Common Digital Audio Formats
Format Extensions Notes
PCM (uncompressed) wav, aiff CD audio is PCM at a 44.1kHz sampling rate, 16-bit resolution, and two channels
MPEG Audio Layer-III (MP3) mp3 An ISO standard for compressed audio; the most widely supported compressed format
mp3PRO mp3 An extension to MPEG Audio Layer-III that provides higher quality at lower bit rates
MPEG AAC aac, mp4, m4a An ISO standard format that offers higher quality than MP3 at the same bit rate
AAC with Fairplay DRM m4p The copy-protected format for music purchased from Apple's iTunes Music Store
Ogg Vorbis ogg A high-quality open source format
Apple Lossless m4a A lossless format supported by iTunes
Windows Media Audio wma A proprietary format from Microsoft used by many online music stores
Real Audio ra A proprietary format from RealNetworks used primarily for streaming audio

There are so many types of audio files out there that it's all too easy to end up with the wrong one.

Maybe you converted your CD collection to Real Audio or WMA and now want to convert the songs to a nonproprietary format, such as MP3, so you can play them with iTunes.

Or maybe you purchased songs in a copy-protected AAC format from the iTunes Music Store and need to convert them to MP3 so they'll work in your portable player,

handheld computer, home network, or competing jukebox program, such as Musicmatch Jukebox.

Fortunately, any audio that you can hear on your computer can be converted to another format.

How easy that conversion will be depends on whether the files are copy-protected.

 If they are, you won't be able to convert them directly.

However, you can use one of the indirect conversion methods described later in this article.

If the files are not copy-protected and are in a format supported by your jukebox or audio-editing program, it's fairly straightforward to convert them to another format, and I'll cover that too.

Digital audio will continue to exist in multiple competing formats for the foreseeable future.

While some specialized formats are necessary for certain applications, many of the current formats serve the same purpose.

Some offer less restrictive Digital Rights Management (DRM, i.e., copy protection), some sound better at low bit rates, and some are better at streaming.

    But for the average user the main issues are compatibility with hardware and software, and restrictions imposed by DRM systems.

Formats: Lossless vs. Lossy

Lossless formats store digital audio with absolutely no loss of information. Some, like PCM, store just the raw audio data with no compression, while others like Apple Lossless and FLAC use lossless compression techniques to create files about half the size of files that use plain PCM.

Lossy formats like MP3, AAC, Ogg Vorbis, and WMA can achieve much larger reductions in file size than are possible with lossless compression. By discarding unnecessary and redundant information, lossy formats can squeeze an audio file to less than one tenth of its original size without losing much quality.

Above table  lists several common formats for digital audio. You can identify the format of most audio files by the extension (the letters at the end of the file name). However, the same extension is sometimes used for entirely different formats, as is the case with Apple's lossless format and the AAC format used by the iTunes music store. In other cases, such as with AAC, different extensions are used, depending on which program created the file and whether the file is copy-protected.

Sound Quality

When you convert from one lossless format to another, or from a lossy format to a lossless one, you won't lose any quality. But when you convert from one lossy format to another you will lose a bit of information. That's because different encoding schemes throw away different parts of the signal. But that's part of the tradeoff. The same is true if you convert lossy files from one bit rate to another—you lose a little quality each time.

If you convert a lossless format to a lossy format, you will also lose information, but you can control the quality of the converted file by adjusting the bit rate and other settings. Lower bit rates result in smaller files with poorer sound quality, while higher bit rates result in better sound quality, but larger files.

With MP3, a bit rate of 192kbps should sound pretty good for all but the most demanding material. AAC can provide similar quality at 128kbps, which means the files will take up approximately 33% less space.

If you're converting AAC files encoded at 128kbps to MP3, set the bit rate for the MP3 files to 192kbps. Otherwise, you'll lose a lot of information because MP3 is less efficient than ACC and will not be able to store all the information at the same bit rate.