Cardioid microphone - A directional microphone with a heart-shaped pickup pattern used for specialized audio applications.

Carrier - A transmitted electromagnetic pulse or wave at a steady base frequency of alternation on which information can be imposed by increasing signal strength, varying the base frequency, varying the wave phase, or other means. 

CCD - Charge-coupled device. Camera technology that captures video signals.

CCIR-601 - Studio standard format for video. 720 x 480 pixels at 30 frames per second (NTSC ) or 720 x 576 at 25 fps ( PAL .)

CCITT - Consultative Committee on International Telephony and Telegraphy. A body of the International Telegraph Union ( ITU ) which prepares recommendations, commonly refered to as international standards, to resolve technical telegraph and telephone problems.

CELP - Code Excited Linear Prediction. Compression method used in G.728.

Cell -  (53-bytes) Unit of data used in ATM technology for carrying information.

Central Office ( CO ) - In telephony, the phone company switching facility or center, usually a Class 5 end office, at which subscriber's local loop terminate. Handles a specific geographic area, identified by the first three digits of the local telephone number.

Centrex - Central Office based service provided to business customers that allows direct outward dialing from the organization as well as direct inward dialing to phone extensions. Provides most, if not all, PBX type features.

Channel - A separate path through which signals can flow. 

Channel bank - Equipment in a telephone central office that performs multiplexing of lower speed digital channels into a higher speed composite channel. The channel bank also detects and transmits signaling information for each channel, transmitting framing information so that time slots allocated to each channel can be identified by the receiver.

Chroma ( Chrominance ) - Color information in a video image.

CIF - Common Intermediate Format. 352x288 pixels, 30fps clock. H.261 and H.263 optional format.

Clear channel - A channel in which all the 64 kbps are used for transmission. To achieve this bit robbing signals must be eliminated.

Clock Speed - The number of pulses per second by an oscillator that sets the tempo for the processor. Clock speed is usually measured in MHz (megahertz, or millions of pulses per second).

Coaxial cable - A coaxial cable can carry great quantities of information compared to twisted pair copper wire and is typically used by cable providers to carry television signals into houses and schools.

Codec - Coder-decoder. Device to compress and decompress information ( video, audio, data, etc. )

Coder - A component in videoconferencing solutions that translates digital signals into video pictures.

Communications closet ( CC ) - The CC represents the physical connection where information is received via the wiring system to the communications outlets in individual rooms. Each educational facility must construct a space to house at least one CC per floor.

Communications equipment room ( CER ) - The CER is the entry point for communications into the building. It houses the head-end equipment of the school's communications system and is connected via cabling to the communications closets for distribution to the end user. At least one CER is recommended per facility.

Communications outlet ( CO ) - The CO's, which differ from power outlets, are generally designed to receive multi-cable signals and provide access directly to the user. CO's should be installed within six to eight feet of every workstation to preclude the use of extension cords and cables.

Compression - The method of taking a raw data and processing it so that it may be represented with less information ( or bits in the digital world. ) Compression falls into two categories: lossless- the original data may be completely recovered- and lossy - the representation of the original data contains errors.

Continuous Presence - The ability to see more than one far-end site at a time in a multipoint call.

Control Logic - The computer in a switching system (either time-division, space-division, or multistage) that monitors and controls switching to properly route calls. 

CPE - Customer premises equipment. A generic term for communications terminal gear owned by the customer, residing on customer premises.

CS-ACELP - Conjugate Structure algebraic code excited linear prediction. Compression method used in G.729.

CSN - Circuit Switched Network ( for example ISDN, POTS. )

CSU - Channel service unit. A component of CPE used to terminate a digital circuit, such as DDS or T1 at the customer site. Performs certain line-conditioning functions, ensures network compliance per FCC rules, and responds to loopback commands from central office. Also ensures proper ls density in transmitted bit stream and performs bipolar violation correction.

CTI - Computer-Telephony Integration relates to the implementation of traditional telephone - based audio (and sometimes video) services over a data network. CTI may be implemented over systems that guarantee bandwidth, such as ATM, or frame-based networks like Ethernet or frame relay.

D-channel - The ISDN channel that carries signaling information to control the call setup, teardown, or invocation of supplementary services. The D-channel may also be used to provide Packet Mode Data Service.

D/A - Digital to analog.

Data compression - The shrinking of digital information to achieve smaller file size. One would compress information to allow for faster upload/download times or to fit information on a certain size disk.

DCE - Data communications equipment. The portion of a data terminal that provides the interface to the network.

DCT - Discrete-cosine transform. Method used to encode video information in H.261, H.263, MPEG.

DDS - Dataphone digital service. AT&T private line service for transmitting data over a digital system. The digital transmission system transmits electrical signals directly, instead of translating the signals into tone of varied frequencies as with traditional analog transmission systems.  Digital techniques provide more efficient use of transmission facilities, resulting in lower error rates and costs than analog systems.

Decoder - A component in videoconferencing solutions that breaks down video input into digital signals.

Dedicated Line - (1) A communications circuit or channel provided for the exclusive use of a particular subscriber. Dedicated lines are used for computers when large amounts of data need to be moved between points. (2) A transmission circuit installed between two sites of a private network and ""open"", or available, at all times. Synonym private line leased line.

Delta Modulation - A method of sampling and converting analog signals to digital signals. It is based on encoding the direction of signal change, indicating the direction of signal change with a single bit. 

Demodulator - A videoconference receiver circuit which extracts or "demodulates" the "wanted" signals from the received carrier.

DES - Data Encryption Standard.

Desktop videoconferencing - Videoconferencing on a personal computer. Most appropriate for small groups or individuals. ferenci

Digital - An electronic signal coded in binary format as opposed to analog's continuously variable flow. All digital information is ultimately stored in 1/0 signals that computers process. ISDN is digital.

Digital loopback - Technique for testing the digital processing circuitry of a communications device. May be initiated locally or remotely via a telecommunications circuit. Device being tested will echo back a received test message after first decoding and then encoding it. The results are compared with the original message ( compare with analog loopback. )

Digital Switch - A means of supporting multiway conferencing using the signals in their digital format without converting them to analog. Digital switches permit multiple users with similar codecs to conference generally with voice-activated switching.

Distance Learning - The incorporation of video and audio technologies so that students can "attend" classes and training sessions that are being presented at a remote location. Distance learning systems are usually interactive and are becoming a highly-valuable tool in the delivery of training and education to widely-dispersed students or in instances where the instructor cannot travel to the student's site.

Document sharing - A feature supported by many videoconferencing systems that allows participants of a videoconference to view and edit the same computer document. 

Doubletalk - The act of the near and far ends of a call speaking at the same time. ( Good test for the effectiveness of an echo canceller. )

DPCM - Differential Pulse Code Modulation. Coding technique.

DS-3 - High speed network line, operates at 44.73 MBPs.

DSL - Digital Subscriber Line is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. 

DSP - Digital Signal Processor.

DSU - Data service unit. A device providing interface between a data terminal or other data communications device and a digital access line.

DTE - Data terminal equipment. The portion of a data terminal that interfaces to the end-user's equipment. The main difference between DCE and DTE is that pins 2 and 3 are reversed on the RS-232.

DTMF - Dual Tone Modulated Frequency. Touch-tone signals.

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