AAC (Advanced Audio Coding)
AAC is the audio file format used by Apple in their iTunes® Store. Files may appear on your system with the ".m4a" filename extension. AAC employs a lossy compression system which removes frequencies judged to be essentially inaudible. The current version of the AAC codec was developed as part of the MPEG4 standard. (See our MP3 definition for more on this family of audio/video compression standards.)

Auxiliary input
An input that enables you to connect an MP3 player (or any other audio device) to a receiver. The input can be either minijack or stereo RCA jacks. The connection isn't a permanent installation, but if the receiver's inputs are located in the back, it may be more convenient to leave the cable attached. An auxiliary input connection offers very good sound quality.

Bitrate
With audio compression, the average amount of data required to deliver 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).

Bluetooth®
It's a short-range, wireless technology that allows devices to connect to one another and transfer information. Up to eight Bluetooth-enabled devices — such as players, headphones, speakers and cell phones — can connect together wirelessly. Bluetooth networks generally have a range of about 30 feet.

Cassette adapter
This accessory lets you listen to your MP3 player through the cassette player in your car. It connects to the player's headphone output. The adapter requires no permanent installation, and offers acceptable sound quality.

Dock
A cradle for the MP3 player that connects to a computer for charging the battery and transferring music and video. Most docks connect to your computer via a high-speed USB cable. Some docks unite several different kinds of connections. For example, most docks accept an AC line and can be used as a stand-alone charger. Some also permit connection to your audio/video system so you can enjoy your MP3 player's songs through your system's speakers, and show its videos and photos on your TV.

Dock connector
The port on an MP3 player that connects it to a dock, or to a special cable that allows battery charging and music transfer when connected to a computer.

Embedded memory
See Flash memory

Flash memory
A form of digital storage developed in 1988 for use in personal computers and PC peripherals, flash memory gets its name because sections of memory cells within the microchip are erased in one simultaneous action, or "flash." Today, flash memory — also known as embedded memory — is used in MP3 players, as well as in PDAs, digital cameras, and small removable storage devices known as "thumb drives." Because it has no moving parts, flash memory is completely immune to skipping when it's used for audio storage.

Hard drive
A hard drive — just like the one inside a computer, only more compact — is the type of memory used by some MP3 players to store music and data. Hard drive capacities can range from 4GB to 160GB. The larger the capacity, the more music, videos, photos, or information you can save to your player. Because it has moving parts, a hard drive theoretically can skip while playing music; however, almost all hard drive MP3 players have some form of skip protection to minimize this.

iTunes®
iTunes is the combination media library player/organizer and download software that Apple requires for use with the iPod and iPhone. The program is available as a free download from the Apple.com website. It allows easy transfer of music, TV shows, movies, podcasts, games, and other content to your iPod from your computer. iTunes can be installed on both Macs and PCs, and can serve as your computer's media player. It is, however, designed to only sync with iPods. You will not be able to transfer music and videos directly from iTunes to other brands of MP3 players.

Lossless
Lossless encoding is a kind of audio codec that provides full, uncompressed CD-quality audio in about half the space of the original file. Apple's version is known as Apple Lossless (also known as Apple Lossless Audio Codec or ALE). Microsoft® has developed their own version known as Windows Media Lossless.

Lossy
Lossy encoding is a kind of audio codec that reduces an original file to a fraction of its size by dropping out data. This codec uses sophisticated algorithms to carefully select which information to eliminate, keeping as much of the overall sound as possible. Moderate lossy compression can result in much smaller files that sound very close to the original.

MPEG-4
One of the most commonly used codecs for storing and transferring video. MPEG-4 compresses audio and video content into manageable file sizes for Internet use, as well as for storage on computers and portable devices. MPEG-4 video files usually have an ".mp4" or ".m4v" filename extension, and can be viewed on a number of different media players and programs.

MP3 (MPEG1, Audio Layer 3)
The most popular codec for storing and transferring music. Though it employs a lossy compression system that 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.

QuickTime
The QuickTime video format is Apple's proprietary variant of the MPEG-4 codec. Like MPEG-4, QuickTime compresses audio and video content for more efficient storage on portable devices. QuickTime players come bundled with iTunes, and for the most part QuickTime videos can only be used with iPods. QuickTime video files usually have a ".qt" or ".mov" filename extension, and can be viewed primarily through iTunes and some other third-party media player software.

USB port
USB (Universal Serial Bus) is a "plug and play" interface between a computer and your MP3 player (and other devices, including digital cameras, joysticks, keyboards, and printers). High-speed USB — sometimes known as USB 2.0 — transmits data at 480 Mbps, suitable for transferring demanding files like full-motion video, etc. Fortunately, all high-speed USB devices are backwards-compatible with older USB 1.1 devices.

VBR (Variable Bit Rate)
Variable Bit Rate is a way to encode music for better sound quality while still keeping the file size small. Rather than encode at a constant bit rate, this process encodes simple music passages at a low bit rate, and sonically complex ones at a higher rate.

WAV (Waveform audio format)
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.

Windows Media® Player
Windows Media Player is a combination video player/jukebox/music and video download software from Microsoft that works with several different makes of MP3 players. Windows Media Player is a free download from Microsoft.com, and can only be installed on PCs. It can serve as your computer's media player, and allows easy transfer of music, videos, and other content to your MP3 player from your computer. Windows Media Player is compatible with several makes of players, but not with Apple iPods.

WMA (Windows Media Audio)
Microsoft's proprietary audio format, developed to compete with MP3. It employs a lossy compression system 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 a WMA 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 and compressed audio. WMA files have a ".wma" filename extension and can be played on Windows Media Player software and compatible MP3 players. WMA files are not compatible with iTunes.

WMV (Windows Media Video)
Microsoft's proprietary video format. Like MPEG-4, WMV compresses audio and video into manageable file sizes for use on the Internet and with portable digital players. WMV files have a ".wmv" filename extension and can be viewed on Windows Media Player software and compatible MP3 players. WMV files are not compatible with iTunes.

WiFi®
WiFi is a wireless protocol developed to allow various kinds of devices to talk to each other. Cellular phones use this kind of connectivity, as do home wireless computer networks. Many shops, hotels and other businesses offer "open" WiFi hotspots — places where you can access the Internet through your wireless device without needing a password or special software.

Wired FM modulator
A wired FM modulator can connect an MP3 player to any car stereo, and requires permanent installation. It offers good sound quality.

Wireless FM transmitter
A wireless FM transmitter lets you listen to your MP3 player through your car's FM radio. It offers acceptable sound quality and typically does not require permanent installation.