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Compression Terminology


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Compression 101

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Compression A to Z 

AVI (Audio Video Interleaved) - Windows format for saving video with sound. Sometimes used for media files on CDs (1X to 4X CD-Rom speeds). 

Bandwidth - The width of the band over which frequencies can be transmitted. So what's a band? A band is a portion of the electromagnetic spectrum, defined by the lowest and highest frequencies in it. The bandwidth is the difference between the highest and lowest frequencies. The greater the bandwidth, the greater the carrying capacity. Maximum bandwidth is determined by your internet connection speed (Mbps).  The actual bandwidth  is a factor of the maximum bandwidth and the amount of network traffic at that moment. You can test the bandwidth available at your site at  Bandwidth Speed Test.com, the Internet connection speed test center. 

Cinepak - An ancient codec that produces large files. Still used for 24-bit video intended for CD-ROM. This codec attains higher compression ratios and faster playback speeds than the Apple Video codec. You can set the data rate for playback; picture quality drops more noticeably at data rates below 30 KBps.

Cleaner - A software package that provides a graphic-user interface for applying a variety of compression/decompression algorithms.  Cleaner is the industry standard for video compression.

Codec - A mathematical algorithm that COmpresses/
DECompresses digital video. These codecs are usually applied by a specialized software application such as Cleaner or QuickTime Pro.

Compression -The process that reduces the file size (number of bytes, KB, MB)  required to store or transmit digital video. Most compression methods either compare data between frames or eliminate  inter-frame or intra-frame redundancies. The compression may be done by software, hardware or a combination of the two. The data is decompressed during playback on the client machine. So the user must have access to the codec used through some plug-in or player on the client machine.

Dropped Frames  - In digital video, when the computer cannot keep pace with the displayed images, it drops frames in an attempt to catch up. This can occur when the hard drive does not have sufficient room to write the file in one continuous sweep (frames can be dropped between sectors).

Cross-platform - Media files that can be used or executed by computers  with different operating systems. For example, FLASH , a a vector animation program, can produce media files that can play on both   Macintosh and Windows computers. Windows Media Technology is not always cross-platform.

DV - (NTSC or PAL), A Lossless codec used by digital video cameras and on DVDs.  In DV, every frame is a key frame so the files are large but very high quality.

Fast Start. See Progressive Download.

Frames per second (FPS) -The number of picture images (frames) displayed per second in a media file. FPS gives digital video the illusion of motion. Full-motion video, such as analog video, with no dropped frames, is considered 30 FPS (NTSC standard).

Frame Rate - number of images per second displayed in a stream of video.

High Motion -  The amount of motion in a video is either considered high or low.  High motion can be produced by objects moving across the field,  as well as changes in camera angles, zooming in on an object, fading in or out or other transitions.

Interframe - Certain codecs register what information changes between frames.  If a several frames in the video have the same blue sky with no changes (clouds, birds, etc), then this information does not have to be represented in the data for every frame.  It is only represented in keyframes or when there is a change to the data.

.JPG, .JPEG (Joint Photographic Experts Group) - compressed, still-image graphics files. There are two major types of JPEG - progressive, where a fuzzy images emerges into 100% decompression/clarity, and baseline or standard, where a clear image is revealed, part-wise, top-down. Progressive, or interleaved, decompression looks better over networks.  JPEG A or B- codecs compress large picture files and simple jpg animation  

Key Frame- a complete video  frame that contains all the graphic information, not just the changes from the previous frame. If you reduce the number of key frames you will also have a smaller file but sometimes the video looks like the audio is no longer synchronized.  The number of key frames is set by picking a key frame every  # of frames, so reducing the number of key frames is performed by increasing the number.  For example, a key frame every 20 frames has fewer key frames than a key frame every 10 frames.

Lossless Compression - A compression method that makes every frame a keyframe.  The data within the frame is compressed.  For example, if an entire wall is the same color that data can be compressed into a smaller number of bytes. 

Lossy Compression - A compression method which, to save space, does NOT contain all the original video information. Lossy compression reduces data by describing only interframe changes.  The user must specify the number of frames per second and the number of frames between each keyframe.

MP3 (high-end, modem or standard) - An audio codec for music that produces small files for a broad range of instruments and voice.  It is also good for voices that have a slurred or musical quality.

MPEG (Motion Picture Experts Group) - creators of both hardware and software codecs for video files. MPEG 1 and 2 codecs still generate large file sizes, so MPEG is best suited for CD-ROM, NTSC or PAL, applications.  MPEG 4 is the new standard for inter-operable files although not all browser plug-ins are compatible yet.
  • Current MPEG Standards:
    1. MPEG1 - 1/4 broadcast quality translating to 352x240pixels. (consumer-quality video)
    2. MPEG2 - Full-motion quality translating to 704x480 pixels and 30  (broadcast-quality video).
  • There is no MPEG 3
  • MPEG 4 has finished beta testing. WOW!  Small file sizes, brillant color and high motion. Can it get better than this?

Pixels- The number of dots, or points of color that are the unit of measurement for video.  For example, a video clip can be 240 x 180, 360 x 240, 800 x 600, etc.  The most common ratio for video clips is 4:3 (width to height).

Progressive Download - Also called Fast Start.  A file configuration that allows the client machine to start playing the file before it is fully downloaded from the server.  Files are configured for Progressive Download during the compression process when using Cleaner. 

QuickTime - A file format commonly used in video.  QuickTime was developed by Apple by is truly cross-platform. QuickTime files can be served by a Real Time Streaming Protocol server or a web server that has been coded for Fast Start (also called Progressive Download).

QuickTime Pro - A software program available from Apple that provides a graphic user interface for video, audio and animation compression as well as minor video and audio editing.  QT Pro is cross-platform.

Real Time Streaming Protocol - A server format that provides files in real-time.  This format discourages downloading files but disruptions in the web cause lost data. 

RealMedia (audio or video) - A file format commonly used in video and audio.  Based on the QuickTime architecture and  truly cross-platform. RealMedia files can be served by a Real Time Streaming Protocol server or a web server that has been coded for Fast Start (also called Progressive Download).  RealMedia is a proprietary format (not open source like QuickTime) so files in this format can not be converted to other file types.

Sample Rate- The frequency at which audio is sampled for compression.

Sorenson Video - A video codec  that produces high quality files for either CD-ROM or web video. Similar to the Cinepak codec, Sorenson Video can produce high quality at data rates under 200 KBps. Although this codec is capable of better picture 
quality and  creates smaller files than Cinepak, it requires more compression time.

Windows Media (audio or video) - A file format commonly used in video and audio.  Developed by Microsoft and finally cross-platform.  Many streaming radio stations on the web use Windows Media format (such as National Public Radio).  Files can be served by a Real Time Streaming Protocol server or a web server that has been coded for Fast Start (also called Progressive Download).  Windows Media is a proprietary format (not open source like QuickTime) so files in this format can not be converted to other file types.

 

 
   

 

This page last updated: 04/25/2003

http://www.uri.edu/artsci/langlab/documentation/terminology.html

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Page last revised on Friday, April 25, 2003.