The principles employed in television are based on human vision, that is, how the eye perceives the scene before it, including its structure, lights and shadows, details, and colors.

Television must deal with an important difference between the way in which radio communications are transmitted through space and the usual way in which humans receive visual images. The retina of the eye receives an image of the whole scene before it, and hundreds of thousands of fibers in the optic nerve transmit to the brain, individually and simultaneously, signals that together represent the whole scene. Human vision thus uses hundreds of thousands of "channels" at once.

In television transmission by microwave or cable, by contrast, the entire content of the scene must be sent through a single channel. To accomplish this, the scene is broken down into many small pieces (called picture elements - pixels) that look like the half-tone dots used in printed pictures.

In the television camera an electrical signal is formed to represent the brightness (and in color television, also the color) of each picture element. These signals are sent over the channel, one at a time, to the receiver. At the receiver the signals are transformed back into light, and the picture elements are assembled on the viewing screen in their proper relative positions.

In a television system a still picture is presented in less than a tenth of a second, so that a series of still pictures can be presented at a rate greater than ten pictures per second. Motion in the scene is represented, as in motion pictures, by a series of still pictures, each differing slightly from those preceding and following it.
Although ten still pictures per second is an adequate rate to convey the illusion of motion, for such motion to be depicted smoothly, a rate of at least 24 per second is necessary; this is the rate used in professional motion pictures.

The process of breaking down the scene into picture elements and reassembling them on the screen of the television receiver is known as scanning. It is similar to the eye's motion when a person reads a page of printed matter.
The television camera produces a rapid succession of electrical impulses, called the video signal; these impulses correspond to the succession of picture elements scanned in every line of every image. At the television receiver this signal, which has been transmitted through space or by cable, is recovered and used to control the picture tube. The picture tube produces an image that is composed of horizontal lines precisely like those used in the camera. As the camera examines the topmost line, a spot of light produced by the picture tube moves across the screen and produces the topmost line of light on the screen. The video signal causes the spot of light to become brighter or darker as it moves, and thus the picture elements scanned by the camera are reproduced line by line at the receiver, until the whole area of the screen is covered, completing the image. Then the process is repeated. The scanning motions in the camera and those in the receiver must keep in precise step, otherwise the picture elements would appear in the wrong positions on the receiver screen, and the pattern of the image would be distorted or broken up entirely.

The lens (which is often of the zoom type, particularly in sports telecasting) focuses the scene on the front end of the camera tube. The tube that was most widely used in the late 1970s was the vidicon, which is an evacuated glass cylinder. At the front end of the tube is a flat glass plate, the inside of which is coated with a photosensitive material, a sulfur compound of antimony. Underneath the antimony coating is a thin, transparent coating of metal.

The electrical resistance of the antimony compound is lowered when light falls on it. The optical image from the lens falling on the antimony coating causes its resistance to change in proportion to the amount of light reaching it at each point on its surface; that is, a pattern of electrical resistance is formed that matches the pattern of light in the image.

At the opposite end of the camera tube is a structure known as an electron gun. This forms a narrow electron beam that travels down the tube and encounters the charge pattern on the rear of the antimony coating. The focusing coils are arranged to keep the electron beam narrow (that is, sharply focused) so that the beam that strikes the coating has the size and shape of the picture element.
As the electron beam scans the charge image, it thus produces a succession of voltage changes at this terminal, which constitute the video signal. The video signal at the camera terminal is weak, so it is amplified at once within the camera housing. After further processing, the amplified video signal is transmitted to the receiver, where it reaches the picture tube and re-creates the image.

To avoid the hazard of electric shock, the high-voltage supply of the receiver is designed to operate at a low current, a few thousandths of an ampere at most

. The synchronization signals are sent over the air by the television station. Within the receiver the signals are recovered, the horizontal signals being used to control the horizontal scanning motions of the picture tube and the vertical signals controlling the vertical motions. These timing signals are inserted in the video signal. The horizontal pulses occur between successive lines, the vertical ones just after the bottom of the picture is reached.

A separate signal is used to broadcast the sound portion of a television broadcast most commonly by means of frequency modulation, the same method used for FM radio. The microphone associated with the camera picks up the sound, producing a signal that is amplified and transmitted to the broadcast station, where it controls a separate transmitter. The signal transmitted has a frequency that varies with the sound pressure picked up by the microphone. Both picture and sound signals are broadcast over the same channel. The potential for high-quality sound that frequency modulation possesses is usually not realized in television because loudspeakers small enough to fit into television cabinets cannot reproduce the bass notes in proper proportion to the other registers.

For the video signal to be transmitted over the air, it must be carried by a broadcast signal (the carrier signal). The carrier signal is an alternating current of very high frequency. The modulated carrier current is directed through the transmitting antenna, where it creates an electromagnetic wave that radiates through space. The antenna is designed to radiate waves in the horizontal direction toward the surrounding audience, with little or no power wasted in the upward direction.
The amplitude of the radiated wave continually changes in response to the video signal it carries, more power being radiated during the dark portions of the picture, less power during the bright portions, with maximum power output during the synchronization pulses. The radio waves used in television broadcasting travel in essentially straight lines, are intercepted when they strike any massive object, and are greatly weakened when they encounter the horizon.

When the television signal is intercepted by a nearby structure, such as a building or water tower, it is reflected in all directions, including back toward the transmitter. A receiver located between the transmitter and the reflecting structure, then, receives two signals, one directly from the transmitter as intended, the other by reflection from the structure. The reflected signal, having traveled a greater distance, arrives later than the direct signal.
Conditions in which a reflected signal exists, called multipath reception, are common in built-up city areas having many tall buildings. To minimize the effect, the receiving antenna must be as high as possible and oriented so that it discriminates against the reflected signal. One method of avoiding reflections is to feed many receivers by coaxial cable from a single antenna (community antenna) located high above surrounding structures, where it is free from reflected signals.

In a typical black-and-white television receiver, the signal from the antenna is fed to the tuner. Two channel selector switches - one for the VHF (very high frequency) channels 2-13 and the other for the UHF (ultra high frequency) channels 14-83 - connect circuits that are tuned to the desired channels and, at the same time, discriminate against signals from undesired channels. These circuits also form part of an amplifier, which is designed to add as little snow to the signal as is feasible.

The amplified signals from the desired channel are then passed to the mixer, which transposes all the signal frequencies in the channel to different values, called intermediate frequencies. The output of the tuner consists of all the signals in the desired channel, but the intermediate channel is fixed in the frequency band from 41 to 47 MHz, no matter what channel is tuned in.

From the tuner the 41-47 MHz channel, with all picture and sound information present, is passed successively through several additional amplifiers (from two to four intermediate-frequency - or IF - amplifiers), which provide the major portion of the amplification in the receiver.

The next stage is video detector, which removes the high-frequency carrier signal and recovers the video signal. The detector also reproduces (at a lower frequency) the sound carrier and its frequency variations. The sound signal is then separated from the picture signal and passes through a frequency detector, which recovers the audio signal, that is, the signal equivalent to the microphone signal at the transmitter. This signal is amplified further and fed to the loudspeaker, where it re-creates the accompanying sound.

The essential difference is that a color broadcast is in reality three telecasts in one. The screen of a color receiver actually displays three images superimposed on each other; these images present, respectively, the red, green, and blue components of the colors in the scene. This use of three primary colors in the television follows the method used in color photography and color printing, in which three layers of colored dyes (in photography) or three interspersed sets of fine colored dots (in printing) give to the eye the impression of all the natural colors.

Color television achieves reproduction of the wide range of natural colors by adjusting the relative brightness of the red, green, and blue images. If two images are suppressed (for example, red and green), only the remaining color (blue) is seen. If one image is suppressed (for example, blue), the other two (green and red) can cover the range of colors from green to red, including the intermediate colors orange and yellow, by making the green image brighter or dimmer than the red one.

When all three colors are present in the proper proportions, white light is produced. By adjusting portions of the scene to be brighter than the others, the whole range of grays from black to white can be produced. Finally, by allowing one or two of the three colors to predominate, the white light can be given the tint of the stronger colors, and thus pastel shades of all the natural colors can be reproduced.
Moreover, if a black-and-white receiver was to tune in on such a color telecast, it could receive only one of the three channels, and the tonal values reproduced would be unnatural because they would be confined to a monochrome (single-color) rendition of the scene.