Television (TV) is a telecommunicating medium for transmitting and receiving moving images that can be black and white or colored, with or without accompanying sound. "Television"
may also refer specifically to a TV set, TV programming, or Telecommuniation.
. The Broadcast television system is typically disseminated via radio transmissions
on designated channels in the 54–890 MHz frequency band Signals are now often transmitted with stereo or surround sound in many
countries. Until the 2000s broadcast TV programs were generally transmitted as
an analog television signal,
but in 2008 the USA went almost exclusively digital.A standard television set comprises multiple internal electronic circuits,
including those for recieving and decoding broadcast
signals. A visual display device which lacks a tuner is properly called a video monitor, rather than a television.
A television system may use different technical standards such as digital television (DTV) and high definition television(HDTV). Television systems
are also used for surveillance, industrial process control, and guiding of
weapons, in places where direct observation is difficult or dangerous
The first images transmitted electrically
were sent by early mechanical fax machines, including the pantelgraph, developed in the late nineteenth century. The
concept of electrically powered transmission of television images in motion was
first sketched in 1878 as the telephonoscope, shortly after the invention of the telephone.
At the time, it was imagined by early science fiction authors, that someday
that light could be transmitted over copper wires, as sounds were.
The idea of using scanning to transmit images was
put to actual practical use in 1881 in the pantelegraph, through the use of a pendulum-based scanning
mechanism. From this period forward, scanning in one form or another has been
used in nearly every image transmission technology to date, including
television. This is the concept of rasterization, the process of converting a visual image
into a stream of electrical pulses.
In 1884 Paul Gottlieb Nipkow,
a 23-year-old university student in Germany, patented the first
electromechanical television system which employed a scanning device, a spinning disk with a
series of holes spiraling toward the center, for rasterization. The holes were
spaced at equal angular intervals such that in a
single rotation the disk would allow light to pass through each hole and onto a
light-sensitive selenium sensor which produced
the electrical pulses. As an image was focused on the rotating disk, each hole
captured a horizontal "slice" of the whole image
