VIDEOCONFERENCING glossary  

352 x 288 is the resolution of the Common Interchange Format (CIF), the most frequently used resolution for motion video.

640 x 480 is the base resolution used for VGA on PC-compatible computers and on the Macintosh. 640x480 is usually used in computer displays to avoid the cost of higher resolution display. 640 x 480 is also frequently used for still images in videoconferencing.

1024 x 768 is the most common resolution used for desktop PCs and Macintoshes.

10BaseT is the most frequently used wiring scheme for Ethernet. The "10" indicates 10 million bits per second. The "T" indicates use of twisted-pair wires of the sort used for telephone circuits. 10BaseT uses two pairs between the computer interface and the "hub," one pair for transmitting and one for receiving.

100BaseT is the family of wiring schemes used for Fast (100 million bits per second) Ethernet.

Aliasing is the perception as noise of high-frequency sounds or visual components, due to insufficient sampling frequency. Proper filtering avoids aliasing.

Analog signals have voltage directly analogous to the strength of the corresponding physical signal, that is, the loudness of a sound or the brightness of a light source. The voltage of the signal alternates [between positive and negative) at the frequency of the sound or light source.

Aspect ratio is the ratio of the pixel width of the visible display to the height of the display area,

Audiographics is a means to augment telephones with graphics such as shared documents.

Basic rate ISDN (BRI) is the most common form of ISDN. BRI provides two B-channels with one 16,000 bit per second D-channel for signaling, using ordinary telephone wiring.

B-channel is a 64,000 bit per second digital telephone circuit. B stands for "bearer." A 56,000-bit channel is "restricted." A typical telephone line in urban areas is capable of transmitting a pair of B-channels.

BRI stands for "basic rate ISDN."

Cathode-ray tube (CRT) is the type of picture tube used in televisions and for computer displays.

CCITT is the former name of the ITU-T.

Chair control (see conducted conferences).

Chrominance refers to the color components (hue and saturation) of a luminance-based representation of color.

CIF stands for Common Interchange Format.

Circuit switching is what the telephone network is based on, which means that once a call is established, there are circuits (B-channels) dedicated to the call.

Codec is a term for coder/decoder.

Coder is a device for coding.

Coding is one of two terms, along with "compression," used almost inter-changeably in referring to processes for reducing the bit rate.

Composite video is the most common form of electrical signals used to transfer video between components. Luminance, hue, and saturation are multiplexed together on a single signal wire on the sending end and separated at the receiving end. Most inexpensive cameras, VCRs, and televisions use composite video. Composite video usually uses single-pin "phono" connectors, the same as the ones that are typically used for audio connections.

Compression is an alternate term for coding.

Conducted conferences allow one site to be designated as the conductor of the conference. Conducted conferences are sometimes called "chaired" or "chair controlled."

Continuous presence refers to the capability in a multipoint conference to make video from many of the sites continuously present on the screen.

CRT stands for cathode-ray tube.

D-channel is an ISDN circuit used for dialing and other signaling.

DCT stands for discrete cosine transform.

Digital signals are a representation of physical signals using numbers, usually called "samples," to represent amplitude or intensity. The range of the numbers in a sample is the major determiner of the signal-to-noise ratio.

Discrete cosine transform (DCT) is a key step in many video coding techniques.

Document sharing is a conferencing capability to allow the participants to view and manipulate the same document.

Document stand is a device with a built-in video camera for capturing images of paper documents and similar items.

E1 is a dedicated telephone circuit, similar to T1, but with a 1,928,000 bit per second bandwidth.

Echo cancellation is used in speakerphones and videoconferencing to eliminate echoes caused by microphones capturing sounds produced by corresponding microphones. Echo cancellation works by "remembering" what signal has been put through the speaker, and then subtracting it, appropriately attenuated and delayed, from the signal entering the microphone. The difficulty is in attenuating and delaying the correct amount. Echo cancellation requires much computation, and likely adds cost for processor capability to perform the cancellation algorithms.

Ethernet is the most widely used form of local-area network (LAN). In the original form, signals are broadcast on coaxial cables, analogous to radio transmission through the mythical "ether." 10BaseT wiring is now preferred over coaxial cable in most environments.

Filtering is the removal of undesired frequencies. Most commonly, filtering is used to remove high-frequency audio and video signals to avoid aliasing in sampling. From a digital perspective, the filtering has not reduced the amount of data, since there are the same number of samples, each with the same number of bits, as without filtering. The filtering, if done properly, results in output signals sounding or looking better.

Focal length is the distance from the optical center of a lens to the point where light rays converge (are in focus). Shorter focal lengths correspond to a wider field of view and vice versa, i.e., a "telephoto" lens has a longer focal length.

Frequency is the number of cycles per unit of time (of a sound or radio wave, for example).

Full duplex is simultaneous bidirectional transmission, as opposed to alternating bidirectional transmission (half duplex).

G.711 is the default audio representation used in H.320, providing roughly 3.5-KHz frequency response in a 64,000-bit communication channel.

G.722 is an optional audio representation used in H.320, providing roughly 7-KHz frequency response in a 48,000-, 56,000-, or 64,000-bit communication channel.

G.723 is an audio representation used in H.324, providing roughly 3.5KHz frequency response in a 6,300-bit communication channel.

G.728 is an optional audio representation used in H.320, providing roughly 3.5-KHz frequency response in a 16,000-bit communication channel.

Graphics tablet is a device for computer input of physical coordinates, commonly used in computer-aided design.

Grayscale refers to monochrome representation of light with multiple intensity values (more than on and off, corresponding to white and black).

H.221 "Frame Structure for a 64 to 1920 Kbit/s Channel in Audiovisual Teleservices" defines the usage of P B-channels to transmit multiplexed audio, video, other data and control signals.

H.261 "Video Codec for Audiovisual Services at P x 64 Kbit/s" has been known informally as "P x 64" because it defines video coding based on P 64,000-bit per second channels. (P is typically two or more.).

H.263 "Video Coding for Low Bit Rate Communication" is the coding method designed for H.324, using the techniques of H.261 plus significant enhancements.

H.320 "Narrowband Visual Telephone Systems and Terminal Equipment" is the summary ITU-T recommendation for standard videoconferencing using ISDN or similar telephone circuits.

H.323 "Visual Telephone Systems and Terminal Equipment for Local Area Networks which Provide a Non-Guaranteed Quality of Service" (ver. 2 - "Packet-based multimedia communication systems") is the summary ITU-T recommendation for standard videoconferencing with conventional local-area networks.

H.324 "Terminal for Low Bit Rate Multimedia Communication" is the summary ITU-T recommendation for standard videoconferencing using POTS.

Half duplex is bidirectional transmission in alternating (not simultaneous) directions.

Hue is the relative proportion of green and red in a luminance-based representation. The hue control on a television is often called "tint."

Internet is the proper name for the worldwide computer network that evolved from the 1970s ARPANET.

Interframe coding is coding among related video frames.

Intraframe coding is coding within a video frame.

Inverse multiplexor (IMUX) is a device for separating one higher bandwidth communication channel to appear as multiple B-channels.

IP stands for Internet Protocol, the network level protocol used in the Internet and other computer networks.

IPX/SPX stands for "Internet Packet eXchange/Sequenced Packet eX-change," the family of protocols originally used on NetWare networks. (Some NetWare networks use TCP/IP instead of IPX/SPX, and many NetWare networks use both families of protocols.)

Iris is the device behind a camera lens that controls the amount of light admitted, and thus controls brightness of images.

ISDN stands for "Integrated Services Digital Network." ISDN is a standardized approach to providing digital service from digital telephones.

ITU-T is the International Telecommunications Union—Telecommunication Standardization Sector, which establishes "recommendations" for standard protocols.

JPEG is the Joint Photographic Experts Group standard for coding of still images.

Local-area network (LAN) is a computer network, designed for a small geographic area, that is capable of higher speed connections than typical networks for wider geographical areas.

Luminance is the brightness intensity of a visual image. The luminance control on a television is often called the "picture."

MCU (see Multipoint control unit).

Modem (MOdulator/DEModulator) is a device used to transmit digital data across analog telephone circuits.

Motion estimation is the estimation of which pixels in a frame are different from those in the previous frame.

Motion Picture Experts Group (MPEG) is a standard form of coding used for stored video and television.

Multimedia is data that include multiple forms of natural media, typically including audio and video.

Multiplexing is the process of combining separate streams or channels into one logical stream of data.

Multipoint is the usual term for conferencing with more than two sites.

Multipoint control unit (MCU) is a device for implementing multipoint conferences.

Noise-gated microphone is one that can effectively turn itself off when the sounds entering the microphone are below a predetermined threshold (and thus likely to be "extraneous") and turn itself on (instantly) when louder sounds are present (when someone is speaking near the microphone).

NT1 (network terminator one) is a device for BRI that converts the two off-premises signal wires to four-wire connections suitable for an ISDN telephone. An NT1 is often designed to support more than one of these four-wire connections, for example, one for a telephone and one for a fax machine.

NTSC (National Television Standards Committee) is the standard for television broadcasting in North America.

Overlaying video (see video overlay).

Packet switching is the approach used in most computer networks for carrying different logical streams of data on the same shared physical network.

PAL (phase alternating lines) is the standard for television broadcasting in Europe and other countries.

Pixel ("picture element") is one of many small dots used to represent a picture.

Plain Old Telephone Service (POTS) is a common term for conventional analog telephony.

PRI stands for primary rate ISDN.

Primary rate ISDN (PRI) is a high-bandwidth form of ISDN. A PRI circuit has 23 B-channels and a 64,000-bit D-channel for signaling (1,536,000 bits per second) in the United States, and 30 B-channels in Europe and Asia.

Private branch exchange (PBX) is a telephone switchboard designed for private use (as opposed to a "central branch exchange" used in telephone company offices).

Program sharing is a conferencing mechanism allowing more than one person, each on different computers, to use the same copy of the same program, working with the same document, and seeing the same displays on screen.

QCIF (Quarter Common Interchange Format) is a variant on CIF with 176 x 144 resolution.

Resolution is a measure of detail in a digital visual image, measured either in number of pixels in the horizontal and vertical directions, or in pixels per unit of physical measure of the display device.

RGB is a representation of color using the intensities of the primary colors of light, red, green, and blue. Appropriate mixtures of these primary colors can be used to represent any color. For example, black is represented by R(ed), G(reen), and B(lue) all at zero intensity, white is R, G, and B all at one (full intensity), yellow is B at zero, R and G at one, etc. This representation facilitates design of both hardware and software for manipulation of images and colors at the pixel level.

Router is a device in packet-switched networks for routing packets from one subnetwork to another.

Sample is a number representing the strength of a signal at a particular time, used with other samples in digital signal representation.

Saturation is the overall intensity of color in a luminance-based representation of a color image. On a television, the saturation control is often called "color."

Scaling is the process of converting resolution.

Scan converter is a device used to convert the computer video output to a television representation, or vice versa. This device must perform the appropriate scaling to convert between square pixels and rectangular pixels.

Student response terminals is a device, typically a keypad, for a student to communicate to an instructor.

S-Video is a form of electrical signal used to transfer video between components. S-Video keeps the luminance and chrominance component signals separate on separate signal wires, using multipin connectors. S-Video leads to higher image quality, compared to composite video, because the degradation that results from combining and separating the components is avoided.

Switched 56 is a 56,000-bit per second circuit, designed for dial-up customer use, which became available in the United States before ISDN.Though similar to BRI, Switched 56 is harder to attach to than BRI, because of the equipment needed at the customer premises. A typical videoconfer-ence requires two Switched 56 circuits versus one BRI to get comparable bandwidth. As BRI becomes readily available it is expected that use of Switched 56 will diminish.

T1 is a higher speed telephone circuit used in the United States for dedicated connections, capable of 1,544,000 bits/second. The electrical connection uses two pairs of telephone wires. T1 circuits are widely used for connecting PBXs to central telephone offices. T1 circuits are also used to connect distant LAN segments.

T.120 "User Data Transmission Using a Multi-Layer Protocol (MLP)" is a comprehensive ITU-T recommendation for data conferencing.

TCP/IP (Transmission Control Protocol/Internet Protocol) is the name usually used for the family of protocols used on the Internet and in many other networks.

Telecommuting is the practice of working at home but "commuting" to an office by computer networking and/or videoconferencing.

Telemedicine is the practice of medicine across a distance by computer networking and/or videoconferencing

Temporal filtering is the process of dropping frames and otherwise filtering excess detail across successive frames as part of video coding.

Terminal adapter (TA) is an ISDN device to manage dialing and answering the call (using the D-channel) and convert the bit-serial data on the B-channels to and from a form suitable for management by the conferencing software.

Touch panel is a touch-sensitive display device for controlling equipment.

VGA stands for "video graphics array." VGA first appeared on the IBM PS/2 in 1987 and subsequently became the de facto standard for graphics subsystems in PCs. The default resolution for VGA is 640x480. Higher resolutions of 800 by 600, 1,024 x 768, and 1,280 x 1,024 are known as "super" VGA.

Video follows voice is the concept of a videoconferencing system that automatically points the camera at the person speaking.

Video overlay is the combining of multiple images in mosaic fashion. In broadcast television, a common example is that of showing a forecaster in front of a weather map. In computer-based videoconferencing, a common example is that of showing a video image of people "in front of" a computer-generated image.

Videoconferncing is an exchange of digitized video images and audio between two or more separate sites. The transfer images may include video streams, still pictures of objects, information or data from graphics, files or applications. This allows conference participants to see, hear and collaborate, in real-time with their counterparts.

Virtual circuit is a logical connection across a packet-switched network that temporarily has the appearance of a dedicated physical circuit.

Visual artifacts is a term for discrepancies between a coded image and the original source.

Voice-activated switching refers to multipoint conferences where the sites generally display the video from the site with the strongest audio signal, with that site seeing the video from the previously selected site.

YIQ is the luminance-based representation of color used in NTSC.

YUV is the luminance-based representation of color used in PAL and in many video coding approaches.