Radio is the technology of using radio waves to carry information by systematically modulating properties of electromagnetic energy waves transmitted through spaceh. When radio waves strike an electrical conductor, information in the waves can be extracted and transformed back into its original form.
Early attempts at developing a system of amplitude modulation for voice and music were demonstrated in 1900 and 1906, but had little success. World War I accelerated the development of radio for military communications, and in this era the first vacuum tubes were applied to radio transmitters and receivers. Electronic amplification was a key development in changing radio from an experimental practice by experts into a home appliance. After the war, commercial radio broadcasting began in the 1920s and became an important mass medium for entertainment and news.
David Sarnoff, an early entrepreneur in broadcast radio, persuaded the Radio Corporation of America to begin an AM broadcasting service which rapidly grew in popularity. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. After the war, the experiments in television that had been interrupted were resumed, and it also became an important home entertainment broadcast medium. Stereo FM broadcasting of radio was taking place from the 1930s onwards in the United States and displaced AM as the dominant commercial standard by the 1960s.
Today, radio takes many forms, including wireless networks and mobile communications of all types, as well as radio broadcasting. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment (the era from the late 1920s to the mid-1950s is commonly called radio's "Golden Age"). Radio was unique among methods of dramatic presentation in that it used only sound.
With the advent of the internet and digital media, Broadcast.com drove early adoption of online media as an audio streaming format. With Apple's invention of the iPod, files were increasingly distributed in audio format and the term podcast was introduced to society.
The generator of a podcast maintains a central list of the files on a server as a web feed that can be accessed through the Internet. The listener or viewer uses special client application software on a computer or media player which accesses this web feed, checks it for updates, and downloads any new files in the series. This process can be automated to download new files automatically, which may seem to users as though new episodes are broadcast or "pushed" to them. Files are stored locally on the user's device, ready for offline use. There are many different mobile applications available for people to use to subscribe and to listen to podcasts. Many of these applications allow users to download podcasts or to stream them on demand as an alternative to downloading. Many podcast players (apps as well as dedicated devices) allow listeners to skip around the podcast and control the playback speed.
Some have labeled podcasting as a converged medium bringing together audio, the web, and portable media players, as well as a disruptive technology that has caused some people in the radio business to reconsider established practices and preconceptions about audiences, consumption, production, and distribution. Podcasts are usually free of charge to listeners and can often be created for little to no cost, which sets them apart from the traditional model of "gate-kept" media and production tools. Podcast creators can monetize their podcasts by allowing companies to purchase ad time, as well as via sites such as Patreon, which provides special extras and content to listeners for a fee. Podcasting is very much a horizontal media form – producers are consumers, consumers may become producers, and both can engage in conversations with each other.