Video Glossary

ASF - Active Streaming Format.  A Microsoft file format for digital video playback over the Internet, or on a stand-alone computer.  Kind of a wrapper around any of a number of compression types, including MPEG.  Part of Netshow, a proprietary streaming media solution from Microsoft.  Biggest competitor is Real Networks.  While this 'wrapper' support many standard formats, ASF files are themselves proprietary example, a regular Ethernet line has a bandwidth of 10 Mbps (10 million bits per second)

CIF - A video format that supports both NTSC and PAL signals.  CIF is part of the ITU H.261 videoconferencing standard.  It specifies a data rate of 30 frames per second (fps), with each frame containing 288 lines and 352 pixels per line.

G.7xx - A family of ITU standards for audio compression.

Gatekeeper - In the H.323 world, the gatekeeper provides several important functions.  First, it controls access to the network, allowing or denying calls and controlling the bandwidth of a call.  Second, it helps with address resolution, making possible email type names for end users, and converting those into the appropriate network addresses.  Also handles call tracking and billing, and call signaling.

Gateway - Gateways provide a link between the H.323 world and other video conferencing systems.  A common example would be a gateway to a H.320 (ISDN) video conferencing system.

H.261 - ITU standard for video coding for videoconferencing.  H.261 is a discrete cosine transform (DCT) based algorithm for video in the 64kb/s to 2mb/s range.  All H.323 compliant video conferencing systems are required to support this codec.

H.263 - ITU standard for video coding within videoconferencing.  H.263 offers better compression than H.261, particularly in the low bit-rate range used by modems.

H.320 - ITU standard for videoconferencing over ISDN and fractional T1 lines.  

H.323 - ITU standard for videoconferencing over networks that do not guarantee bandwidth, such as the Internet.  H.323 is the standard that this cookbook is recommending that most users in the education community should be using.  For more detailed information on this and the other ITU standards see the bibliography of this document.

H.324 - ITU standard for video conferencing over standard phone lines.

 IGMP - Internet Group Management Protocol.  This protocol is used in multicasting.

IP Multicast - A system for sending IP transmissions out only one time, but allowing for multiple users to receive it.  This would reduce the bandwidth required for audio and video broadcasting over the Internet, but it is not widely used yet.

Lossless compression - Refers to data compression techniques in which no data is lost.

Lossy compression - Refers to data compression techniques in which some amount of data is lost.  Lossy compression technologies attempt to eliminate redundant or unnecessary information.  Most video compression technologies, such as MPEG, use a lossy technique.

MPEG - Moving Picture Experts Group is a series of ISO standards for digital video and audio, designed for different uses and data rates.

u       MPEG-1 - The initial MPEG standard, designed to encode full motion video so it could be played back off of a CD (150 kb/s).  The bit rate of a standard MPEG1 is 1.5Mbps.  MPEG-1 has a frame size of 352x240 pixels, which gives a picture quality slightly better than VHS videotape.

u       MPEG-2 was a follow-on standard supporting higher data rates, and thus higher quality.  MPEG-2 is the standard used in DVD video players, most digital satellite systems in North America, and in the new North American Digital TV system.

u       MPEG-3 was abandoned, as its planned functionality was included in MPEG-2.

u       MPEG-4 delivers video at comparable quality to MPEG1 at a much lower bit rate.  MPEG-4 also supports a wide variety of elements that can be transmitted separately and combined to form the video frame, such as a talking head in one stream and the background in another.

Multipoint Conferencing Server (MCS) (also MCU) - A hardware or software H.323 device that allows multiple video conferencing (or audio or data) users to connect together.  Without an MCS typically only point-to-point conferences can take place.

Streaming Media - Sending video or audio over a network as needed, such as Real Audio/Video or Microsoft NetShow, instead of forcing the user to download the entire file before viewing it.

T.120 - T.120 is an ITU-T standard (International Telecommunications Union) for document conferencing.  Document conferencing allows two or more people to concurrently view and edit a document across a network.

Transcoding - Converting a data stream from one format to another, such as MPEG 1 to H.263, or an H.320 videoconferencing session to H.323.

Unicast - Sending each user a personal copy of a video (or other data) stream.  As opposed to Multicast, where one copy is sent and whoever wants it listens to that copy

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