A podcast is a collection of digital media files which is distributed over the Internet using syndication feeds for playback on portable media players and personal computers. The term, like "radio", can refer either to the content itself or to the method by which it is syndicated; the latter is also termed podcasting. The host or author of a podcast is often called a podcaster.
The term "podcast" is a portmanteau of the words "iPod" and "broadcast",[1] the Apple iPod being the brand of portable media player for which the first podcasting scripts were developed (see history of podcasting). These scripts allow podcasts to be automatically transferred to a mobile device after they are downloaded
Though podcasters' web sites may also offer direct download or streaming of their content, a podcast is distinguished from other digital media formats by its ability to be syndicated, subscribed to, and downloaded automatically when new content is added, using an aggregator or feed reader capable of reading feed formats such as RSS or Atom.
Podcasting is an automatic mechanism whereby multimedia computer files are transferred from a server to a client, which pulls down XML files containing the Internet addresses of the media files. In general, these files contain audio or video, but also could be images, text, PDF, or any file type.
The content provider begins by making a file (for example, an MP3 audio file) available on the Internet through some known Uniform Resource Identifier. This is usually done by posting the file on a webserver; however, BitTorrent trackers also have been used. The files may be available to the general public or may only be available to certain users.
The content provider then announces the existence of that file by referencing it in another file known as the feed. The feed is a list of the URLs by which episodes of the podcast may be accessed. This list is usually published in RSS format (although Atom can also be used), which provides other information, such as publish date, titles, and accompanying text descriptions of the series and each of its episodes. The feed may contain entries for all episodes in a series, but is typically limited to a short list of the most recent episodes, as is the case with many news feeds. Standard podcasts consist of a feed from one author. More recently, multiple authors have been able to contribute episodes to a single podcast feed using concepts such as social podcasting.
The content provider posts the feed on a webserver. The location at which the feed is posted is expected to be permanent. This location is known as the "feed URL" or "feed URI". The content provider makes this feed known to the intended audience.
A podcast specific aggregator is usually an always-on program which starts when the computer is started and runs in the background. It works exactly like any aggregator which checks each feed at a specified interval, such as every two hours. If the feed data has substantially changed from when it was previously checked (or if the feed was just added to the application's list), the program determines the location of the most recent item and automatically downloads it. The downloaded episodes can then be played, replayed, or archived as with any other computer file. Many applications also automatically transfer the newly downloaded episodes available to a user's portable media player, which is connected to the PC running the aggregator, perhaps via a USB cable.
The publish/subscribe model of podcasting is a version of push technology, in that the information provider chooses which files to offer in a feed and the subscriber chooses among available feed channels. While the user is not "pulling" individual files from the Web, there is a strong "pull" aspect in that the receiver is free to subscribe to (or unsubscribe from) a vast array of channels. Earlier Internet "push" services (e.g., PointCast) allowed a much more limited selection of content.
In March 2006 it was reported that 80% of podcast "episodes" are "consumed" on the PC onto which they are downloaded, i.e. they are never actually transferred to a portable player, or are deleted from the PC without being listened to To conserve bandwidth, users may opt to search for content using an online podcast directory. Some directories allow people to listen online and become familiar with the content provided from an RSS feed before deciding to subscribe. For most broadband users, bandwidth is generally not a major consideration
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