Format

File Extension

Definition

M P E G

.mpg

Short for Moving Picture Experts Group, and pronounced m-peg, a working group of ISO. The term also refers to the family of digital video compression standards and file formats developed by the group. MPEG generally produces better-quality video than competing formats, such as Video for Windows, Indeo and QuickTime. MPEG files can be decoded by special hardware or by software.

MPEG achieves high compression rate by storing only the changes from one frame to another, instead of each entire frame. The video information is then encoded using a technique called DCT. MPEG uses a type of lossy compression, since some data is removed. But the diminishment of data is generally imperceptible to the human eye.

There are three major MPEG standards: MPEG-1, MPEG-2 and MPEG-4.

The most common implementations of the MPEG-1 standard provide a video resolution of 352-by-240 at 30 frames per second (fps). This produces video quality slightly below the quality of conventional VCR videos.

MPEG-2 offers resolutions of 720x480 and 1280x720 at 60 fps, with full CD-quality audio. This is sufficient for all the major TV standards, including NTSC, and even HDTV. MPEG-2 is used by DVD-ROMs. MPEG-2 can compress a 2 hour video into a few gigabytes. While decompressing an MPEG-2 data stream requires only modest computing power, encoding video in MPEG-2 format requires significantly more processing power.

MPEG-4 is a graphics and video compression algorithm standard that is based on MPEG-1 and MPEG-2 and Apple QuickTime technology. Wavelet-based MPEG-4 files are smaller than JPEG or QuickTime files, so they are designed to transmit video and images over a narrower bandwidth and can mix video with text, graphics and 2-D and 3-D animation layers. MPEG-4 was standardized in October 1998 in the ISO/IEC document 14496.

AVI

.avi

Short for Audio Video Interleave, the file format for Microsoft's Video for Windows standard.

Quick Time

.mov

QuickTime is a multimedia framework developed by Apple Inc., capable of handling various formats of digital video, media clips, sound, text, animation, music, and several types of interactive panoramic images.

The most recent version is 7.1.5 and is available for Mac OS X and Microsoft Windows.

ASF

.asf

Advanced Systems Format (formerly Advanced Streaming Format) is Microsoft's proprietary digital audio/digital video container format, especially meant for streaming media. ASF is part of the Windows Media framework.

The format does not specify how (i.e. with which codec) the video or audio should be encoded; it just specifies the structure of the video/audio stream. This is similar to the function performed by the QuickTime, AVI, or Ogg container formats. One of the objectives of ASF was to support playback from digital media servers, HTTP servers, and local storage devices.

ASF is based on serialized objects which are essentially byte sequences identified by a GUID marker.

The most common filetypes contained within an ASF file are Windows Media Audio (WMA) and Windows Media Video (WMV). Note that the file extension abbreviations are similar in name to the codecs of the same name but are different things.

ASF files can also contain objects representing metadata, such as the artist, title, album and genre for an audio track, or the director of a video track, much like the ID3 tags of MP3 files.

Files containing only WMA audio can be named using a .wma extension, and files of only audio and video content may have the extension .wmv. Both may use the .asf extension if desired.

Certain error-correcting techniques related to ASF are patented in the United States (United States Patent 6,041,345 Levi, et al. March 21, 2000) by Microsoft. Although the format is publicly documented by Microsoft, its license limits implementations to closed-source development projects only. Apple's iTunes software (for Windows) now has the capability to convert WMA files to any iTunes-supported format.[1]

The ASF container provides the framework for digital rights management in Windows Media Audio and Windows Media Video. An analysis of an older scheme used in WMA reveals that it is using a combination of elliptic curve cryptography key exchange, DES block cipher, a custom block cipher, RC4 stream cipher and the SHA-1 hashing function.

ASF files have MIME type application/vnd.ms-asf or video/x-ms-asf. (Advanced Stream Redirector (ASX) files also have MIME type video/x-ms-asf.)

The ASF container is usually streamed on the net through the RTSP protocol.

WMV

.wmv

WMV (*.wmv) files use Microsoft's Advanced Systems Format (ASF) container format (digital). These files can be played by players such as MPlayer or Windows Media Player, the latter being only available for Microsoft Windows and Macintosh systems. Many third-party players exist for various platforms such as Linux that use the FFmpeg implementation of the WMV codecs.

The extension .wmv typically describes ASF files that use Windows Media Video codecs. The audio codec used in conjunction with WM Video is typically some form of Windows Media Audio, or in rarer cases the somewhat deprecated Sipro ACELP.net audio codec. Microsoft recommends that ASF files containing non-Windows Media codecs use the more generic .asf file extension.

VRML

(Virtual Reality Modeling Language) A 3D graphics language used on the Web. After downloading a VRML page, its contents can be viewed, rotated and manipulated. Simulated rooms can be "walked into." The VRML viewer is launched from within the Web browser.

The first VRML viewer was WebSpace from SGI, whose Open Inventor graphics library was the basis for developing VRML. Cosmo Player, WorldView and Cortona are other popular Windows viewers, and SimVRML and Virtus Voyager are well known Mac viewers.