Three technical eras
Television has gone through three broad technical eras. In the first, pictures were scanned mechanically with spinning discs. In the second, electronic cameras and picture tubes sent analog signals over the air, a method that dominated from the late 1930s to the early 2000s. In the third, pictures are turned into digital data, compressed, and broadcast or streamed. This article follows the changes in how the picture is captured, carried and shown.
Mechanical scanning
The idea behind every television system is scanning: breaking a scene into a sequence of tiny dots or lines that can be sent one at a time and reassembled at the other end. In 1884 Paul Nipkow patented a spinning disc with a spiral of holes that could scan an image line by line. Several inventors tried to turn this into a working system over the following decades, and John Logie Baird in Britain, Charles Francis Jenkins in the United States and others demonstrated mechanical television in the 1920s.
The pictures were crude, typically using 30 lines or so, and the equipment was noisy and limited in brightness. Mechanical systems did run experimental public services, including a Baird-based service in Britain in the early 1930s. But the discs could not provide enough detail, and they were soon overtaken by electronic methods. Those methods, built around cathode ray tubes, are what replaced the discs.
The analog electronic era
Electronic cameras and picture tubes made much higher resolution possible. Britain started regular high-definition service in 1936, with 405 lines, and the United States settled on 525 lines in 1941. Many other countries later used 625 lines. Each of these is an analog system, meaning the picture brightness is represented by a continuously varying voltage, which is modulated onto a radio carrier. The viewer’s set demodulates it and paints the picture, line by line, on the tube.
Analog television had some strengths. Receivers were relatively simple, and a weak signal degraded gradually into a snowy picture instead of failing outright. It also had limits. Each channel used a large slice of the radio spectrum for a single program. Ghosting from reflected signals, interference and noise were common. Color television, added in the 1950s and 1960s in several incompatible standards, was a layer fitted onto a signal designed for black and white. And the pictures were fixed at roughly 4:3 proportions, with a few hundred visible lines.
Early digital steps
The shift to digital began in studios, before it reached viewers. In the 1970s and 1980s broadcasters adopted digital equipment for storing frames, converting between standards, and creating effects. Digital videotape formats appeared in the 1980s. By the 1990s much of a studio’s internal chain was digital, even though the final broadcast was still analog.
Meanwhile, researchers worked on higher-definition pictures. Japan’s public broadcaster developed an analog high-definition system in the 1980s. In the United States, regulators and companies were considering how to introduce better pictures, and a decisive change was a proposal by a group of companies in the early 1990s for an all-digital system. Digital compression made this possible. Schemes such as MPEG-2, standardized in the 1990s, remove redundant information from video, so that a picture that would need an enormous number of bits if sent raw can be sent in a small fraction of that.
Digital broadcasting
Compression is the basic reason digital television matters. A channel that once carried one analog program can carry several digital programs, or one high-definition program, depending on how the operator allocates the bits. Digital transmission also allows:
- sharper pictures, including wide-screen 16:9 formats;
- multichannel sound;
- program guides, subtitles and other data sent alongside the picture; and
- error correction, so a picture is either clean or breaks up, instead of slowly degrading.
Standards were developed regionally. Europe’s DVB family covers terrestrial, satellite and cable delivery, North America adopted the ATSC standard, and Japan and Brazil developed related systems. Each region chose its own scheme, so the details differ. The practical process of retiring analog signals, called the digital switchover, took place country by country, mostly between the mid-2000s and the 2010s.
The screen changes too
The move to digital broadcasting happened at the same time as a change in displays. The cathode ray tube was bulky, and it was difficult to make large screens with it. Liquid crystal and plasma panels offered thin, light sets that suited the wider formats and the higher resolutions of digital pictures. By the 2010s the typical living-room screen was a flat panel, often with several times the pixels of an old analog picture. Newer formats, such as 4K displays, grew from this combination of digital sources and flat panels.
Beyond broadcasting
Digital television blurred the line between television and other media. The same compressed video that travels over the air also moves through cable systems, satellite links and the internet. Streaming services deliver programs on demand to televisions, computers and smartphones, and the idea of a fixed broadcast schedule is no longer the only way to watch. The history of the medium has been one of repeated redefinition: from a spinning disc, to an electronic tube, to a stream of numbers.