The Television: A Change in
Entertainment
For ages Man dreamt about the possibility of transmitting pictures over great distances, but not until he had learnt to master the electron was there any real hope of turning dream into practical reality.
And it all happened by chance...
1873. Ireland. A young telegraph operator, Joseph May, discovered the photoelectric effect: selenium bars, exposed to sunlight, show a variation in resistance. Variations in light intensity can therefore be transformed into electrical signals. That means they can be transmitted.
1875. Boston, USA. George Carey proposed a system based on the exploration of every point in the image simultaneously: a large number of photoelectric cells are arranged on a panel, facing the image, and wired to a panel carrying the same number of bulbs.
This system was impracticable if any reasonable quality criteria were to be respected. Even to match the quality of cinema films of that period, thousands of parallel wires would have been needed from one end of the circuit to the other.
In France in 1881, Constantin Senlecq published a sketch detailing a similar idea in an improved form: two rotating switches were proposed between the panels of cells and lamps, and as these turned at the same rate they connected each cell, in turn, with the corresponding lamp. With this system, all the points in the picture could be sent one after the other along a single wire.
This is the basis of modern television: the picture is converted into a series of picture elements. Nonetheless, Senlecq's system, like that proposed by Carey, needed a large number of cells and lamps.
1884, the German Paul Nipkow applied for a patent covering another image scanning system: it was to use a rotating disk with a series of holes arranged in a spiral, each spaced from the next by the width of the image; a beam of light shining through the holes would illuminate each line of the image.
The light beam, whose intensity depended on the picture element, was converted into an electrical signal by the cell. At the receiving end, there was an identical disc turning at the same speed in front of a lamp whose brightness changed according to the received signal.
After a complete rotation of the discs, the entire picture had been scanned. If the discs rotated sufficiently rapidly, in other words if the successive light stimuli followed quickly enough one after the other, the eye no longer perceived them as individual picture elements. Instead, the entire picture was seen as if it were a single unit.
The idea was simple but it could not be put into practice with the materials available at the time.
Other scientific developments were to offer an alternative. The electron, the tiny grain of negative electricity which revolutionised physical science at the end of the 19th century, was the key. The extreme narrowness of electron beams and their absence of inertia caught the imagination of many researchers and oriented their studies towards what in time became known as electronics. The mechanical approach nevertheless stood its ground, and the competition lasted until 1937.
The cathode ray tube with a fluorescent scene was invented in 1897. Karl Ferdinand Braun, of the University of Strasbourg, had the idea of placing two electromagnets around the neck of the tube to make the electron beam move horizontally and vertically. On the fluorescent screen the movement of the electron beam had the effect of tracing visible lines on the screen.
A Russian scientist, Boris Rosing, suggested this might be used as a receiver screen and conducted experiments in 1907 in his laboratory in Saint Petersburg.
As early as 1908 the Scotsman A. A. Campbell Swinton outlined a system using cathode ray tubes at both sending and receiving ends. This was the first purely electronic proposal. He published a description of it in 1911 :
The methods proposed by Nipkow and Campbell Swinton were at the time theoretical ideas only. The available cells were not sensitive enough and they reacted too slowly to changes in light intensity. The signals were very weak and amplifiers had not yet been invented.
But science marched on. There was the potassium cell, which reacted much more rapidly than the selenium cell. Then came the triode, manufactured in large quantities from about 1915, the development of which owed much to the new-born "wireless". There was also the neon lamp, whose light intensity could be varied rapidly, making it suitable for use in disc receivers.
It was Nipkow's ideas which were the first to benefit from these inventions, and were the first to become practical realities.
In 1925, an electrical engineer from Scotland, John Logie Baird, exhibited in Selfridges department store in London an apparatus with which he reproduced a simple image, in fact white letters on a black background, at a distance. It was not really television because the two discs which served to transmit the image and to reproduce it, were mounted on the same shaft. However Baird did effectively demonstrate that the principle of successive scanning could be applied in practice. He did it again in 1926, in his laboratory, with the first transmission of a real scene the head of a person. The picture was scanned in 30 lines, with 5 full pictures every second.
Similar machines were built in Germany. A smaller mechanical apparatus was presented at the Berlin Radio Show in 1928 by Denes von Mihaly. It was called the "Telehor",. Here too the picture was scanned with 30 lines, but at a picture rate of 10 frames/second.
In France, some time later, the "Semivisor" appeared. It also used 30-line scanning and was built by René Bartholemy.
It was about this time that the first tests with the radio-electric transmission of television took place, using the medium-wave radio band.
These transmissions attracted the attention of many amateur enthusiasts who built their own disc receivers. The public slowly became aware of the research that was under way.
Manufacturers joined in the new adventure, organising systematic studies in their laboratories. New companies were born, such as "Fernseh" in Germany (1929).
But what happened to Rosing's experiments? Had everyone forgotten them?
In fact many researchers kept his work in mind but they had to wait for developments in the design of cathode-ray tubes before these could be put to any practical use.
Around 1930, a number of researchers independently developed the principle of interlaced scanning, which involves exploring first all the odd-numbered lines, followed by the even-numbered lines; this technique avoids flicker.
Industry developed techniques to achieve a very great vacuum in tubes. Receivers with cathode-ray tubes came onto the market in 1933.
However, the use of cathode-ray tubes at the transmission source, where the picture was scanned, remained the stumbling-block for many years. Initially, the spot of light produced on the fluorescent screen was made to substitute for the light beam in the Nipkow system. In Germany, Manfred von Ardenne built the first "flying spot" cathode-ray tube, thereby enabling transparencies to be scanned. A complete transmission system was presented at the 1931 Berlin Radio Show. This scanning method was subsequently used for all television films.
The process nonetheless posed enormous problems when applied to real scenes because the light beam had to operate in a darkened environment. Outdoor scenes, for example, were totally impossible. Another process, known as the "intermediate film" system, provided a roundabout solution to this problem for a number of years. Scenes were shot on film, and this was immediately developed and scanned by a disc or flying spot scanner.
The solution to the problem of out-door shooting came from across the Atlantic.
Following up an idea he had had in 1923, Vladimir Zworykin (one of Rosing's assistants who had emigrated to the United States) invented the "lconoscope". This was a globe-shaped cathode-ray tube and it contained the first photoelectric mosaic made from metal particles applied to both sides of a sheet of mica.
This first camera tube was more compact than the disc, easier to use and more sensitive. The electron beam, which "visits" the elements of the mosaic at a considerable speed, collects from each point all the photoelectric charge which has accumulated there since the last visit, whereas in the mechanical systems the photoelectric cell receives the light from each point only during the very short period while it is actually being scanned.
Zworykin presented the first prototype iconoscope at a meeting of engineers in New York in 1929. The apparatus was built by RCA in 1933. It scanned the image in 120 lines, at 24 frames/second.
Progress was then rapid: as early as 1934, 343-line definition had been achieved and interlacing was being used.
In England, lsaac Schoenberg (another Russian emigrant and childhood friend of Zworykin) led developments in the EMI company on a camera tube similar to the iconoscope. This was the Emitron and it had certain advantages over its rival. EMI, too, adopted interlacing. Also, as early as 1934, EMI. Schoenberg was aiming at a greater number of lines than RCA the target was 405 lines.
The system of mechanical analysis, based on the Nipkow disk, nevertheless continued to hold favour with some.
In 1929, Baird convinced the BBC that it ought to make television transmission outside normal radio programme hours using a 30-line system giving 12˝ frames per second. He marketed his first disc receivers, known as "televisors". He steadily improved his equipment, increasing the scanning to 60, 90, 120 and even 180 lines.
In France, René Bartholemy embarked on the development of a particular variant of the disc. During 1931, he gave two demonstrations, which brought him considerable renown, involving 30-line transmission and reception.
Bartholemy's system, which had been tried by certain German engineers, had a mirror drum instead of a disc with holes. The mirrors, which served to illuminate the subject with light from a bright source, were inclined to an increasing degree with respect to the drum axis. They therefore scanned the subject in a series of parallel lines. Potassium cells collected the light reflected from the subject.
Baird, too, built similar systems. However the mirror drum was bulky and was unsuitable for the high speeds that had to be used to achieve a large number of lines. It was therefore abandoned in 1933 and work on Nipkow disc systems was resumed.