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Digital Music in Your Car Glossary

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AAC (Advanced Audio Coding)
A compression format for digital audio. In terms of sound quality and data efficiency, AAC solidly beats the still-popular MP3 format — not surprising, since AAC is a newer, more advanced form of compression. According to some listening tests, AAC files encoded at lower bitrates (like 96 Kbps) sound as good or better than MP3s encoded at higher bitrates (like 128 Kbps) despite their notably smaller size.

The current version of the AAC codec was developed as part of the MPEG4 standard (a flexible MPEG codec that's used for both streaming and downloadable Web content, and is also the video format employed by a growing number of portable video recorders). AAC is the audio file format used by Apple in their popular iTunes Music Store. Files may appear on your system with the ".M4A" filename extension.

Apple Lossless
Apple Lossless Encoding (also known as Apple Lossless, Apple Lossless Audio Codec or ALE) is an audio codec developed by Apple Computer that provides full, CD-quality audio in about half the space of the original file.

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Bitrate
With audio compression, the average amount of data required to store one second of music (expressed in kilobits per second, or Kbps). Some codecs like MP3, WMA, and AAC allow files to be encoded at different bitrates. Generally, as bitrate decreases, so does the sound quality of the resulting file (and also the amount of memory required to store it).

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MP3 (MPEG1, Audio Layer 3)
The most popular codec for storing and transferring music. Though it employs a "lossy" compression system which removes frequencies judged to be essentially inaudible, MP3 still manages to deliver near-CD sound quality in a file that's only about a tenth or twelfth the size of a corresponding uncompressed WAV file. When creating an MP3 file, varying amounts of compression can be selected, depending on the desired file size and sound quality. For more info on this topic, see our article on the MP3 format.

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Ogg Vorbis — see "Vorbis"

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Variable Bitrate (VBR)
Many newer audio and video codecs employ a technology known as variable-bitrate encoding, which allows resulting files to look and sound better, while still retaining a compressed, convenient file size. Essentially, VBR encoding assigns more bits to complexly detailed passages in the original source, and fewer bits to the simpler passages.

By contrast, constant-bitrate (CBR) encoding uses about the same amount of memory for simple and complex passages — so the user is more likely to experience audible or visible loss of quality during complex parts, especially with lower-bitrate files.

Vorbis (Ogg Vorbis)
Vorbis is an "open-source" digital audio compression format — that is, it exists in the public domain and is completely free for commercial or non-commercial use. Because Vorbis is most often used in conjunction with a digital A/V container format known as "Ogg," it's usually referred to as "Ogg Vorbis."

Vorbis, like MP3, is a "lossy" compression system, removing frequencies deemed inaudible. Both formats offer variable-bitrate encoding options, for better efficiency. But the algorithms Vorbis uses to decide which information to discard differ from those used by MP3. Proponents claim that the Vorbis format outperforms MP3, producing files that are significantly smaller than MP3s of similar sound quality (or files that sound better than similarly sized MP3s).

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WAV
A standard audio format for Windows operating systems, often used for storing high-quality, uncompressed sound. WAV files can contain CD-quality (44.1 KHz/16-bit) audio signals. However, CD-quality WAV files require relatively large amounts of memory — roughly 10 MB per minute of music.

WMA (Windows Media Audio)
Developed by Microsoft, Windows Media Audio is one of today's most popular 'Net audio formats. Though not as popular as MP3, WMA tends to outperform MP3 in the area of sound quality, particularly with files encoded at lower bitrates like 64 or 96 Kbps. This performance advantage makes it handy for applications like portable digital audio players, where total play time is limited by a finite amount of internal memory.

The Windows Media Audio format features built-in copy protection abilities, unlike MP3. Windows XP, Microsoft's current flagship operating system software, contains native support for WMA encoding, enabling users to create their own WMA music files.

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