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Saving a copy of a song in a new file format
You can convert a song to a different file format while keeping a copy of the original. For example, you can save a copy of a compressed song file such as MP3 or AAC in an uncompressed song format (AIFF or WAV).
When converting from a compressed to uncompressed file format (for example, from MP3 to AIFF) you shouldn't notice any reduction in sound quality. However, when converting between compressed formats (for example, MP3 and AAC), you may notice a reduction in the sound quality. For the best results, if you want your music encoded in a different file format, import the music again from the original source using the new encoding format.
To convert a song's file format:
Choose Edit > Preferences, then click the Advanced tab at the top of the window and click Importing.
From the Import Using pop-up menu, choose the encoding format that you want to convert the song to, then click OK to save the settings.
Select one or more songs in your library, then choose Advanced > Convert Selection to MP3, Convert Selection to AAC, Convert Selection to Apple Lossless, Convert Selection to AIFF, or Convert Selection to WAV. (The menu item changes to show what's selected in your Importing preferences.)
To convert all the songs in a folder or on a disk, hold down the Shift key and choose Advanced > "Convert Selection to," then choose the folder or disk containing the songs you want to convert. All the songs in the folder or on the disk will be converted except audio files you purchased from the iTunes Store. (Purchased songs are encoded using a protected AAC format that prevents them from being converted.)
The song in its original format and the newly converted song appear in your library.
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That was off of iTunes help.
