Inevitably, shortly after it became feasible to transmit intelligence through the air using electromagnetic waves, people began thinking about and experimenting with using the wireless to transmit visual images.
Commercial radio broadcasting is generally reported to have begun in the United States with radio station KDKA, in 1920. Just a few short years after this historic event, the man widely recognized as the father of television, John Logie Baird, was able to transmit rudimentary television pictures through the air in his laboratory in London.
Baird developed a form of television in which the subject was scanned mechanically, using a device called the "Nipkow Disk," a spinning disk that had a spiral pattern of holes drilled in it. Light was reflected off the subject toward the spinning disk, each hole in which described a circular pattern as the disk spun.
On the opposite side of the disk from the subject, selenium photocells were used as transducers to convert the light that passed through the disk holes into modulated electrical signals. To properly reconstruct the images, Baird's system relied on keeping a disk in the receiver spinning at the same speed as the disk spinning in the transmitter.
PERFORATED DISK
It should be noted here that the invention of television, not unlike many other inventions throughout history, is typically credited to a single person, but as usual the story is a little more complex. Baird used as the basis for his invention the perforated disk invented in the mid-1880s by the German inventor Paul Nipkow. In fact, Nipkow himself invented his disk to transmit representations of moving images electrically, and he, too, used selenium photocells to perform conversions between light and electricity.
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The spinning Nipkow disk generated circular scan lines; the image scanned by it was viewed within a relatively small angular sector of the disk called the viewport. Within the viewport area, the scan lines described sections of circles. The resolution afforded by a Nipkow disk depended on the number of holes it contained-the larger the number of holes, the greater the resolution. Baird's system is said to have used about 30 holes, imparting a resolution capability of about 30 lines. Historical reports indicate that disks having 30 to 100 holes were typically used, with a "high-definition" disk containing 200 holes having been developed.
Using his mechanical system, Baird was initially able to transmit silhouettes, progressing to the first televised transmission of a human face in 1925, and of a moving object in 1926.
In 1929, the BBC began transmitting television pictures and even rudimentary programs using the Baird system, but events overtook Baird in 1936 as the BBC began using the 405-line, electronically scanned television system developed by EMI. (See Table 1.)
Here again, we may infer that while Baird was "inventing television," EMI was taking a different path to the same goal. Compared to Baird's system, 405 lines was high definition, indeed.
The BBC 405-line, 50 Hz electronic system was soon joined by other systems that variously used 441, 450 and 455 scanning lines. The field/frame repetition rates for these systems were either 50 Hz or 60 Hz, depending on the local power-line frequencies.
During World War II, most television broadcasting was suspended in England and elsewhere, and development work on television as an entertainment medium ground to a halt. During the course of that war, however, television systems boasting resolutions around 1,000 lines were being experimented with in France and Germany.
In wartime Germany, Fernseh demonstrated a 1,029-line system that was promoted as a system for transmitting maps for military purposes. This analog system had the disadvantage of requiring 15 MHz of bandwidth.
In France, experimental work had previously been done on two systems that had 567 and 1,000 lines respectively; but after World War II, France began broadcasting with an 819-line system. In the mid-1960s, all of Europe adopted color television transmission systems that used either PAL or SECAM, both of which have 625 total lines and about 576 active lines.
ACROSS THE ATLANTIC
In the United States in 1935, the Radio Corporation of America demonstrated a 343-line television system. In 1936, two committees of the Radio Manufacturers Association (RMA), which is now known as the Consumer Electronics Association, proposed that U.S. television channels be standardized at a bandwidth of 6 MHz, and recommended a 441-line, interlaced, 30 frame-per-second television system. The RF modulation system proposed in this recommendation used double-sideband, amplitude-modulated transmission, limiting the video bandwidth it was capable of carrying to 2.5 MHz. In 1938, this RMA proposal was amended to employ vestigial-sideband (VSB) transmission instead of double sideband. In the vestigial-sideband approach, only the upper sidebands-those above the carrier frequency-plus a small segment or vestige of the lower sidebands, are transmitted. VSB raised the transmitted video bandwidth capability to 4.2 MHz. Subsequently, in 1941, the first National Television Systems Committee adopted the vestigial sideband system using a total line rate of 525 lines that is used in the United States today.
In 1954, the second NTSC adopted the compatible NTSC color television system that is used for analog television transmission in the United States and many other countries. This system, used on the 6 MHz U.S. television channel, permits the transmission of a limited horizontal resolution of about 333 TV lines per picture height, and of a frame composed of 525 total lines, 483 active lines, and a frame repetition rate of 29.97 per second. Today, all non-NTSC countries use some form of PAL or SECAM analog television transmission. All the 441-, 405- and 819-line systems have been supplanted by the 625 total lines and approximately 576 active lines of PAL or SECAM, most varieties of which have a frame rate of 25 per second. Most of these systems use channels that are wider than those of the United States, either 7 or 8 MHz, affording video bandwidth capabilities of 5.75 MHz and thus higher horizontal resolution, as well as higher line rates.
We have seen that the HDTV line rates of today are not revolutionary developments, in that a German company is said to have developed a full television system that used more than 1,000 lines during World War II, and the French, who had a functioning transmission system that broadcast 1,000 lines at about this same time, instituted regular broadcasting using an 819-line system after World War II.
The major problem with these analog high-line-rate systems was that they required large video bandwidths, and commensurately wide-bandwidth RF channels. In the digital television broadcasting age, digital compression technologies have made it possible to store and transmit these high line-rate signals while occupying far less spectral bandwidth.
