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 an uncompressed song file such as AIFF or WAV to a compressed format like MP3, AAC, or Apple Lossless Encoder.
Saving a copy of a song in a new file format
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, you should import the music again from the original source using the new encoding format.
To convert a song’s file format:
To convert all the songs in a folder or on a disk, hold down the Option key and from the Advanced menu, choose Convert Selection to. All the songs in the folder or on the disk will be converted. Some purchased songs are encoded using a protected AAC format that prevents them from being converted. iTunes Plus purchases are not protected and can be converted.
The song in its original format and the newly converted song appear in your library.
When you convert a song, some data may be lost due to the way certain formats compress data. For this reason these formats are sometimes called “lossy” formats. The advantage of using a “lossy” format is that the file sizes are much smaller, which means you can store more songs in the same amount of disk space. The disadvantage is that the sound quality may not be as good as the original, uncompressed format. Depending on the song, the speakers or headphones, and the player you use to play the song, you may not be able to tell the difference between a compressed “lossy” song and a song that is not compressed.
Once a song is compressed (meaning some of its data is lost) you cannot retrieve the data by uncompressing it. If you convert a song from a “lossy” format to a uncompressed format, the quality of the song will not improve and the file will only take up more disk space. For example, if you convert a song in MP3 format (a compressed format) to AIFF (an uncompressed format) the song will take up much more space on the hard disk, but it will still sound the same as the compressed file. In order to take advantage of uncompressed formats you should only import songs using these formats.