upkeep of the hardware) and broadcasters were compensated primarily by receipt of advertising revenue, increasingly United States television consumers obtain their programming by subscription to cable television systems or direct-to-home satellite transmissions. In the United Kingdom, France, and most of the rest of Europe, on the other hand, operators of television equipment must pay an annual license fee, which is usually used to fund (wholly or partly) the appropriate national public service broadcaster/s (e.g. British Broadcasting Corporation, France Télévisions, etc.).

Technology

Elements of a television system
OT-1471 Belweder, Poland, 19571. power switch / volume
2. brightness
3. pitch
4. vertical synchro 5. horizontal synchro
6. contrast
7. channel tuning
8. channel switch


The elements of a simple television system are:
An image source - this may be a camera for live pick-up of images or VTR or a Film Chain-Telecine-flying spot scanner for transmission of films.
A sound source picked up by a microphone
A transmitter, which modulates one or more television signals with both picture and sound information for transmission.
A receiver (also called a tuner) which recovers and processes the picture and sound signals from the television broadcast signal.
A display device, which turns the electrical signals into visible light and images
A small amplifier and loudspeaker, which turns electrical signals into sound waves (speech, music, and other sounds) to accompany the images.

Practical television systems include equipment for selecting different image sources, mixing images from several sources at once, insertion of pre-recorded video signals, synchronizing signals from many sources, and direct image generation by computer for such purposes as station identification.

Transmission was originally over the air from land-based transmitters. The quality of reception varies greatly, and this led to the proliferation of large antennas on house roofs for best signal in the 1960s. In most cities today, cable systems deliver television over metal or optical cables, but for a fee. It can be delivered by radio from synchronous satellites, which were large for analog, and smaller dishes for digital broadcast, also for a fee, often less than cable systems, which has led to the appearance of small dishes outside of houses and apartments.

Digital systems may be inserted anywhere in the chain to provide better image transmission quality, reduction in transmission bandwidth, special effects, or security of transmission from reception by non-subscribers. A home today might have the choice of receiving analog or HDTV over the air, analog or digital cable with HDTV from a cable television company over coaxial cable, or even from the phone company over fiber optic lines. On the road, television can be received by pocket sized televisions, recorded on tape or digital media players, or played back on wireless phones over a high speed network or the internet.

Display technology

See also: Liquid crystal display television

Thanks to the advances in display technology, there are now several kinds of video displays used in modern TV sets:
CRT (cathode-ray tube): The most common screens were direct-view CRTs for up to roughly 100 cm (40 inch) (in 4:3 ratio) and 115 cm (45 inch) ((in 16:9 ratio) diagonals. These are the least expensive, and are a refined technology that can still provide the best overall picture quality value. As they do not have a fixed native resolution, they are capable of displaying sources with different resolutions at the best possible image quality. The frame rate or refresh rate of a typical NTSC format CRT TV is 29.97 Hz, and for the PAL format, 25 Hz, both are scanned with two fields per frame in an interlaced fashion. A typical NTSC broadcast signal's visible portion has an equivalent resolution of about 640x480 pixels. It actually could be slightly higher than that, but the vertical blanking interval (VBI), allows other signals to be carried along with the broadcast. High lead content.
Rear projection: Most very large screen TVs (to 100 inches [254 cm] or more) use projection technology. Three types of projection systems are used in projection TVs: CRT-based, LCD-based, and DLP (reflective micromirror chip) -based. Projection television has been commercially available since the 1970s, but at that time could not match the image sharpness of the CRT; current models are vastly improved, and offer a cost-effective large-screen display.
A variation is a video projector, using similar technology, which projects onto a screen.
A modern Philips LCD TV
Flat panel (LCD or plasma): Modern advances have brought flat panels to TV that use active matrix LCD or plasma display technology. Flat panel LCDs and plasma displays are as little as 25.4 mm (1 inch) thick and can be hung on a wall like a picture or put over a pedestal. Some models can also be used as computer monitors.
LED technology has become one of the choices for outdoor video and stadium uses, since the advent of bright LEDs and driver circuits. LEDs enable scalable ultra-large flat panel video displays that other technologies may never be able to match in performance.

Each has its pros and cons. Flat panel LCD display can have narrow viewing angles and so may not suit a home environment. Rear projection screens do not perform well in daylight or well-lit rooms and so are only suited to darker viewing areas. A comparison of these technologies is available here.

Terminology for televisions

Pixel resolution is the amount of individual points known as pixels on a given screen. A typical resolution of 720×480 means that the television display has 720 pixels across and 480 pixels on the vertical axis. The higher the resolution on a specified display the sharper the image. Contrast ratio is a measurement of the range between the brightest and darkest points on the screen. The higher the contrast ratio, the better looking picture there is in terms of richness, deepness, and shadow detail. The brightness of a picture measures how vibrant and impacting the colors are. Measured in cd / m2 equivalent to the amount of candles required to power the image.

On the other hand, the so-called brightness and contrast adjustment controls on televisions and monitors are traditionally used to control different aspects of the picture display. The brightness control shifts the black point, or shadow level, primarily affecting the contrast ratio or gamma of the image, while the contrast control primarily controls the image intensity or brightness.[7]