Glossary
This page contains definitions of common terms relating to video technology.
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ACELP
Industry standard audio compression technology developed by VoiceAge Corporation.
ADSL
Asymmetric Digital Subscriber Line
Consumer level broadband Internet connection technology. Download speeds up to 2Mbs and upload speeds up to 256kbps.
AIFF
Audio Interchange File Format
An audio file format standard used for storing sound data. The format was co-developed by Apple Computer in 1988 and is most commonly used on Apple Macintosh computer systems.
ANSI
American National Standards Institute
Has conducted detailed analysis of videoconferencing signal quality.
Artefacts
A term used to describe the distortions added to the original signal during the coding and decoding processes.
ASF
Advanced Streaming Format
Microsoft streaming multimedia file format.
ASX
Files with the '.asx' extension are Advanced Stream Redirectors used to redirect users to streaming media content. These are simple text metafiles containing media and server information.
AVI
Audio Video Interleave
A container format used by Microsoft's Video for Windows multimedia framework.
Asynchronous
A video signal that is not synchronised to the local reference (or camera) signal.
ATM
Asynchronous Transfer Mode
The international standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media, such as E3, SONET, and T3.
Bandwidth
The capacity of a communication link (normally measured in bit/s).
Bit rate
Measured in Kilobits per second (kbit/s) or Megabits per second (mbit/s), this determines the quality of video or audio reception. A higher bit rate means better quality, typically demonstrated as near-TV quality video and near-CD quality audio. Similarly, a low bit rate can produce poor quality files with poor image quality and 'telephone'-like sounding audio.
C
Used to denote the chrominance (colour) portion of a video signal.
Capture card
The adaptor board (PCI/AGP) used to input audio/video onto the encoding computer's hard disk.
CCD
Charge Coupled Device
Used in television cameras as a photo-sensitive device to convert light into an electrical signal.
Channel (in H.323 transmission)
Communication path wide enough to permit a single RF transmission. Multiple channels can be multiplexed over a single cable in certain environments.
CIF
Common Intermediate Format
A format commonly used in videoconferencing systems, used to standardise the horizontal and vertical resolutions of the colour components of an image.
Client (in streaming)
Device, typically a computer, used to play streams from a server.
CODEC
COder DECoder unit
The equipment that provides the compression and signal processing to enable high bandwidth sound and vision signals to be transmitted and received over limited bandwidth transmission paths.
Composite Video
A method of transmitting video information where the luminance, chrominance and synchronisation components of a television signal are combined into a single signal. Composite video is transferred between video devices using a single interconnect cable with an RCA connector. (Also see Video Sources).
Congestion
Increased retry rates, latency and failures observed when network traffic exceeds capacity.
Congestion Avoidance
Mechanism by which an ATM network controls traffic entering the network to minimise delays. In order to use resources most efficiently, lower-priority traffic is discarded at the edge of the network if conditions indicate that it cannot be delivered.
Congestion Collapse
Condition in which the retransmission of frames in an ATM network results in little or no traffic successfully arriving at the destination. Congestion collapse frequently occurs in ATM networks composed of switches that do not have adequate and effective buffering mechanisms complemented by intelligent packet discard or ABR congestion feedback mechanisms.
CORBA
Common Object Resource Broker Architecture
A Microsoft convention to facilitate the interworking of certain software objects.
DA
Distribution Amplifier
Amplifiers that split a single input to produce several isolated outputs identical to the input, enabling signals to be routed to many destinations without signal degradation.
De-interlace
De-interlacing provides a way around the production of unwanted artefacts (see above) that may result from, for example, changing the frame rate or resizing an image. Common methods of de-interlacing are Duplication and Interpolation (see below).
DNS
Domain Name Server
A device holding a directory of IP addresses and able to recognise the IP addresses of another PC logging in to the server; the system used on the Internet for translating names of network nodes into addresses.
Duplication
A de-interlacing method (see above). By duplicating the lines from one field in an image to create the other, we end up with an image, which can appear a lot less "torn", but with a lower effective vertical resolution.
DV
Digital Video
See Video sources. Process of converting audio/visual source material into format compatible with streaming technologies.
DVD
Digital Versatile Disk or Digital Video Disk.
Echo Cancellation
The CODEC delays the vision signal by approximately 200 milliseconds. To maintain sound/vision coincidence the audio signals are delayed by a similar amount. This time delay produces unacceptable echo into the conference. Echo cancellation is introduced electronically to reduce this echo to a workable level. The conference environment also influences the amount of echo, so echo cancellers need to adjust to the acoustics of the conference room in use: this process is termed "training".
Encode
Ethernet
Baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet networks use CSMA/CD and run over a variety of cable types at 10Mbit/s. Ethernet is similar to the IEEE 802.3 series of standards. Fast Ethernet is a variant running at 100Mbit/s.
FIFO
First-in, first-out
Refers to a buffering scheme where the first byte of data entering the buffer is the first byte retrieved by the CPU. In telephony, FIFO refers to a queuing scheme where the first calls received are the first calls processed.
Firewall
Router or access server, or several routers or access servers, designated as a buffer between any connected public networks and a private network. A firewall router uses access lists and other methods to ensure the security of the private network.
Frame/Picture Store
A means of storing electronically one complete television picture, i.e. one frame of information.
FTP
File Transfer Protocol
Application protocol, part of the TCP/IP protocol stack, used for transferring files between network nodes.
Gatekeeper
The component of an H.323 conferencing system that provides address translation and control access to the LAN for H.323 terminals and gateways. The gatekeeper can provide other services to the H.323 terminals and gateways, such as bandwidth management and locating gateways. A gatekeeper maintains a registry of devices in the multimedia network. The devices register with the gatekeeper at start-up and request admission to a call from the gatekeeper.
Gateway
In the IP community, an older term referring to a routing device. Today, the term router is used to describe nodes that perform this function, and gateway refers to a special-purpose device that performs an application layer conversion of information from one protocol stack to another.
GSTN
General Switched Telephone Network
G.711
ITU.T Standard for encoding audio signals for voice grade transmission (300-3400 Hz) encoded at data rates of 56 or 64 kbit/s. The main encoding algorithm for digital telephony.
G.723.1
ITU.T standard for the narrow-band encoding of audio signals with data rates of 5.3 kbit/s or 6.4 kbit/s.
GUI
Graphical User Interface
Program control system – composed of ‘windows’, check boxes, toolbars, drop down menus, etc. Typically employed in Microsoft Windows/Apple Macintosh operating systems as opposed to traditional command line interfaces employed in the Unix operating system.
H.320
The umbrella ITU-T standard for narrow band videoconferencing interoperability over ISDN networks.
H.323
The umbrella ITU-T standard for narrow band videoconferencing interoperability over IP networks.
H.324
The umbrella ITU-T standard for narrow-band video encoding and compression.
H.261
An ITU-T standard for video coding (or compressing the video signal).
H.263
An ITU-T standard for video coding, specifically designed for operating at low data rates, i.e. 64 to 128 kbit/s.
HDTV
High-Definition Television System
Generic term used to describe the next generation of broadcast television systems.
Hi-8
A modified form of the Sony 8mm video recording format that produces higher quality by splitting the video signal into black and white (Y) and colour (C) information, i.e. it records/replays S-video.
ICM
Intelligent Call Management
The ability of a CODEC to detect a fault in one ISDN line (during an ISDN 6 connection) and continue the conference at a lower data rate.
IDE
Integrated Drive Electronics
A standard in the PC industry for economical interfaces to disk drives, in which the drive-specific elements are physically integrated with the drive mechanics. SCSI interfaces can give better performance.
IEE
Institution of Electrical Engineers
IEEE
Institute of Electrical and Electronics Engineers
IEEE 1394
Technical name for the high speed PC connectivity technology used to connect consumer electronics audio/video (A/V) appliances, storage peripherals, other PCs, and portable devices together. Known as 'FireWire' on the Macintosh.
IP
Internet Protocol
The basic protocol for the transmission of information in the Internet. IP specifies how data is fitted into packets and provides features for addressing, type-of-service specification, fragmentation and reassembly, and security. Defined in RFC 791.
Interpolation
Interpolation involves estimating the value of a second field in an image by working out mid-values from the lines above and below it from the first field. This leads to a smoother looking image.
IP Address
32-bit address assigned to systems in IP networks such as the Internet. IP4 addresses are normally written as 4 octets seperated by periods (dotted decimal format) -- for instance 128.86.1.20. Each address consists of a network number, an optional subnetwork number, and a host number. The network and subnetwork numbers together are used for routing, while the host number is used to address an individual host within the network or subnetwork. A subnet mask is used to extract network and subnetwork information from the IP address.
IP Datagram
Fundamental unit of information passed across the Internet. Contains source and destination addresses along with data and a number of fields that define such things as the length of the datagram, the header checksum, and flags to indicate whether the datagram can be (or was) fragmented.
ISDN
Integrated Services Digital Network
Communication protocol offered by telephone companies that permits telephone networks to carry data, voice, and other source traffic.
ISO
International Organisation for Standardization
Established 1947. Mission of ISO is to promote the development of standardization with a view to facilitating the international exchange of goods and services, and to developing cooperation in the spheres of intellectual, scientific, technological and economic activity.
ITU
International Telecommunications Union
An organisation established to set international telecommunications standards and to allocate frequencies for specific uses.
ITU-T
The telecommunications section of the International Telecommunications Union, dealing with videoconferencing and other standards.
JANET
The private network for the UK's education and research community.
JANET(UK)
The company which runs JANET.
Jitter
- The inter-packet delay variance; that is, the difference between inter-packet arrival and departure. Jitter is an important QoS metric for voice and video applications
- Analogue communication line distortion caused by the variation of a signal from its reference timing positions. Jitter can cause data loss, particularly at high speeds.
JVCS
JANET Videoconferencing Service
LAN
Local Area Network
High-speed, low-error data network covering a relatively small geographic area (up to a few thousand metres). LANs connect workstations, peripherals, terminals, and other devices in a single building or other geographically limited area. LAN standards specify cabling and signalling at the physical and data link layers of the OSI model. Ethernet, FDDI, and Token Ring are widely used LAN technologies.
Latency
The time delay introduced into a videoconference by the time taken to code and compress (and decode and decompress) the vision signals in the CODEC.
Logical channel
Non-dedicated, packet-switched communications path between two or more network nodes. Packet switching allows many logical channels to exist simultaneously on a single physical channel.
MPEG-1 Video
Audio and video compression format developed by the MPEG group in 1993. Video files in this format may have any of the following file extensions: .mpg, .mpeg, .mp1, .mp2, .mp3, .m1v, .m1a, .m2a, .mpa, or .mpv.
M3U
A plain text file format for storing multimedia playlists. Similar to '.asx' files.
MAN
Metropolitan Area or regional Network
Generally, a MAN spans a geographic area larger than a LAN, but smaller than a WAN. It is normally under single management and may be of higher capacity than WANs.
MCU
Multipoint Control Unit
A device which enables a multisite (i.e. more than two sites) videoconference to take place.
MIDI
Musical Instrument Digital Interface
A protocol that enables electronic musical instruments, computers, and other equipment to communicate.
Multicast
Network transmission technology. Multicasting sends a single copy of the data in a stream to those clients who request it. Multiple copies of data are not sent across the network, nor is data sent to clients who do not want it as is the case with Unicast.
MP3
MPEG-1 Audio Layer 3
Part of MPEG-1 video encoding format, developed by MPEG group as a means to store good quality audio in small files. Audio files in this format have the file extension .mp3.
MPEG
Motion Picture Experts Group
A working group in charge of the development of standards for coded representation of digital audio and video.
MVQ
Motion Vector Quantization
Video compression method designed for low bitrate transmissions.
N-ISDN
Narrowband ISDN
Communication standards developed by the ITU-T for baseband networks. Based on 64-kbit/s B channels and 16- or 64-kbit/s D channels.
NTSC
National Television Systems Committee
The United States' system for coding colour information onto the composite video signal.
Packet
A unit of data exchanged between end systems (host computers). Includes a header containing control information and (usually) user data. Packets most often are used to refer to network layer units of data. The terms datagram, frame, message, and segment also are used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.
PAL
Phase Alternate Line
The system used in the United Kingdom for coding colour information onto a composite video signal.
PCM
Pulse Code Modulation
Method of digitising audio.
PPM
Peak Programme Meter
Used to measure audio signal amplitude. The meter characteristics are weighted to produce a fast rise but a very slow fall of the needle movement while following sound signals. This characteristic makes the measurement of rapidly varying sounds (e.g. music) more accurate.
Point-to-Point
The simplest method of videoconferencing, where one site communicates with one other.
Proxy
In computer networks, this is a system that services the requests of its clients by forwarding requests to other servers.
PSTN
Public Switched Telephone Network
General term referring to the variety of telephone networks and services in place worldwide.
Protocol
An agreed-upon format for transmitting data between two devices.
PTZ
Pan Tilt Zoom
Proprietary PictureTel multipin connector.
QCIF
Quarter CIF
The minimum vision quality used in H.320 conferencing systems, with a resolution of 180 x 144 and a repetition rate of 15 or 7.5Hz.
QoS
Quality of Service
Measure of performance for a transmission system that reflects its transmission quality and service availability.
QuickTime
Apple Computer's cross-platform, videoconferencing, collaborative computing, and multimedia communications technology.
RAID
Redundant Array of Inexpensive Disks
Various schemes for achieving resilience and performance in disk subsystems by integrating many physical drives.
RAM (also DRAM, SDRAM)
[[Synchronous] Dynamic] Random Access Memory
Semiconductor memory which is faster than storage such as disks and CDs, but more expensive.
RAS
[In the context of H.323 conferencing] Registration, Admission, and Status Protocol
Protocol that is used between endpoints and the gatekeeper to perform management functions. RAS signalling function performs registration, admissions, bandwidth changes, status, and disengage procedures between the VoIP gateway and the gatekeeper.
Router
Network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information. Occasionally called a gateway (although this usage is becoming outdated).
RTCP
RTP Control Protocol
Protocol that monitors the QoS of an RTP connection and conveys information about the on-going session.
RTP
Real-Time Transport Protocol
Commonly used with IP networks. RTP is designed to provide end-to-end network transport functions for applications transmitting real-time data, such as audio, video, or simulation data, over multicast or unicast network services. RTP provides such services as payload type identification, sequence numbering, time stamping, and delivery monitoring to real-time applications.
RTSP
Real Time Streaming Protocol
Enables the controlled delivery of real-time data, such as audio and video. Sources of data can include both live data feeds, such live audio and video, and stored content, such as pre-recorded events. RTSP is designed to work with established protocols, such as RTP and HTTP.
SCSI
Small Computer Serial Interface
A series of interface standards for disk drives and other peripherals, usually offering better performance than the IDE interface standard in PCs but with more complexity and at higher cost.
SECAM
Sequential Colour with Memory
The French system for coding colour information onto the composite video signal.
SPL Meter
Sound Pressure Level Meter
Used to measure ambient sound levels: required for setting-up high quality echo cancellers.
SuperJANET
High speed telecommunications network linking academic and research sites in the UK.
SVGA
Super Video Graphic Array
The standard specification for a high resolution PC display (1024 x 768 pixels).
S-VHS
A modified form of the JVC VHS video record format that produces higher quality results by recording/playing back S-video (i.e. Y,C) signals.
S-video
The luminance and chrominance information of a colour television signal are transmitted as separate components to improve quality (see Composite video).
T.120
An ITU-T standard that describes data conferencing. H.323 provides for the capability to establish T.120 data sessions inside an existing H.323 session.
TCP
Transmission Control Protocol
Connection-oriented transport layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack.
TCP/IP
Transmission Control Protocol over IP
Specifies a connection-based service through the Internet so that streams of data can be delivered complete and in the right order; the establishment of such streams and other facilities for managing them. Defined in RFC 793.
TERENA
Trans-European Research and Education Networking Association
Promotes and develops high-quality international network infrastructures to support European research and education.
Training (of echo cancellers)
To operate effectively, echo cancellers need to monitor the echo received from a remote site within the local conference room and generate a correction signal to cancel the echo. This alignment procedure is termed 'training'. This process considers not only echo from the remote site but also the acoustic characteristics of the local conference room. If the local room has poor acoustics, e.g. a high ambient noise level or very long term echo, then the echo canceller may not be able to achieve a satisfactory alignment. The local site will then introduce annoying echo into any conference in which it participates.
Transport Layer
Layer 4 of the OSI reference model. This layer is responsible for reliable network communication between end nodes. The transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control. Corresponds to the transmission control layer of the SNA model.
UDP
User Datagram Protocol
Connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.
UKERNA
United Kingdom Education and Research Networking Association.
The previous trading name of JANET(UK).
UMatic SP
A proprietary video cassette recording format developed by the Sony Corporation.
Unicast
The bulk of the traffic on today's networks is Unicast. A separate copy of the data is sent from the source to each client that requests it.
Unstable (video signal)
A video signal is termed unstable when it emanates from an electro-mechanical replay device such as a video player and has not received electronic time base correction to render it stable.
USB
Universal Serial Bus
A PC interface. Sometimes used to directly connect a television camera for videoconferencing.
VBR
Variable Bit Rate (See Bit rate).
VBR files vary the amount of output data per time segment. A higher bitrate is allocated to the more complex segments of media files while less space is allocated to less complex segments.
VCD
Video Compact Disk
Stores video in MPEG-1 format, up to 74 minutes at VHS quality.
VCR
Video Cassette Recorder
Vectorscope
An instrument that displays colour vectors, and thus enables colour phase to be measured.
Vertical Interval Switching (VITS)
A method of switching within video switchers/mixers that arranges for the actual transition to occur in the time between consecutive television pictures so as to minimise disturbance to the images.
VGA
Video Graphic Array
The standard specification for a normal resolution PC display (640 x 480 pixels).
VHS
Video Home System
Half-inch magnetic videotape cartridge format developed by JVC for home use with the ability to record and play back analog video and audio signals.
Video Server
Term used generically to describe a computer system that is capable of holding volumes of moving image material in a digital format and delivering that material in real time to clients across a network connection.
Video Sources
Depending on the capture card (see above) in use, material for streaming may be captured from different video sources using the following common formats: Composite; DV – Digital Video, S-Video – Super Video.
VNC
Virtual Network Computing
A method of linking a PC over a LAN to a network device e.g. a CODEC using software rather than a hard wired connection to the device.
VPN
Virtual Private Network
Enables IP traffic to travel securely over a public TCP/IP network by encrypting all traffic from one network to another. A VPN uses 'tunnelling' to encrypt all information at the IP level.
VTAS
Video Technology Advisory Service
One of JANET(UK)'s Video Services.
VU(M)
Volume Unit Meter
Used to measure audio signal amplitude. For measuring constant signal levels (e.g. tone) it is adequate. For the measurement of audio signal levels that are rapidly changing (e.g. music) it is unsatisfactory (See PPM, above).
WAN
Wide-Area Network
Data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers.
Waveform Monitor
An instrument that displays a video waveform to enable amplitude measurements to be measured.
WAV
Waveform Audio
Microsoft and IBM audio file format standard for storing an audio bitstream on PCs. Audio files in this format have the file extension .wav.
WAX
Windows Audio Media Redirector
Plain text files used to describe multimedia files and their presentation.
WMA
Windows Media Audio
Proprietary audio data compression technology developed by Microsoft.
WWW
World Wide Web
The inter-linking of multimedia services i.e. text, pictures, moving pictures and sound through a standardised set of data formats which has enables a wealth of information to be sent and received over the internet.
Y
Used to denote luminance (brightness) of a video signal.
YC Luminance Chrominance
A synonym for S-video.