Television
One type of mass media that uses electronic transmission to transfer moving images and sound from a source to a recipient is television (TV). Television has had a significant impact on society by expanding the senses of hearing and vision beyond the confines of physical space. Originally envisioned in the early 20th century as a potential medium for interpersonal and educational communication, it developed into a dynamic broadcast medium by the middle of the century, delivering news and entertainment to listeners worldwide via the broadcast radio model. There are many different ways to watch television these days: "over the air" via terrestrial radio waves (cable TV); via coaxial cables (traditional broadcast TV); and/or reflected from direct broadcast satellite, or DBS, TV, satellites in geostationary Earth orbit.
Mechanical systems
The human imagination is as old as the desire to see far-off places. In ancient Greece, priests examined the innards of birds, attempting to decipher what the birds had observed as they soared over the horizon. They thought their gods could see human activity happening anywhere in the world while perched comfortably atop Mount Olympus. The play Henry IV, Part 1 by William Shakespeare also opens with the introduction of Rumour, a character the other characters rely on to keep them informed about events occurring in remote parts of England.
Electronic systems
Ultimately, the insurmountable issues with mechanical scanning were its limited scan rate, which resulted in a flickering image, and the comparatively large size of each disc hole, which caused poor resolution. Instead of using spinning discs to solve the problems, Scottish electrical engineer A.A. Campbell Swinton wrote in 1908 that "two beams of kathode rays can probably be employed." Electron beams produced in a vacuum tube are known as cathode rays. According to Swinton, they could "paint" a transient image on the glass screen of a tube that had an internal phosphorescent coating by being guided by magnetic or electric fields. The rays would avoid the flickers because they travel at almost the speed of light.
Color television
Colour television was not a novel concept by any means. A.A. Polumordvinov, a Russian scientist, created a system of revolving Nipkow discs and concentric cylinders with slits covered in red, green, and blue filters in the late 1800s. However, even the most basic black-and-white television was decades away, so he was far ahead of the times in terms of technology. Using a Nipkow disc with three spirals of 30 apertures—one spiral for each primary colour in order—Baird demonstrated a colour system in London in 1928. Two gas-discharge tubes made of helium and mercury vapour for the colours green and blue, and neon for the colour red, made up the light source at the receiver.
Working principles
A glass vacuum tube is the CRT in a TV. Tiny phosphor dots coated in the three primary colours of red, green, and blue light up the inner surface of the screen. When an electron beam strikes these phosphor dots, it glows, creating the images we see on the screen.
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