http://support.apple.com/kb/HT1550
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.
Products Affected
iTunes
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:
Open iTunes Preferences.
Windows: Choose Edit > Preferences.
Mac: Choose iTunes > Preferences.
Click the General button, then click the Importing Settings
button in the second section of the window.
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 from the
Advanced menu, choose one of the following (The menu item changes to show
what's selected in your Importing preferences):
Create MP3 version
Create AAC version
Create AIFF version
Create WAV version
Create Apple Lossless version
If you haven't imported some songs into iTunes yet, you can
import and convert them at the same time. This will create a converted copy of
the file in your iTunes Library based on your iTunes preferences. To convert
all the songs in a folder or on a disk, hold down the Option key (Mac) or Shift
key (Windows) and choose Advanced > Convert Import preference setting.
The Import preference
setting will match what you chose in step 3. iTunes will prompt you for the
location of the folder or disk you want to import and convert. All the songs in
the folder or on the disk will be converted. Note: 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.
Additional Information
About compression
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.
A note about copyright
iTunes software may be used to reproduce materials. It is
licensed to you only for reproduction of non-copyrighted materials, materials
in which you own the copyright, or materials you are authorized or legally
permitted to reproduce. If you are uncertain about your right to copy any
material, you should contact your legal advisor. See the iTunes Store Terms of
Service for additional information.
Need to convert mp4 to mp3
22947 Views 3 Replies Latest reply: Jan 25, 2011 5:02 PM by nemateoz 
Level 2 (385 points)
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