What makes a picture hard to send
Live transmission of pictures is a harder job than live sound. Sound is relatively easy to broadcast. A speech or a song needs a band of a few thousand to a few tens of thousands of cycles per second. A television picture changes brightness at each point many times a second, so a picture of useful detail needs a much wider band, millions of cycles per second in an analog system. A broadcasting channel for television is therefore many times wider than one for radio, and it must be placed at high frequencies where there is room.
The signal must carry more than the picture. The receiver needs to know when each line starts and each frame begins, so synchronization pulses are added. Sound is sent with the picture, either on a separate carrier or in a combined signal. A color signal adds still more. Designing a signal that does all this and can be sent from a transmitter to many different kinds of receiver was a major engineering effort in the 1930s and after.
First experiments
The earliest transmissions were mechanical-scan experiments. Baird’s company sent low-definition pictures over the air in the late 1920s, and the BBC carried experimental broadcasts of this kind beginning in 1929. Experimental stations in the United States, including some run by Jenkins and by General Electric, also broadcast mechanically scanned programs in the late 1920s and early 1930s.
Electronic systems took over in the mid-1930s. A transmitter sends the picture on one carrier and sound on another, and a receiver picks both. Because the frequencies used are high, the waves travel in straight lines and are not reflected back by the upper atmosphere, as lower-frequency signals can be, so range is limited to roughly the horizon. Tall masts and hilltop locations extended the reach. The Alexandra Palace station in London, which began regular service in 1936, used a mast on a hill, and in the United States the Empire State Building later carried antennas for several New York stations.
Covering events live
An early test of the medium’s potential was live coverage outside a studio. In 1937, the BBC covered the coronation procession of King George VI using mobile cameras, in one of its early outside broadcasts. Germany broadcast part of the 1936 Olympic Games to viewing rooms in Berlin. In 1939, American viewers saw the opening of the New York World’s Fair and baseball games.
Outside broadcasts need a camera, a means to send the signal back, and a transmitter at the studio. The signal from the event was typically sent to the studio by cable or by a microwave link, and the studio sent it out. For small events, a single van carried the equipment. For bigger ones, multiple cameras and a control unit were set up on site.
Linking cities
Local transmitters could reach a city and its surroundings, but they could not reach the whole country. To provide the same program in many places, engineers needed to carry the signal over long distances.
- Coaxial cable: A buried cable with a central conductor can carry the wide band that television needs, with amplifiers every few miles. AT&T built coaxial links among American cities in the late 1940s.
- Microwave relay: Towers about 30 miles apart pass high-frequency beams from one to the next, line of sight. The system is cheaper than a cable across rough country.
- Transcontinental service: A coast-to-coast network, using both techniques, began carrying live programs in September 1951. A speech by President Truman at a conference in San Francisco was an early well-known broadcast on the system.
Similar networks developed in Europe. The Eurovision link, established in 1954, connected several countries’ broadcasters so they could exchange live programs and share events. These connections rely on carrying signals over different national standards, which sometimes required converters.
Across oceans
Cables and towers cannot span an ocean, and the space age that began with the Sputnik launch of 1957 opened a new option. Early live transatlantic television had to wait for satellites. In July 1962, Telstar, a satellite launched in the United States, relayed the first widely noted live transatlantic pictures, though the satellite was only in range of a ground station for short periods because it orbited the Earth in a low orbit. Later geostationary satellites, such as Early Bird in 1965, offered continuous coverage. Ever since, communications satellites have linked broadcasters across continents, and live events like the Olympic Games and the 1969 Moon landing reached global audiences. The Apollo 11 pictures were sent back in a special format and converted at ground stations so that regular broadcast equipment could display them, according to most accounts, and the converted picture was of limited quality.
Recording and editing
Live transmission was also shaped by the arrival of recording. Before videotape, which records on magnetic tape, the only way to store a program was to film a monitor. Ampex’s videotape recorder, shown to broadcasters in 1956, made it possible to record the picture and show it later. Some programs still went live, especially news, sports and special events, which is why the word “live” became a feature of the broadcast. Videotape also supported instant replay, a technique used in sport broadcasting from the 1960s.
Digital delivery
Digital technology changed delivery without removing the basic requirement: a signal must get from the event to the viewer without too much delay or loss. Fiber links and satellite uplinks carry compressed digital signals, and mobile units can send video using cellular networks. Live streaming over the internet adds a delay of several seconds, and for events such as football matches some viewers notice that a neighbor’s broadcast arrives earlier. The history from the first experimental transmission to today’s live coverage is a long chain of improvements in range, picture quality and speed.