Going Back to the Beginning of HDTV
Think HDTV began with the adoption of the ATSC standard in 1994? With the development of Hi-Vision in the late 1970s? Nope, and nope. Read on.
Just a few short years after the beginning of commercial radio broadcasting in the U.S., 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 mechanical television that did not use electronic scanning. Rather, it broke images into light samples and reconstructed those samples using a device called the Nipkow disk, a spinning disk that had spiral patterns of holes drilled in it.
Selenium photocells were used as transducers to make the conversions between light samples and electrical signals.
In order to reconstruct the images, the spinning disk in Baird's receiver had to be synchronized with the disk in his transmitter, and this was achieved by synchronizing their drive motors to the power line frequency of 50 Hz.
As Baird's system developed, its resolution increased. His first experiments were carried out with 8 scanning lines at 8 frames per second. He soon increased this to 12 scanning lines at 12.5 frames per second, followed by 30 lines and then 25 frames per second. His frame rate stayed at 25 per second, and his line count progressed from 30 to 60, 125, and 240 lines.
Baird was initially able to transmit silhouettes, which progressed to the first televised transmission of a human face in 1925, and a moving object in 1926. In 1929, the British Broadcasting Corp. began transmitting pictures and even rudimentary programs using the Baird mechanical system.
But technological developments overtook Baird in 1936, as the BBC replaced his 240-line mechanical system with the 405-line (377 active lines), electronically scanned television system developed by EMI. And so it goes.
The BBC's 405-line, 50 Hz television 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 frequency.
Technological advances have definitely changed the image quality available since this television set was built. World War II halted the development of television as an entertainment medium. During the course of that war, however, television systems boasting resolutions in excess of 1000 lines were being experimented with in France and Germany.
In wartime Germany, the Fernseh AG television company demonstrated a 1029-line system that they promoted as a medium for transmitting maps for military purposes. This analog system had the disadvantage that it required 15 MHz of RF bandwidth.
In pre-war France, experimental work had been done on two systems that had 567 and 1000 lines respectively; after World War II France began broadcasting with an 819-line system. In the mid-1960s, Western Europe adopted color television transmission systems that used either PAL or SECAM, both of which have 625 total lines, and about 576 active lines.
In the United States, in 1935, the Radio Corp. of America demonstrated a 343-line television system. In 1936, two committees of the Radio Manufacturers Association (RMA), the organization now known as the Consumer Electronics Association, proposed that U.S. television channels be standardized with a bandwidth of 6 Megahertz, and recommended a 441-line, interlaced, 30 frame-per-second television system.
The video modulation scheme proposed in this recommendation used double sideband amplitude modulated (DSB-AM) transmission, which limited the video bandwidth it was capable of carrying to about 2.5 MHz.
In 1938, this RMA proposal was amended to employ vestigial sideband (VSB) transmission. The VSB approach takes advantage of the fact that the upper and lower sidebands of an amplitude-modulated signal contain identical information and are thus redundant.
In VSB only the upper sideband, the sideband occupying spectrum above the carrier frequency, plus a small segment or vestige of the lower sideband—the sideband occupying spectrum below the carrier frequency—are transmitted. VSB raised the transmitted video bandwidth capability to 4.2 MHz.
Subsequently, in 1941, the first National Television Systems Committee (NTSC) adopted the vestigial sideband system using a total line rate of 525 lines per frame, establishing the basis for the NTSC television system used in the United States for more than six decades.
In 1954, the second National Television Systems Committee adopted compatible NTSC color television. This system, when used on the 6 MHz U.S. television channel, permits the transmission of a limiting horizontal luminance resolution of about 333 TV lines per picture height (TVL/PH), employing 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 transmission, both of which are to a greater or lesser degree based on the principles employed in NTSC.
All the 441-, 405-, and 819-line systems that were previously used have been supplanted by the 625 total lines and approximately 576 active lines of PAL or SECAM, most varieties of which have frame rates of 25 per second. Most of these systems use channels that are wider in RF bandwidth 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.
The HDTV line rates of 1080 and 720 were not revolutionarily high. A German company developed a full television system that used over 1000 lines during World War II, and the French, who had a functioning transmission system that broadcast 1000 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 wide-bandwidth RF channels. In the digital television broadcasting age, digital compression technologies have made it possible to store and transmit high-line-rate signals while occupying far less spectral bandwidth.
BACK TO THE FUTURE
The modern high-definition era began in the early 1980s, with the analog Hi-Vision system developed by the Japanese broadcaster NHK in collaboration with Japanese equipment manufacturers.
This system had a total line count of 1125, with 1035 active lines, interlaced, at a frame rate of exactly 30.00 per second. The Hi-Vision system initially had an aspect ratio of 15:9, but was later changed to 16:9 to conform to the world HDTV aspect ratio standard.
The currently used HDTV scanning formats are digital, and include 1920 horizontal pixels x 1080 active lines (1125 total lines), in both progressive and interlaced formats, and 1280 horizontal pixels x 720 active lines (750 total lines) progressively scanned.
These formats have frame rates of 30 (29.97), 60 (59.94), 25, 50, or 24 (23.98) per second, depending on the application and television system in which they are used.
It is apparent that, although resolutions and line rates were stable from the beginning of commercial television broadcasting in the United States following World War II, and from the implementation of PAL and SECAM broadcasting in Europe in the 1960s, until HDTV became a factor in 1994, they were anything but stable in the developmental era of television.
Although the resolution capabilities of the earliest systems were modest, steady improvements were made. And, supporting the Biblical declaration that "there's nothing new under the sun," even today's HDTV resolution capabilities were available in the World War II era.
Randy Hoffner, a veteran of the big three TV networks, is a senior consulting engineer with AZCAR. He can be reached through TV Technology.
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