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 PlayStation3 and 4 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.
An audio format commonly used for posting sound
clips on the Internet. AU files can be played back on Windows, Macintosh, and
other operating systems.
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 and 4.
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
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 codec is a way of compressing and decompressing
digital files. Each codec uses a slightly different set of algorithms to
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
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.
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 bit-rates.
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.
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
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
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
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
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.