iTunes is a proprietary digital media player application, used for playing and organizing digital music and video files. The program is also an interface to manage the contents on Apple's popular iPod digital media players as well as the iPhone. Additionally, iTunes can connect to the iTunes Store via the Internet to purchase and download music, music videos, television shows, applications, iPod games, audiobooks, podcasts, feature length films and movie rentals (not available in all countries), and ringtones (available only in the USA). It is also used to download applications for the iPhone and iPod touch as long as they are running iPhone OS 2.0 or later.

iTunes was introduced by Apple Inc. on January 9, 2001, at the Macworld Expo in San Francisco. The latest version, iTunes 8, was announced at Apple's September 2008 keynote "Let's Rock".

iTunes is available as a free download for Mac OS X, Windows 7, Windows Vista, and Windows XP from Apple's website. It is also bundled with all Macs, and some HP and Dell computers. Older versions are available for Mac OS 9, OS X 10.0-10.3, and Windows 2000.

Features

iTunes is an application that allows the user to manage audio and video on a personal computer, acting as a front end for Apple's QuickTime media player. Officially, using iTunes is required in order to manage the audio of an Apple iPod portable audio player, although alternative software does exist. Users can organize their music into playlists within one or more libraries, edit file information, record Compact Discs, copy files to a digital audio player, purchase music and videos through its built-in music store, download free podcasts, back up songs onto a CD or DVD, run a visualizer to display graphical effects in time to the music, and encode music into a number of different audio formats. There is also a large selection of free internet radio stations to listen to.

Additionally, users can add PDF files to their library (to add digital liner notes to their albums, for example), but the PDFs cannot be transferred to or read on an iPhone or iPod. However, iPhone/iPod Touch apps exist to sync any type of file to and from the device to an "iDisk" using Apple's MobileMe service.

In the most recent version, iTunes 8.0, the preferences menu was given a complete makeover. The result added very few new options, but instead removed several options. For example, iTunes once gave users the option to display arrows beside the selected song's title, artist, album, and genre that link directly to the iTunes Music Store. Now these arrows are not removable, except through the direct editing of a preferences file.

Media management

iTunes keeps track of songs by creating a virtual library, allowing users to access and edit a song's attributes. These attributes, known as metadata, are stored in two separate library files.

The first is a binary file called iTunes Library and it uses a proprietary file format ("ITL"). It caches information like artist and genre from the audio format's tag capabilities (the ID3 tag, for example) and stores iTunes-specific information like play count and rating. iTunes typically reads library data only from this file.

The second file, iTunes Music Library.xml, is refreshed whenever information in iTunes is changed. It uses an XML format, allowing developers to easily write applications that can access the library information (including play count, last played date, and rating, which are not standard fields in the ID3v2.3 format). Apple's own iDVD, iMovie, and iPhoto applications all access the library.

If the first file is corrupted, iTunes will attempt to reconstruct it from the XML file. Detailed third-party instructions regarding this are documented elsewhere. There have been some concerns, voiced by Mark Pilgrim, that this feature will create an "undocumented binary blackhole" because the recovery from the XML file may not work.

It has also been noted that iTunes does not automatically track changes to actual files in the library. If a file is moved or deleted, iTunes will display an exclamation mark beside the library entry and the user will need to manually amend the library record. There have been a number of third party tools created to address this problem.

iTunes supports ripping from CDs, but not from DVDs. However, in 2008, Apple and select movie studios introduced "iTunes Digital Copy," a bonus feature on some DVDs that provides a copy-protected, iTunes-compatible file for select films. As with any digital music management, users must use an analog-to-digital converter to import analog recordings (such as audio cassettes or vinyl records) to their iTunes libraries.

Library views

Cover Flow allows users to browse their libraries visually by cover art.iTunes users may choose to view their music and video libraries in one of four ways: as a list, as a list with accompanying album artwork, in Cover Flow (a side-scrolling catalog of album artwork), or in Grid View.

The standard list view displays library files with many optional detail fields, including name, artist, album, genre, user rating, play count, and so forth. Item backgrounds alternate between white and a light blue-gray for readability.

The list with accompanying album artwork is much the same, only the list is broken up by albums, with the artwork as a header to the list. Although this allows users to browse content more visually, sorting the list view by name will accordingly break up the library into redundant instances of each album. Accordingly, as with Cover Flow view, the second view mode is most appropriate for users who sort their libraries by album.

Cover Flow displays all of the user's album art as CD covers in a slideshow format. It sorts the albums into artist, genre, etc. Compilation albums are only shown as a single album cover if the compilation tag for each of the album's tracks is turned on. If the song(s) from the album were imported from a 'mix' CD, the album artwork will be displayed as a default music note pictures. Cover flow was first introduced in version 7.0.

Grid View is similar to Cover Flow, displaying the user's cover art in a grid rather than a side-scrolling format. Albums can also be sorted into groups by artist, genre, or composer.

iTunes also sorts with secondary parameters, album by artist and album by year, to make its artwork-centered interfaces more intuitive.

Library sharing

A user's iTunes Library can be shared over a local network using the closed, proprietary Digital Audio Access Protocol (DAAP), created by Apple for this purpose. DAAP relies on the Bonjour network service discovery framework, Apple's implementation of the Zeroconf open network standard. Apple has not made the DAAP specification available to the general public, only to third-party licensees such as Roku. However, the protocol has been reverse-engineered and is now used to stream playlists from non-Apple software (mainly on the Linux platform). DAAP allows shared lists of songs within the same subnet to be automatically detected. When a song is shared, iTunes can stream the song but won't save it on the local hard drive, in order to prevent unauthorized copying. Songs in Protected AAC format can also be accessed, but authentication is required. A maximum of five users may connect to a single user every 24 hours. The multiple, alternate "View" options normally available to iTunes users including "Cover Flow" are disabled when viewing a shared library over a network.

Library sharing was first introduced with iTunes 4.0, where users could freely access shared music anywhere over the Internet, in addition to one's own subnet, by specifying IP addresses of remote shared song libraries. Apple quickly removed this feature with version 4.0.1, claiming that users were violating the End User License Agreement.

With the release of iTunes 7.0, Apple changed their implementation of DAAP. This change prevents any third-party client, such as a computer running Linux, a modified Xbox, or any computer without iTunes installed, from connecting to a remote iTunes repository. iTunes will still connect as a client to other iTunes servers and to third-party servers.

File format support

iTunes 8 can currently read, write and convert between MP3, AIFF, WAV, MPEG-4, AAC and Apple Lossless.

iTunes can also play any audio files that QuickTime can play (as well as some video formats), including Protected AAC files from the iTunes Store and Audible.com audio books. There is limited support for Vorbis and FLAC enclosed in an Ogg container (files using the FLAC container format are not naturally supported) or Speex codecs with the Xiph QuickTime Components. Because tag editing and album art is done within iTunes and not Quicktime, these features will not work with these QuickTime components. iTunes currently will not play back HE-AAC/aacPlus audio streams correctly. HE-AAC/aacPlus format files will play back as 22 kHz AAC files (effectively having no high end over 11 kHz), and HE-AAC streaming audio (which a number of Internet radio stations use) will not play back at all. The latest version of iTunes (Win/Mac) supports importing audio CDs with the default iTunes standard file format of AAC at 256 kbit/s, but users can choose from 16 kbit/s to 320 kbit/s constant bit rates (CBR) in either AAC or MP3.

Importing of audio CDs into MP3 or AAC formats can also be accomplished using variable bitrate (VBR) encoding. However, a double-blind experiment conducted in January 2004 of six MP3 encoders noted that the iTunes encoder came last, in that the quality of the files produced by iTunes was below par. It was stated in the final results that these tests only covered VBR encodings, thus iTunes may have performed better with a Constant bitrate (CBR).

The Windows version of iTunes can automatically transcode DRM-free WMA (including version 9) files to other audio formats, but does not support playback of WMA files and will not transcode DRM protected WMA files. Telestream, Inc. provides free codecs for Mac users of QuickTime to enable playback of unprotected Windows Media files. These codecs are recommended by Microsoft.

File metadata

For MP3 files, iTunes writes tags in ID3v2.2 using UCS-2 encoding by default, but converting them to ID3v2.3 (UCS-2 encoding) and ID3v2.4 (which uses UTF-8 encoding) is possible via its "Advanced" > "Convert ID3 Tags" toolbar menu. If both ID3v2.x and ID3v1.x tags are in a file, iTunes ignores the ID3v1.x tags.

AAC and Apple Lossless files support Unicode metadata, stored in the MP4 container as so-called "Atoms". The QuickTime plugin that supports the OGG container format has no support for tag editing or album art.

iTunes uses the Gracenote interactive audio CD database to provide track name listings for audio CDs. The service can be set to activate when a CD is inserted into the computer and an Internet connection is available. Track names for albums imported to iTunes while not connected to the Internet can be obtained during a later connection, by a manual procedure. For any album loaded into iTunes for which there is not an existing Gracenote track listing, the user can choose to submit track name data to Gracenote.

Sound processing

iTunes includes sound processing features, such as equalization, "sound enhancement" ("sound improvement" in some languages) and crossfade. There is also a feature called "Sound Check" which automatically adjusts the playback volume of all songs in the library to the same level; this is usually called volume leveling or volume normalization. Like "sound enhancement", this can be turned on in the 'Playback' section of iTunes' preferences.

Video support

On May 9, 2005, video support was introduced to iTunes with the release of iTunes 4.8. Users can drag and drop movie clips from the computer into the iTunes Library for cataloguing and organization. They can be viewed in a small frame in the main iTunes display, in a separate window, or fullscreen. Before version 7 provided separate libraries for media types, videos were only distinguished from audio in the Library by a small icon resembling a TV screen and grouped with music in the library, organized by the same musical categories (such as "album" and "composer").

On October 12, 2005, Apple introduced iTunes 6.0, which added support for purchasing and viewing of video content from the iTunes Music Store. The iTunes Music Store initially offered a selection of thousands of Music Videos and five TV shows, including most notably the ABC network's Lost and Desperate Housewives. Disney Channel shows (The Suite Life of Zack & Cody and That's So Raven) were also offered 24 hours after airing, as well as episode packs from past seasons. Since then, the collection has expanded to include content from numerous television networks. The iTunes Music Store also gives the ability to view Apple's large collection of movie trailers.

As of September 5, 2006, the iTunes Store offers over 550 television shows for download. Additionally, a catalog of 75 feature-length movies from Disney-owned studios was introduced. As of April 11, 2007, over 500 feature-length movies are available through iTunes.

Originally, movies and TV shows were only available to U.S. customers, with the only video content available to non-U.S. customers being music videos and Pixar's short films. This feature is being extended to other countries as licensing issues are resolved.

Video content available from the store used to be encoded as 540 kbit/s Protected MPEG-4 video (H.264) with an approximately 128 kbit/s AAC audio track. Many videos and video podcasts currently require the latest version of QuickTime, QuickTime 7, which is incompatible with older versions of Mac OS (only v10.3.9 and later are supported). On September 12, 2006, the resolution of video content sold on the iTunes Store was increased from 320x240 (QVGA) to 640x480 (VGA). The higher resolution video content is encoded as 1.5 Mbit/s (minimum) Protected MPEG-4 video (H.264) with a minimum 128 kbit/s AAC audio track.

Playlists

In addition to static playlist support, version 3 of iTunes introduced support for smart playlists.[28] Smart playlists are playlists that can be set to automatically filter the library based on a customized list of selection criteria, much like a database query. Multiple criteria can be entered to manage the smart playlist.

Any user of iTunes can publish a playlist to the iTunes Store with his or her own preferences, which is called an iMix.

Introduced in iTunes 4.5, the "Party Shuffle" playlist was intended as a simple DJing aid. By default, it selects tracks randomly from other playlists or the library, but users can override the automatic selections by deleting tracks (iTunes will choose new ones to replace them) or by adding their own via drag-and-drop or contextual menu. This allows a mixture of both preselected and random tracks in the same meta-playlist. The playlist from which Party Shuffle drew could be changed on the fly by the computer user, but doing so will cause all randomly chosen tracks to disappear and be replaced.

Party Shuffle was renamed iTunes DJ in iTunes 8. When iTunes was updated to 8.1 quite a few features were added to iTunes DJ. The free Apple Remote application for the iPhone and iPod Touch was also update at this time that added a new iTunes DJ option in the settings screen when the user is connected to a Wi-Fi network and a new song request feature is enabled in iTunes DJ on the hosts. Along with the song request feature voting on songs in the queue was added, the more votes a song gets the high in the queue it will be and sooner it will be played. Song voting can only be done when song requesting is enabled and in two ways: the first by right clicking on a song in the iTunes DJ queue on the hosts computer in iTunes, the second is in the Remote application ether connected with the iTunes DJ option by a guest or by the host in the full playlist section. When song requesting is enabled a customizable welcome message is displayed below the host's shared library name in the button used to connect to iTunes DJ.

Playlists can be played randomly or sequentially. The randomness of the shuffle algorithm can be biased for or against playing multiple tracks from the same album or artists in sequence (a feature introduced in iTunes 5.0, and later discontinued in iTunes 8.0). iTunes DJ can also be biased towards selecting tracks with a higher star rating. With this bias enabled, each star rating increases the preference for that particular song about 4% over that of a one-star-less rated song.[citation needed] Unrated songs are the least likely to be played. Inter-star ratings (Songs assigned an additional "half star," which is visible in iTunes as a ½ symbol in the place of a star but can only be assigned by a third-party program) are stored by iTunes, but only affect this feature in the range of zero to one star.

Genius

The Genius feature, introduced in iTunes 8, automatically generates a playlist of songs from the user's library which are similar to the selected song. Genius playlists are created by the ratings system and collaborative filtering. An iTunes Store account is required because information about the user's library must first be sent anonymously to Apple's database. Algorithms determine which songs to play based on other users' libraries, and Genius becomes more intelligent given a larger data set. The resulting Genius playlist can contain 25, 50, 75, or 100 songs and can be refreshed for new results or saved. The Genius Sidebar will similarly recommend selections for purchase from the iTunes Store based on the selected library track. Once Genius becomes active in iTunes, it can be used on current generations of the iPod Classic, iPod Nano, iPod Touch or the iPhone.

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