Television

The information to be transmitted in a television system is represented by the differences in illumination (and eventually in color) of a changing scene. Since the ordinary optical camera obscura always produces a flat image of the scene, the task becomes the transmission of illumination differences across a flat image. The problem is somewhat similar to that found in cinematography. In cinematography, the projection of moving scenes is achieved by taking (and then projecting) 24 photographs per second, each of which differs only slightly from the previous one. The phenomenon of visual persistence prevents the viewer from noticing the substitution of one photograph for another, creating the illusion of continuity in the projected movement.

A television system may therefore be considered as a telephotographic system capable of transmitting at least 24 photographs per second (in practice 25 or 30). However, at the receiving end, the photograph itself is replaced by the direct reconstruction of the flat image on a specially prepared screen. The required transmission speed, however, forces the use of transduction and emission methods different from those used in telephony.

The transducer of the television transmitter consists essentially of a cathode-ray tube equipped with an electron gun and a deflection system. The screen of this cathode-ray tube consists of a mica sheet that is metalized on one side and coated with droplets of photoelectric material on the other. Each droplet (there are about half a million) forms one of the tiny capacitors that constitute the mosaic of the screen. The discharge currents thus obtained converge into a common connection on the metalized side and are passed through a resistor on the return path to the cathode of the electron gun. This is the signal current that is then applied to the transmission channel.

At the receiver, the signal current is applied to another cathode-ray tube, also equipped with an electron gun and a deflection system. The screen of the receiving tube is the flattened face of the bulb, coated internally with fluorescent phosphors—materials that emit light when struck by the electron beam. If the electron beam scans the screen in perfect synchronism with the beam of the transmitting tube, and its intensity (and therefore the brightness of the point of impact) is controlled by the received signal, the desired reproduction of the transmitted image appears on the screen.

One of the most important problems in television is maintaining perfect synchronism between the scanning beam of the transmitter and that of the receiver. The problem is solved by controlling the movement of both beams through electrical synchronization pulses, which act directly on the transmitter tube and are superimposed on the image signal to be sent along with it to the receiver. There, the synchronization pulses are separated from the image signal by means of special electronic circuits and are applied to control the currents that govern the movement of the electron beam in the receiving tube. The sound accompanying the television image is transmitted completely independently; for this reason television refers to a video or picture signal and an audio or sound signal.

Color Television

In monochrome or black-and-white television, the only information transmitted is the brightness of each point in the scene. To obtain color television, it is necessary to add information related to color. Colorimetry principles are used, particularly the fact that almost any color can be produced by mixing the primary colors red, green, and blue in different proportions. Consequently, in addition to brightness, color television must convey the way in which that brightness is distributed among the primary colors. Thus, two signals must be transmitted: a luminance signal, carrying brightness information, and a chrominance signal, carrying color information. The transmitter uses three pickup tubes whose optical systems are equipped with red, green, and blue filters. The three outputs, summed together, provide total brightness. Chrominance information is reduced to two colors (for example red and blue), and the missing color is obtained at the receiver by subtracting these from the brightness signal.

The receiving tube contains a triple electron gun, each component controlled by its corresponding recovered color signal. On the screen, groups of three phosphor dots—red, green, and blue—are arranged geometrically. With the aid of a perforated mask, the beam from the gun assigned to blue, for example, strikes only the blue dot in each triad. The same applies to the red and green beams. All three beams move simultaneously and in synchronism with the beams from the three tubes used in the transmitter.