technology that makes television work is a complex subject, explained
here in a basic, introductory fashion. Though television seems a
thoroughly modern invention, available only since mid-20th century,
the concept of recreating moving images electrically was developed
much earlier than is generally thought. It can be traced at least
to 1884 when Paul G. Nipkow created the rotating scanning disk which
provided a way of sending a representation of a moving image over
a wire using varying electrical signals created by mechanically
scanning that moving image.
Mechanical scanning of an image involved a spinning disk, with a
spiral grouping of holes, located at both the sending and receiving
ends. At the sending end, a photocell-like device varied the strength
of an electrical signal at a rate representing the amount of light
hitting the cell through the holes in the disk; the greater the
amount of light, the greater the strength of the electrical signal.
At the receiving end, a source of light varied in intensity at the
rate of the electrical signal it received and could be seen through
the holes in the rotating disc, thereby recreating a crude copy
of the image scanned at the sending end. Today, moving images are
scanned electronically as described below and the varying electronic
signal representing the scanned images can be transmitted or sent
though wire to be recreated at the receiver or monitor.
earliest practical mechanical scanning and transmitting of moving
images occurred in the mid-1920s and by the early 1930s electronic
scanning had generally replaced the mechanical scanning methods.
At first the images were crude, little more than shadow-pictures,
but as the potential for television as a profit-making medium became
apparent, more money and effort went into television experimentation
and improvements continued throughout the 1930s.
1941, technical standards for the scanning and transmission of television
images in America had been agreed upon and these standards have,
in general, been maintained ever since. The American standard, known
as National Television System Committee (NTSC), utilizes 525-line,
60-field, 30-frame, interlaced scanning. This means that images
are scanned in the television camera and reproduced in the television
receiver or monitor 30 times each second. Each full image, or frame,
is scanned by dividing the image into 525 horizontal lines, and
then sequentially scanning first all the even lines (every other
line) from top to bottom creating one field, and then scanning the
odd numbered lines in the same manner creating a second field. The
two fields, when combined (interlaced), create one frame. Therefore,
30 complete images or frames, each made up of two fields, are created
each second. Because it is not possible to perceive individual changes
in light and image happening so quickly, the 30-times-per-second
scanned images are perceived as continuous movement, a trait known
as "persistence of vision," similar to motion picture viewing. The
NTSC standard is used in Canada, parts of Asia including Japan,
and much of Latin America as well as in the United States. But there
are two other "standards" in common use today. The PAL systems,
a 25-frame-per-second standard with a number of variants, are used
throughout most of Western Europe and India as well as other areas.
The SECAM 25-frame-per-second standard is used in many parts of
the world including France, Russia and most of Eastern Europe. As
may or may not be obvious, countries that use 60 Hertz (cycle) A.C.
(alternating current) power have adopted a 30 frame-per-second television
system. Countries that utilize a 50 Hz. power system have a 25 frame-per-second
television system. In all these television systems the frame-per-second
rate is equal to half the A.C. power frequency.
"aspect ratio" of the television screen, the ratio of the horizontal
dimension to the vertical dimension, is 4:3. For instance, if a
TV receiver screen is 16 inches wide, the screen will be 12 inches
high. (TV picture tubes are defined by their diagonal measurement
so in the example above the screen would be described as a 20-inch
TV). Often, motion pictures are shown on television in a "letter-box"
format. Because motion pictures are usually shot in an aspect ratio
greater than 4:3 it is necessary to leave a black space at the top
and bottom of the television screen so that the film can be viewed
in a form resembling its theatrical dimensions, without cutting
off the sides. "High Definition" television also utilizes a greater
aspect ratio, generally 16:9.
television camera consists of a lens to focus an image onto the
front surface of one or more pick-up-devices, and, within the camera
housing, the pick-up-device(s) and the electronics to make the camera
work. A viewfinder to monitor the camera's images is normally mounted
in or on the camera. The pick-up-device, either a camera tube or
charge-coupled device (CCD), reads the focused visual image and
converts the image into a varying electronic signal that represents
the image. On high quality cameras, three pick-up-devices are often
utilized; one to pick up each of the three primary colors (blue,
green and red) that make up the color image.
face of the camera tube has a photoemissive material that gives
off electrical energy when exposed to light. The stronger the light
at any given point, the more energy created. By reading the amount
of energy on the surface of the camera tube at each point, an electronic
representation of the visual image can be created. The camera tube
"reads" the amount of energy created on its surface by the focused
image by scanning the image, both horizontally and vertically, with
a moving electronic beam. The scanning occurs because of precise
magnetic deflection of the beam.
CCD replaces the camera tube in most modern cameras, commonly called
chip cameras. This solid-state device measures the energy at the
points on its surface, known as pixels, converts and stores this
information and then sends out this varying electronic signal that
represents the image. CCD image pick-up devices are becoming more
popular due to their small size, long life, greater sensitivity
and light tolerance, minimal power requirements, less image distortion,
In the receiver's, or monitor's, picture tube the camera tube process
is essentially reversed. The face of the picture tube is coated
with a phosphor-like material that glows when struck by a beam of
electrons. The glow lasts long enough to make the scanned image
visible to the viewer. An electron gun shoots the thin beam of electrons
at the face of the screen from within the picture tube. The beam's
direction is varied in a precise manner by magnetic deflection in
a way that matches or synchronizes with the original image scanned
by the television camera. Color picture tubes can have one electron
gun, such as in the Trinitron, or three guns, one for each primary
color. One major difference between a receiver and a monitor should
be mentioned here. A receiver is able to tune in a television station
frequency and show the images being transmitted. A monitor does
not have a tuning component and can receive video signals by wire
a television station, the electronic signal from a television camera
can be combined and/or mixed with video signals from other devices
such as video tape players, computers, film chains or telecines
(motion picture and slide projector units whose outputs have been
converted to video signals) using what is known as a switcher. The
switcher is also used to create various special visual effects electronically.
The video output from the switcher can then be recorded, sent to
another studio or master control room, or sent directly to a transmitter.
complete video signal sent to a transmitter or through wire to a
monitor consists of signals representing the picture (luminance),
color (chrominance), and synchronization. Synchronizing signals
force the receiver to correctly lock onto (sync-up) and reproduce
the original image correctly. Otherwise, for example, the receiver
might begin to scan an image that begins half-way down the screen.
stations are assigned a specific transmitting frequency and operating
power. In the United States, VHF (very-high frequency) television,
channels 2-13, occupies a portion of the frequency spectrum from
54-216 MHz (million Hertz or cycles-per-second). Channels 2-6 are
located between 54 and 88 MHz. The FM radio band, 88-108 MHz, is
located between television channels 6 and 7. Channels 7-13 are located
between 174 and 216 MHz. UHF (ultra-high frequency) television,
originally channels 14 to 83, was assigned the frequency range from
470 to 890 MHz. In 1966 the Federal Communications Commission (FCC)
discontinued issuing licenses for UHF television stations above
channel 69. In 1970 the FCC took away the frequency range from 807-890
MHz for other communication uses and so the UHF band now consists
of channels 14-69, from 470 to 806 MHz. The upper end of this current
range is being coveted for other frequency spectrum uses and it
appears that the number of available channels in the UHF band will
continue to decrease. Each television channel has a frequency bandwidth
of 6 MHz. So, for instance, channel 2 has a frequency bandwidth
from 54 MHz. to 60 MHz. Within its assigned band each station transmits
the video signal as described earlier, an audio signal, and specialized
signals such as closed-captioning information.
In the television transmitter a "carrier wave" is created at an
assigned frequency. This carrier wave travels at the speed of light
through space with specific transmission or propagation characteristics
determined by the individual frequency. The video signal is piggy-backed
onto the much higher frequency carrier wave using a process known
as modulation. Modulation, in the simplest terms, means that the
carrier wave is modulated, or varied slightly, at the rate of the
signal being piggy-backed. In a television transmission the video
signal varies the amplitude or strength of the carrier wave at the
rate of the video signal. This is known as amplitude modulation
(A.M.) and is similar to the method used to transmit the audio of
an A.M. radio station. However, the television station audio signal
is piggy-backed onto the carrier wave using frequency modulation
(F.M.). With television audio the carrier wave's frequency (instead
of its amplitude) is varied slightly at the rate of the audio signal.
modulated television station carrier wave is sent from the transmitter
to an antenna. The antenna then radiates the signal out into space
in a pattern determined by the physical design of the transmitting
antenna. Traditionally the transmitter and antenna were terrestrially
located, but now television signals can be radiated or delivered
by transmitters and antennas located on satellites in orbit around
the earth. In this case the television signal is transmitted to
the satellite on a specific frequency and then retransmitted at
a different frequency by the satellite's transmitter back to the
delivery by carrier wave transmission, television is often sent
through cable directly to homes and businesses. These signals are
delivered by satellite, over-the-air from terrestrial antennas,
and sometimes directly from video players to the distribution equipment
of cable television (CATV) service providers for feeding directly
into homes. The signals are sent at specific carrier wave frequencies
(sometimes called R.F., or radio frequencies) as chosen by the cable
television receiver picks up the transmitted television signals
sent over-the-air or by cable or satellite, removes the necessary
video and audio signals that had been piggy-backed on the carrier
wave, discards the carrier wave, and amplifies and converts the
video and audio signals into picture and sound. A television monitor
accepts direct video and audio signals to provide pictures and sound.
As mentioned above, a monitor cannot receive carrier waves.
primitive experimentation in the 1920s and 1930s through the advent
of commercial television in the late 1940s, to color television
as the standard by the mid 1960s, television has grown quickly to
become perhaps the most important single influence on society today.
From a source of information and entertainment to what some have
dubbed the real "soma" of Huxley's Brave New World, television
has become the most influential medium of the second half of the
twentieth century. While the medium continues to evolve and change,
its importance, influence and pervasiveness appear to continue unabated.
How will new technology change the face of television? Once the
realm of science fiction, we are now seeing new delivery systems,
on-call access, a greater number of available channels, two-way
interaction, and the coupling of television and the computer. We
are in the process of experiencing better technical quality including
improved resolution, HDTV, the convenience of flatter and lighter
television receivers, and digital processing and transmission. And
yet, the basic standard for television broadcast technology has
been with us, with only minor changes and improvements, for well
over 50 years.
K. Blair. Television Engineering Handbook. New York: McGraw-Hill,
Robert. Basic TV Technology: A Media Manual. Boston: Focal
Press, 1990; 2nd edition, 1995.
A.F. Video Engineering. White Plains, New York: Knowledge
Industry Publications, 1992.
Fraidoon, editor. Telecommunications Engineer's Reference Book.
Stoneham, Massachusetts: Butterworth-Heinemann, 1993.
Robert L. Electronic Communication. New York: McGraw-Hill,
1959; 6th edition, 1990.
Boris, and Kenneth Jackson, editors. TV & Video Engineer's Reference
Book. Stoneham, Massachusetts: Butterworth-Heinemann, 1991.
See also All
Channel Legislation; Color
Power Television; Microwave;
States: Cable Television; United
States: Networks; Video