What is Podcasting?
Podcasting is a method of publishing sound files to the Internet, allowing users to subscribe to a feed and receive new audio files automatically. Podcasting is distinct from other types of audio content delivery because it uses the RSS 2.0 file format. This technique has enabled many producers to create self-published, syndicated radio shows.
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Users subscribe to podcasts using "podcatching" software (also called "aggregator" software) which periodically checks for and downloads new content. It can then sync the content to the user's portable music player, hence the portmanteau of Apple's "iPod" and "broadcasting". Podcasting does not require an iPod; any digital audio player or computer with the appropriate software can play podcasts.
On June 6, 2005 as part of the WWDC Keynote, the CEO of Apple Computer, Steve Jobs, announced that automatic podcasting subscriptions would be supported on forthcoming versions of iTunes and iPod.
Origin of podcasting
By 2003, a number of blogs already published audio online, and the RSS file format was widely used for summarizing or syndicating content. Using RSS, former NPR host Christopher Lydon attached audio files to his weblog. Lydon's full-length interviews, which focused on blogging and coverage of the 2004 U.S. presidental campaigns, helped to inspire Adam Curry's iPodder script. Indeed, blogs would become an important factor in the popularization of podcasting.
One of the first uses of the term "podcasting" was in an article in The Guardian  (http://www.guardian.co.uk/online/story/0,3605,1145689,00.html) on February 12, 2004, though it didn't detail the RSS file format or automatic synchronization. In September of that year, Dannie Gregoire used the term to describe the automatic download and synchronization idea that Adam Curry had developed  (http://groups.yahoo.com/group/ipodder-dev/message/41). Gregoire had also registered multiple domain names associated with podcasting. That usage was discovered and reported on by Curry and Dave Slusher of the Evil Genius Chronicles website.
Differences from traditional broadcasting
Unlike radio or streaming media, podcasts are time-shifted, meaning that listeners have control over when they hear the recording similar to a VCR playing back a pre-recorded TV show. Using a portable player, such as the iPod or a cell phone, podcasts are instantly available and can be listened to progressively while tending to other matters, therefore educating a busy listener at no time expense.
From the producer's perspective, podcasts cannot have live participation or immediately reach large audiences as quickly as radio can. However, podcasting allows individuals to easily transmit content worldwide without the need for expensive equipment or licenses, and is frequently used together with an online interactive bulletin board or blog.
Differences from other forms of online audio
Podcasting differs from broadcasting and webcasting in the way that content is transmitted. Instead of a central audio stream, listeners download audio files remotely and automatically. Podcasts can also include metadata such as dates, titles, and descriptions. Podcasting differs from autocasting in terms of content; podcasts are generally voice broadcasts while autocasting is a speech-synthesized version of regular text blogs. Audioblogs can be easily made into podcasts if they add support for RSS to facilitate automatic retrieval.
Podcasting's initial appeal was to allow individuals to create their own "radio shows", but the system is increasingly used for other reasons, including:
Radio stations podcasting their broadcasts, which apparently started in the U.S. on October 4, 2004 when Leo Laporte began re-broadcasting his KFI Los Angeles radio show as a podcast feed. On June 3rd, 2005, Rush Limbaugh began podcasting his daily radio program.
In some instances, podcasts have been turned into unofficial audio tours of museums. New York Times article (http://www.nytimes.com/2005/05/28/arts/design/28podc.html)
Musselburgh Grammar School, Scotland, is trialling podcasting to deliver foreign language audio revision and homework. The pupils were allegedly the first school in Europe to launch a regular podcast.
St Mark's Anglican Church, Clayfield Australia (http://www.stmarksclayfield.org), is trialling Podcasting as a new device to broadcast talks and sermons from their evening service.
Audioblogging is a variant on the blogging trend of online self-publishing, using audio to reach the audience instead of text used by traditional blogs. Audioblogs have similar form as blogs, using post-based entries cataloged by time and date. There is usually a title and brief description, but the bulk of content is in the linked audio file. Usually audioblogs are MP3 format, but occasionally in Ogg Vorbis, AAC, or Macromedia Flash formats.
Many audiobloggers are also text bloggers and use both types of posts on their blogs. It can be claimed that if a blog post contains a linked audio file, the weblog is also technically an audioblog.
The year 2004 was the tipping point for audioblogging. A group of audiobloggers started to unite around the use of RSS enclosures with their audioblog posts and a radio like content format. As media attention grew aroung this new style of audio distribution, a community started to grow. Audioblogs that have come to use RSS are technically known as podcasts.
Autocasting is an automated form of podcasting that allows bloggers and blog readers to generate audio versions of text blogs from RSS feeds. Autocasting software uses XML parsers, TTS (text-to-speech) engines, and audio conversion utilities to convert text blogs into audio files that can be placed on a blog for download, synchronized to a portable audio device, or played on a desktop computer.