Television Technology Demystified

A Non-technical Guide

Aleksandar Louis Todorovic

• Publisher: Routledge
• Year: 2014
• ISBN:
  9781136068539
• Edition: 1
• Topic: Media & Performing Arts
• Subtopic: Film & Video

"Television Technology Demystified" is written for non-technical television production professionals. Journalists, program producers, camera persons, editors, and other television professionals need to know how equipment works, which performance levels are achievable, how to evaluate the technical quality of picture and sound, and other aspects of production; this book presents these and other essential concepts in a simple and non-mathematical way. Aleksandar-Louis Todorovic, a highly respected and well-known figure in the broadcasting community, has succeeded in making complex technology understandable.

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Development of Television
Technology—A Sweep through
History

Today, in the first years of the twenty-first century, we take for granted a number of technological marvels that are irreplaceable components of our everyday life. Having lived in the century that witnessed the development of marvels such as aviation, telecommunications, cars, and television, we tend to forget that almost all the core discoveries and breakthroughs that were essential for the development of these technologies were made during that fascinatingly entrepreneurial nineteenth century. Television is undoubtedly one of these marvels.

The simplest definition of television is that it is the transmission of moving images at a distance and that its workings can be compared to that of the human visual system. The eye captures the light reflected from an object in the surrounding world and transforms that photo energy into neural impulses. These impulses travel to the brain where they are deciphered and, through processes still only partially understood, transformed into a mental reproduction of the original object. In television, as in human vision, the first step in the process of achieving transmission is to transform the light reflected from the world around us into another form of energy. In this case it is transformed into electric energy, which is then handled, memorized, or transmitted by means of specific methods and techniques.

1.1 Optoelectric Transformation

The first of the series of nineteenth-century discoveries that would eventually become the basis of television was the essential discovery of optoelectric conversion. Interestingly, this important discovery was made by chance rather than as a result of serious scientific research. In 1873 an Irish telegraph operator named Leonard May observed that his telegraph behaved differently depending on the time of day. Upon further investigation he realized that a selenium bar, which was part of the apparatus, changed its resistance in relation to the amount of sunlight falling on it. In full sunlight there was less resistance, but resistance increased as the sun moved toward the horizon. This photoelectric (or optoelectric) phenomenon was not to be used in television technology for another 80 years or so, but it should be recognized as one of the most important milestones—the first registered transformation of the energy of light into electric energy. The phenomenon discovered by May (who, incidentally, never profited from it) would later be named photoconductivity.

Photoelectric phenomena, that is, changes in the behavior of electrons due to variations in amount of light illuminating a given material, were at that time being scrutinized by a number of scientists. In 1888 a German physicist, Wilhelm Hallwachs, discovered another very important photoelectric phenomenon— photoemission. Photoemission is the act of releasing free electrons in the surrounding space. Namely some materials have a greater or more modest capacity to release, or as it is usually said, emit free electrons under the impact of light. The number of emitted electrons is directly proportional to the intensity of the incoming light. In other words, the brighter the light illuminating the piece of material, the more free electrons will appear. This physical property would be used to develop the first television experiments some 30 years later.

The first theoretical descriptions of a hypothetical television system, proposed by George Carey in the United States as early as 1875, advocated the use of a mosaic structure similar to the structure of the human eye. He proposed to assemble two mosaics—one of photosensitive cells possessing the capacity to produce at each point an electric charge proportional to the amount of light falling on that particular element, and the other of cells that would display a reverse effect, that is, would produce a certain quantity of light proportional to the received electric impulse. According to Carey, if all cells belonging to these two panels were mutually connected with pairs of wires, one to one, it would be possible to transmit an optical moving picture at a given distance.

Theoretically, such a parallel channel system could allow the transmission of moving images but only theoretically as the obstacles to its realization were numerous. First, in 1875 the necessary technology for its materialization was not available. Second, and even more important, there were so many elements that had to be connected at the same time; a simultaneous system of transmission was and still is very cumbersome and very difficult to achieve in practice. Using a large number of parallel channels for the transmission of one single piece of information was not a viable solution, either economically or technically. One of the basic economic principles in communications is to use always the minimum channel capacity, i.e. the narrowest channel possible, or in some instances the minimum number of channels for the transmission of maximum information. From the technical point of view, it is always preferable to use one single channel instead of a number of parallel ones since all parallel channels should behave identically under all circumstances, and, practically speaking, that is almost impossible to achieve.

The only viable alternative to a simultaneous capturing and transmission system is to analyze, or scan, the picture to be transmitted by dissecting it into a series of tightly spaced consecutive pieces of information and sending them through one single channel. Since these discrete elements will be displayed at the receiving end in very quick succession, the human visual system will not see them as a series of separate pieces of information but will integrate them into a single picture. Experiments have shown that the most appropriate method of scanning is linear scanning, that is, the analysis of individual picture elements, one by one, disposed on consecutive parallel horizontal lines. The number of picture elements analyzed and the number of scanning lines used will determine the resolution of the system, that is, its capacity to reproduce fine details.

1.2 The Nipkow Disk

The first scanning device was developed in 1884 by Paul Nipkow, a German physicist of Polish origin. He made a special perforated disk designed for a point-by-point analysis, that is, for scanning of optical pictures. This mechanical scanning device disassembled simultaneous optical pictures into a number of discrete partial elements. It consisted of a flat circular plate that rotated around an axis located at its center. It was perforated with a number of holes following a spiral path from the center to the outside of the disk (see Figure 1.1.).

If the light reflected from a picture to be transmitted is projected with an optical lens to a certain area of the rotating disk, only discrete values of points of illumination will pass through the perforations to the other side of the disk, there to fall on a photosensitive element. That element will consequently generate a quick succession of electric charges proportional to the quantity of light falling on it at a given moment. The holes on the rotating disk will, in fact, scan the projected optical picture line by line, and the photosensitive element will generate a continuous electric stream of variable intensity. The net result of that operation is that the simultaneous optical picture is transformed into a continuous stream of discrete, sequentially transmitted information whose transmission requires just one channel. One full rotation of the disk scans one full optical picture. The number of holes in the disk will determine the number of lines used to scan or analyze one picture and the number of sequentially analyzed pictures will therefore depend on the rotational speed of the disk—a faster rotational speed will mean more

Figure 1.1 Nipkow disk: The first television scanning device.

full rotations in a unit of time and consequently more scanned/analyzed pictures during that same unit of time. At the same time it is well known that a quick succession of static pictures is integrated by the human visual system as a continuous reproduction of movement.

The electric signal thus generated can be transmitted over a distance by any standard wire or wireless transmission method. However, at the end of the chain it has to be transformed again into a light picture so that the human eye can perceive it. The first experimental television installations used two Nipkow disks—one to scan the optical picture and the other to perform the role of a display device. The latter rotated between the eyes of the viewer and the light source as it changed its radiated light power in relation to the intensity of the incoming electric signal. If both rotating disks were fully synchronous (meaning that they start and stop rotating at the same time and that their rotational speeds are identical) and in phase (meaning that the positions of the holes on both disks compared to the scanned object are always identical, i.e. when the hole number one of the first disk is at the upper left corner of the projected picture the hole number one of the second disk should be at the corresponding position of the reproduced picture), the holes of the display disk would reconstruct, line by line, the transmitted picture. Such a system was acceptable for the early experiments in which the picture was reduced to black shadows on a white background and the resolution was limited to about 60 lines. But the systems based on a Nipkow disk were hobbled by the limitations of the disks: they were crude mechanical devices, burdened by inertia and synchronization problems as well as by the inability of early artificial light sources to react adequately to the extremely fast changes of the incoming signal. In order to reproduce all the tones from black to white passing by different shades of gray (the gray scale), the light source would have to have been capable of changing its intensity several hundred times in the course of one television line, that is, during one 60^th of the duration of one picture, which means during one fraction of a second. However, there was practically no incandescent light source capable of such performance.

1.3 The Cathode-Ray Tube

Further research in the development of television shows that even by the standards of the period, the Nipkow disk was not a viable solution for the display of transmitted moving images. Fortunately, by the end of the nineteenth century another German physicist, Ferdinand Braun, developed the cathode-ray tube (CRT), the essential and basic television display device. This device, shown in Figure 1.2, is equipped with two electrodes connected through an electric circuit. The first of these electrodes is the cathode—an element capable of emitting free electrons under the effect of thermal heating. The other electrode, called the anode, is installed at the opposite side of the glass tube and is at a positive electric potential and therefore attracts these negatively charged electrons. In the neck of the glass tube, past the cathode, there are several plates connected to sources of electricity.

Figure 1.2 Simplified representation of the cross section of a Braun CRT.

Thanks to the electrical power supplied here, a field (called an electrostatic field) is created between the plates and acts as a sort of electrical lens focusing the liberated electrons into a concentrated beam flowing toward the anode that attracts the electrons. Outside the tube, around its neck, two coils are mounted. These coils are connected to another electric source. The current flowing through the windings of the coils creates a magnetic field in the glass tube that has the ability to move the electron beam. The magnetic fields created by these two coils are in fact responsible for the scanning movement of the beam over the front surface of the tube. The anode part of the tube is covered with a phosphor coating—a special material that emits visible light at the point of impact of the electron beam. The intensity of that light is directly proportional to the intensity of the beam at the moment of impact. Adequately created electromagnetic fields can control the movement of the electron beam, making it scan the front end of the tube at a selected scanning speed.

By the beginning of the twentieth century, the three essential elements of a television chain—the optoelectric conversion, the scanning of the optical picture, and the electro-optic transformation—were in place. However, several elements were still missing: a better understanding of the behavior and propagation of electromagnetic waves; the mastering of the amplification of electrical signals; and the expertise, discoveries, and developments that would be achieved by great scientists like Marconi, Tesla, Lee de Forest, and Branly. Also missing at the time was the person who would seize the moment when relevant discoveries reached critical mass and who would have the courage and ability to envision bringing together these discoveries in order to transform a science-fiction toy into a physical reality. More than 20 years would elapse before the first crude moving images would be transmitted between two adjacent rooms on Frith Street in London.

1.4 The Birth of Television

By the early 1920s two visionaries began almost simultaneously but independently to develop a chain capable of transmitting moving images. In 1923 Charles Francis Jenkins in the United States and John Logie Baird in United Kingdom (see Figure 1.3) presented the results of their experiments. The images they showed were just black shadows, cruder than children’s shadow puppets against a white wall, and the distance of transmission was limited to several meters between the transmitter and the receiver that were located in adjacent rooms, but it was the first time that moving images were transmitted. Encouraged by these first results, both men continued with their work and by 1925 were able to demonstrate pictures with halftones, that is, pictures with different shades of gray corresponding to different grades of illumination from the black level to the brightest white.

Figure 1.3 John Logie Baird.

The resolution and the quality of the transmitted gray scale was certainly very limited, but nevertheless these were “real” pictures, and, consequently, 1925 is considered the year when television was born.

Even after that auspicious beginning, 10 additional years of research and de...

Table of contents

• Cover
• Half Title
• Full Title
• Copyright
• Contents
• Introduction
• 1 Development of Television Technology-A Sweep through History
• 2 Light and the Human Eye
• 3 Generating a Television Picture
• 4 Color Television
• 5 Digital Television
• 6 Digital Compression as the Key to Success
• 7 Digital Audio Compression Methods
• 8 Exchanging Program Material as Bitstreams
• 9 Television Cameras
• 10 Video Recording
• 11 Video Editing
• 12 The Networked Production
• 13 Television Graphics
• 14 HDTV
• Acronyms and Selected Abbreviations
• Index