File Formats Glossary
AAC (Advanced Audio Coding)
AAC was developed to improve upon the MP3 audio format, and uses a more advanced form of compression. According to some listening tests, AAC files encoded at lower bitrates (like 96 Kbps) sound as good or as better than MP3s encoded at higher bitrates (like 128 Kbps) despite their smaller size.
The current version of the AAC codec was developed as part of the MPEG4 standard. Versions of AAC are used by Apple in their popular iTunes® Store, as well as Sony in their PlayStation®3 game consoles. Files may appear with the ".m4a" or ".mp4" filename extension. Songs with DRM (digital rights management) purchased from the iTunes Store usually have an ".m4p" extension (with the "p" at the end to denote "protected").
AIFF (Audio Interchange File Format)
An audio format for Macintosh operating systems commonly used for storing uncompressed, CD-quality sound (similar to WAV files for Windows-based PCs). AIFF is considered a lossless container format.
Apple Lossless Encoding (also known as Apple Lossless, Apple Lossless Audio Codec or ALE) is a lossless audio codec developed by Apple Computer to provide full, CD-quality audio in about half the space of the original file.
Developed by Sony engineers in the early 90s for the MiniDisc format, ATRAC is a lossy audio codec which offers near-CD sound quality with relatively small file sizes.
A later version of the ATRAC format that squeezes music into even smaller files. It was used for music storage in some portable Sony digial music players.
The most recent version of the ATRAC codec. ATRAC3plus was found on Sony's Hi-MD portable recorders and offered even better sound quality at lower bitrates than earlier versions.
A high-definition digital video format that can record in 1080i and 720p and still maintain a reasonably small file size. AVCHD files are based on the MPEG4 codec. The advent of high-definition (HD) televisions and displays spurred the development of this format, which uses the same resolution as HDTV signals. AVCHD video files can also be burned to Blu-ray Discs™, and played in compatible devices, such as Blu-ray Disc players and the Sony PlayStation®3.
AVI (Audio/Video Interleaved)
A file format for storing and playing back movie clips with sound on Windows-based PCs. An AVI file is organized into alternating ("interleaved") chunks of audio and video data. AVI is a container format, meaning that it specifies how the data will be organized, but is not itself a form of audio or video compression.
AVI is the type of file that's created when DV clips are imported from a digital camcorder to a PC. (These clips are often referred to as "DV-AVIs" because they contain full-quality digital video content.)
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, as well as the amount of memory required to store it.
BMP (Windows Bitmap Image)
A standard format used for storing images on Windows-based PCs. BMP images can either be compressed or uncompressed. This type of file also sometimes appears with the ".DIB" extension.
A container format is one that holds different kinds of data within its file. Container formats, such as RealAudio and TIFF, are gaining in popularity because of their multimedia applications, as well as their cross-platform compatibility. For example, a single container file can hold chapter information, hyperlinks and subtitles, as well as different kinds of codecs that enable various types of players to read the file.
DV (Digital Video)
DV is the format used by many digital camcorders, usually on Mini DV cassettes. Though the DV format employs a form of lossy video compression (applied in real-time as you record with your camera), it's still memory-intensive. When transferred to a computer, a DV clip requires roughly 1 GB of storage per 5 minutes of video. (Clips are usually stored on the computer as QuickTime or .AVI files.)
Despite its use of compression, DV can provide a clean image with up to 520 lines of resolution. DV uses a type of compression known as "intraframe" — that is, it encodes video at the full standard frame rate of 30 frames per second. This allows frame-by-frame editing. In contrast, video codecs like MPEG1 or MPEG2 tend to handle a video sequence by reducing the number of full frames per second and encoding the differences between frames, making precise editing more difficult. These are known as "interframe" forms of compression.
DivX was developed by DivX, Inc., to compress a great deal of video content into relatively small files and still retain reasonably good image quality when played back. DivX is based on MPEG-4, and is a popular choice for sending video files over the Internet.
FLAC (Free Lossless Audio Codec)
FLAC is a lossless audio codec developed by Josh Coalson and the Xiph.Org Foundation. FLAC can provide full, CD-quality audio in about half the space of the original file. FLAC is considered a lossless container format.
GIF (Graphic Interchange Format)
A format for storing digital images, commonly used for bullets, icons, and other graphics on the Web. The GIF format is limited to 256 colors, so it's not as commonly used as JPEG for storing digital photos. A single GIF file can combine several frames together for basic animated motion.
Named after the Joint Photographic Experts Group, JPEG is a lossy codec for storing and transferring full-color digital images that's often used to post photography and artwork on the Web. JPEG compression takes advantage of the human eye's inability to see minute color changes, removing portions of data from the original picture file. When creating a JPEG file, varying amounts of compression can be selected, depending on the desired file size and image quality.
A form of this codec known as Motion JPEG is used by some digital cameras and camcorders for storing video clips of relatively small file size. With Motion JPEG, each frame of video is captured separately and reduced in size using JPEG compression.
Lossless data compression
As the name implies, lossless compression retains all of the data of the original file as it's converted to a smaller file size. When a lossless file such as a TIFF is opened, algorithms restore all compressed information, creating a duplicate of the source file. Lossless compression is generally preferred for creating high-quality or professional-grade audio and video files where it's important to retain fine detail.
Lossy data compression
With this kind of compression, some of the source file's information is discarded to conserve space. When the file is decompressed, this information is reconstructed through algorithms, usually resulting in some loss of sound quality or image detail when compared to the original. Generally, the higher the resolution of the compressing file, the less the degradation. An MP3 file with a resolution of 256 Kbps, for example, tends to sound more like the source file than one made at 64 Kbps.
MIDI (Musical Instrument Digital Interface)
A MIDI file doesn't contain actual audio data, but rather contains commands that let MIDI-capable synthesizers re-create a specific musical passage. The MIDI protocol has been used for years as a way for electronic musical instruments (like digital keyboards and sequencers) to communicate with each other.
Computer sound cards typically feature the ability to interpret MIDI files into music. Since they don't actually contain the music itself, but rather the commands used to re-create music, MIDI files are a lot smaller than audio files like MP3s, WMAs, or WAVs. MIDI files are small and manageable enough that it's not uncommon to find them embedded in web pages, adding a sonic element to the surfing experience. They usually appear with the ".MID" filename extension.
MPEG stands for Moving Picture Experts Group — a committee that sets international standards for the digital encoding of movies and sound. There are several audio/video formats which bear this group's name. In addition to their popularity on the Internet, several MPEG formats are used with different kinds of A/V gear:
- MPEG1. This format is often used in digital cameras and camcorders to capture small, easily transferable video clips. It's also the compression format used to create Video CDs, and commonly used for posting clips on the Internet. The well-known MP3 audio format (see definition below) is part of the MPEG1 codec.
- MPEG2. Commercially produced DVD movies, home-recorded DVD discs, and most digital satellite TV broadcasts employ MPEG2 video compression to deliver their high-quality picture. MPEG2 is also the form of lossy compression used by TiVo-based hard disk video recorders. It can rival the DV format when it comes to picture quality. Because MPEG2 is a "heavier" form of compression that removes a larger portion of the original video signal than DV, however, it's more difficult to edit with precision. The MPEG2 codec allows for selectable amounts of compression to be applied, which is how home DVD recorders and hard disk video recorders can offer a range of recording speeds. MPEG2 is considered a container format.
- MPEG4. A flexible MPEG container format used for both streaming and downloadable Web content. It's the video format employed by a growing number of camcorders and cameras.
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, you can select varying amounts of compression depending on the desired file size and sound quality. For more info, see our article on the MP3 format.
An updated version of the original MP3 codec. Small, low-bitrate mp3Pro files contain much more high-frequency detail than standard MP3 files encoded at similar low bitrates. The high-frequency portion of the audio signal is handled by an advanced and extremely efficient coding process known as Spectral Band Replication (SBR), while the rest of the signal is encoded as a regular MP3. That means that when you play an mp3Pro file on non-mp3Pro-compatible software, you'll only hear the non-SBR-encoded portions (so you'll lose the highs altogether). However, when encoded and played back using a fully compatible audio program, such as Windows Media Player, mp3Pro files can deliver very good sound quality using low bitrates.
Ogg Vorbis — see Vorbis
PCM (Pulse-code modulation)
A method of creating digital versions of analog sounds. During the encoding process, a digital "snapshot" of the analog sound wave is taken at regular intervals. The process is similar to that of movie film, where each frame is a still photograph taken in a short amount of time. Just as the eye perceives those frames shown in rapid succession as smooth motion, the ear hears the digital samples as a continuous sound wave. The closer together the digital samples (the higher the sampling rate), the smoother and more accurate the sound appears.
QuickTime is a file format for storing and playing back movies with sound. Though developed and supported primarily by Apple, Inc., this flexible format isn't limited to Macintosh operating systems — it's also commonly used in Windows systems and other types of computing platforms. In Windows, QuickTime files usually appear with the ".MOV" filename extension.
An image file of minimally processed data received from a digital camera. Most camera manufacturers have their own proprietary version of the RAW image format, and their own file suffixes. Canon, for example, uses ".crw" or ".cr2" for their version of RAW. Nikon's RAW files end in ".nef," while Sony uses ".arw" and ".srf" suffixes.
Professionals prefer shooting in RAW because the additional information these large files contain allows greater flexibility in post-production editing. Because the image is basically unprocessed (as compared to a JPEG image), RAW files can retain very subtle color variations and fine detail. Color changes, contrast adjustments, and other manipulations of a RAW image yield significantly fewer digital artifacts than the same changes made to a comparable JPEG file.
One of the most popular formats for streaming content on the Internet, RealMedia includes the RealAudio codec for sound clips and RealVideo codec for movies. RealAudio and RealVideo files are often given the common RealMedia ".RM" file extension. RealMedia is a container format that's often heavily compressed for streaming over dial-up Internet connections. RealMedia variable bitrate (RMVB) has been developed for VBR streaming files.
In order to convert an analog signal to a digital file, a series of digital "snapshots," or samples of the signal are made (see:PCM) Collectively, these samples represent the digital version of the anlog sound. The higher the sampling rate, the more accurate the sound reproduction. Sampling rate is usually given in hertz (Hz). CDs use a sampling rate of 44.1 kHz, which represents 44,100 samplers per second. Blu-ray discs can have sampling rates of up to 96 kHz, while SACDs (Super Audio CDs) have a sampling rate of 352 kHz, or 352,000 samples per second.
SDII (Sound Designer II)
An audio format for Macintosh operating systems which is often employed by pro-quality sound editing software applications. SDII files, like AIFF and WAV files, are capable of storing uncompressed CD-quality audio.
Secure Digital Music Initiative (SDMI)
The Secure Digital Music Initiative was established to standardize digital music file specifications throughout the industry. The primary purpose was to create a uniform copyright protection protocol that would work with a variety of digital players, software programs, and download sites. SDMI-compliant devices and files have special coding to recognize and comply with the requirements imposed on copyright-protected materials.
Shorten is a lossless form of compression for digital audio. An SHN file is only about half the size of its original WAV or AIFF source. Unlike lossy audio codecs (such as MP3, WMA, etc.), SHN is capable of reproducing the original audio signal in its entirety, without removing frequencies. Because of this, SHN offers significantly better sound quality than MP3. However, since SHN files are significantly larger than MP3 files, this format isn't nearly as convenient when it comes to storage space or download time.
TIFF (Tag Image File Format)
TIFF is a flexible container format for digital still images, commonly used in desktop publishing. TIFF images can incorporate various forms of compression (like JPEG), or can be uncompressed. Some digital cameras offer a special TIFF mode for capturing uncompressed photos; however, these files require many times more storage space than JPEGs, and can quickly fill up your camera's available memory.
Variable Bitrate (VBR)
Most 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 portions in the original source, and fewer bits to the simpler portions.
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).
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. WAV is a container format.
WMA (Windows Media Audio)
Developed by Microsoft, Windows Media Audio is one of today's most pervasive Internet audio formats. Though not as popular as MP3, proponents of lossy WMA claim that it can outperform MP3 in the area of sound quality, particularly with files encoded at lower bitrates such as 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 Vista, Microsoft's current flagship operating system software, contains native support for WMA encoding, enabling users to create their own WMA music files.
WMV (Windows Media Video)
Microsoft's proprietary lossy compression format for motion video. Windows Media Video is used for both streaming and downloading content via the Internet. Microsoft's Windows Media Player, an application bundled with Windows Vista operating systems, lets you play back and manage a range of audio and video file types, including WMA and WMV.
Xvid is an open-source lossy video codec based on MPEG-4. It was developed in response to DivX, and received its name from the backwards spelling of DivX. Xvid compresses a great deal of video content into relatively small files, and retains a reasonably good video resolution. It can be used with several different operating systems, and is a popular choice for transferring video over the Internet.