Converting MP3 to AAC and Audio File Formats Comparison


To convert MP3 to AAC, do the following:

Open iTunes.
Choose Edit > Preferences (Windows).
Click General at the top of the Preferences window.
Click Import Settings.
From the Import Using menu, choose AAC Encoder
Clok Ok and OK to return to
Itunes main screen

If the file you are converting is already in your iTunes library, select the song, then choose Advanced > Convert Selection to AAC. If the file is not in your iTunes library, hold the Shift key (Windows) while choosing Advanced > Convert Selection to AAC and then locate the file for the song you want to convert on your hard disk.
When the conversion is finished, the new file appears in the iTunes library under Music. To locate the song on your hard disk, see this document.

source: http://support.apple.com/kb/HT1348

Conversely, you can convert AAC files into MP3 or another format:

However, If one of the songs you are trying to convert was purchased from the iTunes Store and isn’t in iTunes Plus format, then you will get an error message saying that protected files cannot be converted to other formats. The best way around this limitation is to burn the protected song(s) to an Audio CD (not a Data CD-ROM!!!) and reimport them back into iTunes.
There are several “underground” applications, like Hymn, that will remove the DRM protection from a song purchased from the iTunes Store. However, Apple is constantly looking for ways to disable songs that have had the DRM tampered with. Don’t waste your time. Just burn an Audio CD and reimport the songs back into iTunes.
Once your song(s) have finished being reimported/converted, search your iTunes library for the files. You should find 2 versions of each song you converted. One is the original file, the other is the converted version. If you only want the newly converted version of the song, then delete the old one. One easy way to tell the 2 songs apart is to view your iTunes Library by Date Modified. The newer song is the one you just converted.
source: http://www.methodshop.com/gadgets/ipodsupport/convertsongs/index.shtml#ixzz0cegIkUUz

More interesting facts about file formats
source: http://images.apple.com/support/itunes_u/docs/iTunes_U_Creating_Content.pdf

Understanding File Formats
“To create and distribute materials for playback on iPod and in iTunes, you need to
get the materials (primarily audio or video) into compatible file formats.
Understanding file formats and how they compare with each other will help you
decide the best way to prepare your materials.
Apple recommends using the following file formats for iPod and iTunes content:
• AAC (Advanced Audio Coding) for audio content
AAC is a state-of-the-art, open (not proprietary) format. It is the audio format of
choice for Internet, wireless, and digital broadcast arenas. AAC provides audio
encoding that compresses much more efficiently than older formats, yet delivers
quality rivaling that of uncompressed CD audio.
• H.264 for video content
H.264 uses the latest innovations in video compression technology to provide incredible video quality from the smallest amount of video data. This means you see crisp, clear video in much smaller files, saving you bandwidth and storage costs over previous generations of video codecs. One of the factors to keep in mind when preparing your materials is file size. Using high-quality compressed formats, such as AAC and H.264, allows files to download faster and take up less space on your audience’s hard disk.
Note: iPod and iTunes support many other formats, including MP3, MP3 VBR, AIFF, Apple Lossless,WAV, AA, MPEG-4, and PDF (iTunes only).

File Formats Comparison:

AAC (Advanced Audio Coding): A state-of-the-art, open audio file format. At any given bit rate, AAC delivers higher quality audio than other older MP3 formats. AAC provides almost twice the clarity of MP3 audio at the same bit rate with equal or smaller file sizes. File size: Usually less than 1 MB for each minute of content.
Use when you want CD quality audio in a highly compressed file.
AAC is the next generation in audio formats that offers:
• Improved compression with higher quality results and smaller file sizes
• Increased quality at a wide range of data rates
• Support for multichannel audio, providing up to 48 full frequency channels
• Higher resolution audio, yielding sampling rates of up to 96 kHz
• Improved decoding efficiency, requiring less processing power for
decoding

MP3: A compressed audio format. Because it is an older format, quality is not as high as with newer formats. File size: About 1 MB for each
minute of content. MP3 Offers good quality audio, but compression and quality are not as good as next generation formats.

MP3 VBR (virtual Bit Rate): An enhanced MP3 format that compresses the audio at varying rates based on the content. The compression is
determined moment by moment; for example, silence and simple sounds compress much more than complex sounds such as reverberation. Unless the entire audio composition is complex, MP3
VBR provides better overall sound quality than MP3 without making the file size too large.

Protect AAC: A file format that is protected with FairPlay, a DRM (Digital Rights Management) system from Apple. Apple uses Protected AAC to encode copy-protected music titles purchased from the iTunes Store.

AIFF (Audio Interchange File Format): File format for storing highquality digital audio and musical instrument information. Uncompressed, CD-quality audio file format File size: About 10 MB for each minute of content. Use for CD-quality audio when size of file is not an issue. If you plan to burn highquality audio CDs with the
content, you should use the Apple Lossless or AIFF format for the best results.

Apple Lossless: CD-quality equal to AIFF and WAV formats in a smaller
file size. Apple Lossless compresses CD audio to 50% of its original size. (MP3s compress to 10% of original.) File size: 5 MB for each minute of content. Use for CD-quality audio when you need a smaller file size than AIFF or WAV, but not as compressed as AAC or MP3.
Can be played in iTunes, applications that support QuickTime, and iPod models that come with a Dock connector. If you plan to burn highquality audio CDs with the content, you should use the Apple Lossless or AIFF format for the best results.

WAV: Native digital audio format in Windows. Uncompressed, CD-quality audio file format. File size: About 10 MB for each minute of content. Use primarily with Windows computers that are not using
iTunes, or computers that do not have MP3 software. Note: iTunes does not write metadata to WAV files.

AA (format 2, 3 and 4): Developed by audible.com for
the spoken word, primarily audiobooks. Use for spoken word and
audiobooks.

H.264: The next generation video compression technology in the MPEG-4 standard The result is crisp, clear video in much smaller files, saving
in bandwidth costs over previous generations of video formats. For example, H.264 delivers up to four times the resolution of MPEG-4 at the same data rate. Use when you want very high quality across the broadest range of bandwidths from 3G mobile devices to iChat AV for
videoconferencing. Because H.264 is an integral part of the QuickTime 7 architecture in Mac OS X v10.4 Tiger, QuickTime-based
applications—including iChat AV, Final Cut Pro, and other third-party applications—can take full advantage of this new video format.

MPEG-4 Video: Defined by MPEG (Moving Picture Experts Group) to
deliver DVD-quality video at lower data rates and smaller file sizes than MPEG-2. (MPEG-2 was defined for DVD video; MPEG-4 was defined for
Internet delivery of digital media.) Based on the QuickTime architecture
Use to create content in a simple, cost-effective “author once, play anywhere” model. You don’t have to manage the same material in multiple formats. Works with a wide variety of devices, including mobile
phones and digital still cameras.

PDF booklets: PDF stands for Portable Document Format. PDF files can be stored and accessed from within iTunes, but not on iPod. Used for text-based books or articles. PDF extension is
required. Note: iTunes does not write metadata to PDF files.

Deciding Which Audio Format to Use
The most common Internet audio file formats you might encounter are AAC and MP3. Although MP3 is widely used, the AAC format offers several advantages, especially when creating content for delivery:
• This newer format uses more advanced technology.
• It offers better compression than MP3, which means higher quality results and smaller file sizes.
• The format is compatible with iTunes and iPod. (Even listeners who do not have iPod can still listen through iTunes.)
• It supports chapters. (Your listeners can jump to a specific section of your content.)
An enhanced podcast can be divided into chapters, allowing listeners to quickly navigate to specific parts of the podcast or content. Each chapter can have an associated piece of artwork.
• The format provides the ability to incorporate URLs and pictures set to appear at certain times during playback of audio files. (Images are visible within iTunes and on iPod with color display. URLS are accessible within iTunes.)
Note: You can choose to make your content available in both formats (MP3 and AAC), or your listeners can use iTunes to convert AAC files to MP3 files.
What Is Metadata? Information about information is called metadata. For example, an audio file contains audio information. The name of the person that created the file, the length of the file, title of the file, description, and so on, are examples of metadata a file might contain.
Metadata is useful for several reasons:

• The iTunes and iPod user interface is built from the metadata. For example, clicking the Artist category would yield useless results if you didn’t enter information for the artist metadata.
• It makes browsing and searching much more efficient and helpful to your audience. Listeners can search by the information in the Artist field, search by your groupings, or any other category.
• It supports and reinforces the content. For example, in an educational context, metadata meets the needs of different learning styles. There are visual learners and those who learn better by reading text. A speech of a political leader could include the text of the speech that listeners can read as they hear the speech, and it could include a picture of the speaker so listeners can connect emotionally to the voice. While some metadata may be entered when you publish your files to the server, it is also important to include some embedded metadata in the file itself. iTunes and many other applications can display and use this metadata for cataloging as well as search and retrieval functions. If you add metadata to file formats that support metadata in iTunes, the metadata stays with the content even if it is moved”

source: http://images.apple.com/support/itunes_u/docs/iTunes_U_Creating_Content.pdf

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