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In the digital music era, people often call any music file an "MP3." But that's not necessarily accurate. MP3 refers to a specific type of audio file and not every digital audio file is actually an MP3. If you use an iPhone, iPod, or other Apple device, there's a good chance that most of your music isn't in the MP3 format at all.
What kind of file are your digital songs, then? This article explains the details of the MP3 filetype, the more advanced and Apple-preferred AAC, and some of the other common audio file types that do and don't work with iPhones and iPods.
All About the MP3 Format
MP3 is short for MPEG-2 Audio Layer-3, a digital media standard designed by the Moving Picture Experts Group (MPEG), an industry body that creates technical standards.
Songs saved in the MP3 format take up less space than the same songs saved using a CD-quality audio format like WAV (more on that format later). MP3s save storage space by compressing the data that makes up the file. Compressing songs into MP3s involves removing parts of the file that won't impact the listening experience, usually the very high and very low ends of the audio. Because some data has been removed, an MP3 doesn't sound identical to its CD-quality version and is referred to as a "lossy" compression format . The loss of some segments of the audio has caused some audiophiles to criticize MP3s as damaging the listening experience.
Because MP3s are more compressed than AIFF or other lossless compression formats, more MP3s can be stored in the same amount of space than CD-quality files.
While the settings used for creating MP3s can change this, generally speaking an MP3 takes up about 10% of the space of a CD-quality audio file. For example, if the CD-quality version of a song is 10 MB, the MP3 version will be around 1 MB.
The audio quality of an MP3 (and all digital music files) is measured by its bit rate, rendered as kbps.
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The higher the bit rate, the more data the file has and the better the MP3 sounds. The most common bit rates are 128 kps, 192 kbps, and 256 kbps.
There are two kinds of bit rates used with MP3s: Constant Bit Rate (CBR) and Variable Bit Rate (VBR). Many modern MP3s use VBR, which makes files smaller by encoding some parts of a song at a low bit rate, while others are encoded using higher bit rates. For example, a section of a song with only one instrument is simpler and can be encoded with a more-compressed bit rate, while parts of a song with more complex instrumentation need to be less compress to capture the full range of sound. By varying the bit rate, the overall sound quality of an MP3 can stay high while the storage needed for the file is kept relatively small.
MP3 may be the most popular digital audio format online, but the iTunes Store does not offer music in that format (more on that in the next section). Despite that, MP3s are compatible with iTunes and with all iOS devices, like the iPhone and iPad. You can get MP3s from:
Many music-file-sharing services.
All About the AAC Format
AAC, which stands for Advanced Audio Coding, is a digital audio file type that has been promoted as the successor to the MP3. AAC generally offers higher-quality sound than an MP3 while using the same amount of disk space or less.
Many people think AAC is a proprietary Apple format, but this is not correct. AAC was developed by a group of companies including AT&T Bell Labs, Dolby, Nokia, and Sony. While Apple has adopted AAC for its music, AAC files can actually be played on a range of non-Apple devices, including the game consoles and mobile phones running Google's Android OS, among others.
Like MP3, AAC is a lossy file format. In order to compress CD-quality audio into files that take up less storage space, data that will not impact the listening experience—again, generally at the high and low end—is removed. As a result of the compression, AAC files do not sound identical to CD-quality files, but generally sound good enough that most people don't notice the compression.
Like MP3s, the quality of an AAC file is measured based on its bit rate. Common AAC bitrates include 128 kbps, 192 kbps, and 256 kbps.
The reasons that AAC produces better sounding audio than MP3s are complex. To learn more about the technical details of this difference, read the Wikipedia article on AAC.
Apple has adopted AAC as its preferred file format for audio. All songs sold at the iTunes Store, and all songs streamed or downloaded from Apple Music, are in the AAC format. All AAC files offered in these ways are encoded at 256 kbps.
The WAV Audio File Format
WAV is short for Waveform Audio Format. This is a high-quality audio file generally used for applications that require high-quality sound, such as CDs. WAV files are uncompressed, and therefore take up more disk space than MP3s or AACs, which are compressed.
Because WAV files are uncompressed (also known as being a "lossless" format ), they contain more data and produce better, more subtle, and more detailed sounds. A WAV file generally needs 10 MB for every 1 minute of audio. By comparison, an MP3 needs about 1 MB for every 1 minute.
WAV files are compatible with Apple devices, but are not commonly used except by audiophiles. Learn more about the WAV format.
The WMA Audio File Format
WMA stands for Windows Media Audio. This is the file type promoted most by Microsoft, the company that invented it. It is the native format used in Windows Media Player, both on Macs and PCs. It competes with the MP3 and AAC formats and offers similar compression and file sizes as those formats. It is not compatible with the iPhone, iPad, and similar Apple devices. Learn more about the WMA format.
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The AIFF Audio File Format
AIFF stands for Audio Interchange File Format. Another uncompressed audio format, AIFF was invented by Apple in the late 1980s. Like WAV, it uses about 10 MB of storage per minute of music. Because it does not compress audio, AIFF is a higher-quality format preferred by audiophiles and musicians. Since it was invented by Apple, it's compatible with Apple devices. Learn more about the AIFF format.
The Apple Lossless Audio File Format
Another Apple invention, the Apple Lossless Audio Codec (ALAC) is a successor to AIFF. This version, released in 2004, was originally a proprietary format. Apple made it open source in 2011. Apple Lossless balances reducing file size with maintaining sound quality. Its files generally are about 50% smaller than uncompressed files, but with less loss in audio quality than with MP3 or AAC. Learn more about the ALAC format.
The FLAC Audio File Format
Popular with audiophiles, FLAC (Free Lossless Audio Codec) is an open-source audio format that can reduce the size of a file by 50-60% without reducing audio quality too much.