Glossary
3:2 Pulldown Process used in telecine to produce 60 Hz video from 24fps film.
AAU Audio access unit (see Access unit).
AC-3 Audio coding technique developed by Dolby used with ATSC (q.v.) and DVD (q.v.); aka Dolby Digital.
ACATS Advisory Committee on Advanced Television Service.
AES/EBU interface Standardized interface for transmitting digital audio information between two devices (see Channel status).
ATM Asynchronous transfer mode; a protocol for transport in broadband digital networks.
ATSC Advanced Television Systems Committee.
ATV Advanced Television; television transmission systems using modern techniques such as compression and error correction.
Access unit The coded data for a picture or block of sound and any stuffing which follows it.
Active line Part of television line on which visible part of image is portrayed. Distinguishes the line blanking area which is not visible.
Aliasing Generation of incorrect samples due to input frequencies exceeding one-half the sampling rate.
Anchor picture Picture used as basis for predictive coding.
Anti-aliasing filter Filter which restricts the frequency range of an analog signal to less than one half the sampling rate.
Aperture effect A loss of resolution caused by the image being scanned or sampled by a non-ideal system in which the scanning beam has finite area.
Artifact Undesirable erroneous effect visible on-screen.
Aspect ratio Ratio of image width to height; SDTV is 4:3, widescreen TV is 16:9. May also refer to spacing of pixels.
Azimuth recording Twisting alternate magnetic heads left and right to record adjacent tracks which can be read without crosstalk. Also called guardbandless recording.
B Video signal representing blue component of image.
BER Bit error rate (see Bit); rate at which incorrect bits are received.
BNC Bayonet Neill-Concelman; coaxial connector used for video signals.
Back porch Blanked area between sync pulse and beginning of video.
Bit Abbreviation for binary digit
Blanking Voltage corresponding to black on-screen. Below blanking = invisible.
Block matching Simple technique used to estimate motion between images in compression and standards conversion.
Bottles Colour ident signals inserted in vertical blanking to synchronize Dr,Db sequence in SECAM. No longer mandatory but still in use.
Bouquet Group of transport streams in which programs are identified by combination of network ID and PID. Part of DVB-SI.
Bowtie Test signal used to check relative timing of analog component video signals.
Breezeway Space between end of burst and active line.
Burst Signal used to synchronize local subcarrier oscillator in composite decoder.
Buzzword A specialist term which performs two functions. (a) To those who understand its meaning it makes technical conversations briefer. (b) To those who do not understand its meaning it is a way of preventing communication.
Byte Set of bits, generally eight.
CAT Conditional Access Table. Packets having PID (q.v.) code of 1 which contain information about the scrambling system (see ECM and EMM).
CCIR International Radio Consultative Committee, now known as ITU-RB (radio branch of International Telecommunications Union).
CCTV Closed circuit television; cameras connected to monitors by private cabling (as opposed to broadcast). Often used for security.
CIE Commission Internationale d’Eclairage; standards body for colorimetry. Branch of ISO (q.v.).
CIF Common Intermediate Format; 352 × 240 pixel format for 30fps videoconferencing.
CRC Cyclic redundancy check; error-detection technique.
Channel code Modulation technique which converts raw data into a signal which can be recorded, or transmitted by radio or cable.
Clamp Circuit which forces video signal to correct level during blanking.
Closed GOP Group of pictures in which the last pictures do not need data from the next GOP for bidirectional coding. Used to make a splice point in a bitstream.
Codeword Entity used in error correction which has constant testable characteristic.
Coefficient Number specifying the amplitude of a particular frequency in a transform or binary number used to control a multiplier.
Coercivity Measure of the erasure difficulty, hence replay energy, of a magnetic recording.
Colour framing A process which synchronizes the subcarrier and sync timing in composite VTRs to a reference. This allows edits without jumps or break-up on replay.
Colour under Recording technique which downconverts chroma signal to lower frequency to save tape.
Companding Abbreviation of compressing and expanding; means for increasing dynamic range.
Composite video (a) (old usage) Video signal carrying sync pulses as well as picture. (b) (modern usage) Video signal carrying subcarrier-based chroma.
Compression Also called bit rate reduction or data reduction. Process which allows pictures to be represented with fewer data. Picture quality may suffer if used to excess.
Concatenation Connecting more than one codec in tandem.
Concealment Method of making uncorrectable errors less visible, e.g. interpolation.
Constellation Pattern seen on test instrument when viewing QUAM (q.v.) signal.
Contouring Video artifact caused by insufficient sample wordlength.
Contribution quality Describes a signal which is going to be further post-produced and so must have high quality.
Convergence Causing all three electron beams in a shadow mask tube to fall in the same place.
Crosstalk Unwanted signal breaking through from adjacent wiring or recording track.
Crushing Some of contrast range is lost typically because of wrongly adjusted brightness control.
Curie temperature Temperature at which magnetic materials demagnetize.
Cutting list Data sheet containing edit frame numbers which is used by film laboratory to conform negative. Equivalent of EDL.
Cylinder In disks, set of tracks having the same radius.
Cylinder address Code sent to disk drive positioner to control radial head movement.
Db Colour difference signal used in SECAM; Db = 1.505(B–Y)
Dr Colour difference signal used in SECAM; Dr = –1.902(R Y)
DCT Discrete cosine transform; mathematical technique which analyses blocks of an image to determine which spatial frequencies are present. Used in compression.
DSB Direct satellite broadcasting; system where a consumer antenna can receive a broadcast.
DSP Digital signal processor; computer optimized for waveform processing.
DTS Decoding time stamp; part of PES header indicating when an access unit is to be decoded.
DVB Digital video broadcasting; the broadcasting of television programs using digital modulation of a radio frequency carrier. This can be terrestrial or from a satellite.
DVB-IRD See IRD.
DVB-SI DVB service information; information carried in a DVB multiplex describing the contents of different multiplexes. Includes NIT, SDT, EIT, TDT, BAT, RST, ST (q.v.).
DVC Digital video cassette.
DVD Digital video disk, aka digital versatile disk; optical disk for consumer video or data applications.
Data integrity General term for any action or strategy which minimizes the proportion of data bits in a system which are corrupted.
Decimation Reduction of sampling rate by omitting samples.
Dielectric Insulating material used between the conductors in transmission lines.
Differential coding Sending a value not in an absolute sense but as the difference between the current and previous values.
Digitizing In non-linear editors the process of transferring input images and audio onto the system disks. May include a compression step.
Distribution (a) In DVTRs, sharing data between two or more heads in a DVTR allows concealment if one head clogs. (b) In statistics, the shape of the probability curve.
Dither Random signal added to analog input to linearize subsequent quantizing step.
Downconvertor Unit which allows HDTV signals to be converted to standard definition.
Drift Error build-up when bit rate is further reduced by additional truncation of coefficients.
EBU European Broadcasting Union.
ECM Entitlement control message; conditional access information specifying control words or other stream-specific scrambling parameters.
EDH Error detection and handling; a data integrity option for SDI digital interface (q.v.).
EDL Edit decision list; used to control editing process with timecode.
EDTV Extended definition television; television systems which enhance the quality of existing TV standards.
EFM Eight to fourteen modulation; the channel code (q.v.) used in D-3 and D-5 DVTRs.
EIT Event information table; part of DVB-SI.
EMC Electromagnetic compatibility; legislation controlling sensitivity of equipment to external interference and limiting radiation of interference.
EMM Entitlement management message; conditional access information specifying authorization level or services of specific decoders. An individual decoder or a group of decoders may be addressed.
ENG Electronic news gathering; term used to describe use of videorecording instead of film in news coverage.
EOB End of block; code sent when all remaining coefficients in a DCT block are zero.
EPG Electronic program guide; program guide delivered by data transfer rather than printed paper.
E–E Electronics to electronics; a mode in a VTR where the tape and heads are bypassed but the signal passes through everything else.
ESP Extended studio PAL; BBC-developed wide bandwidth composite signal compatible with composite digital VTRs.
Edit gap In a DVTR, a space left on the recorded track allowing data on one side of the space to be edited without corrupting data on the other side.
Elementary stream Raw output of a compressor carrying a single video or audio signal.
Embedded audio Digital audio signals multiplexed into ancillary data capacity of SDI (q.v.).
Entropy The unpredictable part of a signal which has to transmitted by a compression system if quality is not to be lost.
Entropy coding Coding system which achieves compression of signals having non-uniform statistics.
Equalizer Circuit needed to compensate for a reduction of certain frequencies in recording or transmission.
Error propagation When a small error in a critical bit results in protracted errors in a decoder.
Event A set of elementary streams, typically audio and video, having common clocks, start times and end times.
Eye pattern Characteristic pattern seen on oscilloscope when viewing channel-coded (q.v.) signals.
FEC Forward error correction; system in which redundancy is added to the message so that errors can be corrected dynamically at the receiver.
FM Frequency modulation; used in analog VTRs.
FSc Frequency of subcarrier.
Faraday effect Rotation of plane of polarization of light by magnetic field.
Ferrite Hard non-conductive magnetic material used for heads and transformers.
Film-for-video System in which material intended for television broadcast or videocassette release is shot on film.
Flash convertor High-speed ADC used for video conversion.
Flatbed A traditional film editing machine having a viewer and means to drive the film to and fro.
Flyback Retrace prior to a further scan.
Flywheel sync System used in TV sets to resist sync loss due to interference. Effectively a phase-locked loop.
Four-field sequence A repetition rate which occurs in NTSC due to the subcarrier frequency having a half-line offset. The subcarrier can only return to a given phase after an even number of lines and this requires two frames or four fields.
Fourier transform Frequency domain or spectral representation of a signal.
Front porch Blanked area after video but before H sync pulse.
Fukinuki hole Space theoretically clear in ideal three-dimensional PAL spectrum.
G Video signal representing green component of image.
Gs Video signal representing green component of image with synchronizing pulses added.
GOP Group of pictures; starts with an I picture and ends with last picture before next I picture.
Galois field Mathematical entity on which Reed–Solomon (q.v.) error correction is based.
Gamma Non-linear relationship between video signal voltage and screen brightness.
Gamut Allowable range of signal voltage.
Gigabyte Measure of data storage; equal to 1024 megabytes.
Guard band In older VTRs a space left between the tracks to reduce crosstalk.
H Horizontal.
H-coherent Having a frequency which is an integer multiple of H.
HDTV High-definition television; television systems which produce a better quality image than SDTV by use of e.g. progressive scanning.
Hamming distance Number of bits different between two words.
Helix angle In VTRs the mechanical inclination of the tape path.
Helper Additional signal which when optionally decoded in a more complex receiver enhances picture.
Horizontal editing Edit process which primarily involves the time axis. (e.g. cuts). Contrasts with vertical editing.
Hue The dominant wavelength in a colour (see Saturation).
Huffman coding Form of entropy coding (q.v.) using variable-length parameters.
I Signal used in NTSC chroma; I = -0.27(B–Y) + 0.74(R–Y).
IEC International Electrotechnical Commission; branch of ISO (q.v.).
IRD Integrated receiver decoder; combined RF receiver and MPEG decoder used to adapt TV set to digital transmissions, aka STB (set-top box).
IRE (unit) Institute of Radio Engineers unit which is 1 per cent of black to white voltage swing.
ISO International Standards Organization.
IT Information technology; computers, software, data storage and communications.
Illuminant Light source having standardized position on chromaticity diagram.
Insertion loss Reduction in signal level due to incorporating a device into a series circuit.
Inter-coding Compression which uses redundancy between successive pictures, aka temporal coding.
Interlaced scan The frame is scanned by two or more fields where not all lines are included in each scan.
Interleaving Reordering of data on medium to reduce effect of defects.
Interpolation Replacing missing sample with average of those either side.
Intra-coding Compression which works entirely within one picture, aka spatial coding.
JPEG (Joint Photographic Experts Group) A compression standard for still images or individual frames.
Jitter Statistical distribution of events on time axis which ideally are equally spaced.
Judder Artifact occurring when motion is incorrectly portrayed.
K-factor Figure of merit for linear distortions in a transmission system. Tested with pulse and bar signal.
Kell factor Degree by which a display approaches ideal resolution set by line spacing.
Kerr effect See Faraday effect.
Keycode Signal recorded on the edge of film which allows individual frames to be located according to the time at which they were shot (see Timecode).
Keying Electronic equivalent of matte in which part of one image is replaced with part of another.
Kilobyte Measure of data storage equal to 1024 bytes.
Latency Access delay in disk drive due to mechanical motion.
Level In MPEG (q.v.), the size of the input picture in use with a given profile (q.v.).
Lightning Screen display of all three colour difference signals allowing rapid assessment of correct adjustment.
Loop-through Connecting the same signal to several destinations in a daisy chain.
MAC Multiplexed analog components; an alternative to composite video for colour broadcast from satellites.
MPEG (Moving Picture Experts Group) A compression technique using inter-coding and intra-coding for moving pictures.
MTF Modulation transfer function; the ratio of output to input contrast index in an optical system.
Macroblock Screen area represented by several luminance and colour difference DCT blocks which are all steered by one motion vector.
Margining Checking performance of system by measuring the amount of deliberate degradation needed to cause a failure.
Masking In human hearing the reduced sensitivity to one sound in the presence of another.
Matrix (a) Circuit for converting between component and colour difference signals. (b) Switching element of a router.
Megabyte Measure of data storage; equal to 1024 kilobytes.
Mezzanine level System using lower compression factor to permit concatenation.
Moiré Artifact in composite analog VTRs caused by high-order sidebands folding into baseband.
Motion compensation Technique used to eliminate judder in standards convertor (see Phase correlation; Block matching; Motion vector).
Motion vector Parameter in a compression system or standards convertor which tells the decoder how to shift pixels from a previous picture so it more nearly resembles the current picture.
NIT Network information table; information in one transport stream which describes many transport streams.
NTSC Never Twice the Same Colour; a television system in which the colours are a function of where the hue control was left by an unskilled viewer.
Non-linear An editing system in which random access storage is used so that the time axis of access to the material can be non-linear.
Null packets Packets of ‘stuffing’ which carry no data but which are necessary to maintain a constant bit rate with a variable payload. Null packets always have a PID (q.v.) of 8191 (all 1s).
OIRT East European equivalent of EBU, merged with EBU in 1992.
Off-line System where low-quality images are used for decision-making purposes. Low quality is not seen by end viewer.
On-line System where the quality seen by the operator is the same as that seen by the end viewer.
Optic flow axis Axis passing through space-time relative to which part of a moving image appears stationary.
Orthogonal Signals or processes which are on independent axes. e.g. U and V in PAL chroma.
Overflow When a buffer memory consistently receives more data than is being removed or when a counter exceeds its maximum value.
Oversampling Temporary use of a higher than necessary sampling rate in convertors in order to simplify analog filters.
PAL Phase alternating line; composite video colour system.
PALPlus 16:9 version of PAL.
PAT Program Association Table. Data appearing in packets having PID (q.v.) code of zero which the MPEG decoder uses to determine which programs exist in a transport stream. PAT points to PMT (q.v.) which in turn points to the video, audio and data content of each program.
Pb Standard colour difference signal which is 0.56433(B–Y).
Pr Standard colour difference signal which is 0.71327(R–Y).
PCM Pulse code modulation; technical term for analog source waveform e.g. audio or video, expressed as periodic numerical samples. PCM is an uncompressed digital signal.
PCR Program clock reference; sample of encoder clock count sent in program header to synchronize decoder clock.
PCRI Interpolated program clock reference; PCR estimated from previous PCR to measure jitter.
PID Packet identifier; thirteen-bit code in transport packet header. PID 0 indicates packet contains PAT (q.v.). PID 1 indicates packet contains CAT (q.v.). PID 8191 (all 1s) indicates null (stuffing) packets. All packets belonging to the same elementary stream have the same PID.
PLUGE Picture line-up generator; a test signal for adjusting monitors.
PMT Program Map Tables; tables in PAT (q.v.) which point to video, audio and data content of a transport stream.
PSI Program Specific Information; information which keeps track of the different programs in an MPEG transport stream and the elementary streams in each program. PSI includes PAT, PMT, NIT, CAT, ECM and EMM.
PSI/SI General term for MPEG PSI and DVB-SI combined.
PTS Presentation time stamp; time at which a presentation unit is to be available to the viewer.
PU Presentation unit; one compressed picture or block of audio.
Pack A set of PES packets; the pack header contains a SCR code.
Packets Beware! the term is used in two contexts and these are not the same. In program streams, a packet contains one or more presentation units. In transport streams a packet is a small fixed-size data quantum.
Padding See stuffing.
Partial response Channel coding (q.v.) technique used in Digital Betacam.
Patch Tube face area illuminated by the flying spot in telecine.
Payload Content of a packet other than the header.
Pedestal In NTSC black level may be raised to 7.5 IRE units instead of zero to guarantee retrace is invisible.
Pel See Pixel.
Phase correlation Technique used for motion estimation in compression and standards conversion.
Phase-linear Describes a circuit which has constant delay at all frequencies.
Phase-locked loop An electronic circuit which extracts the average phase from a jittery signal in a manner analogous to a flywheel. See Reclocker.
Phosphor Substance which emits light when struck by electron beam.
Pixel Short for picture cell; a point sample of a picture. Also called a pel (see also Square pixel).
Power factor In electrical supplies, power factor measures the efficiency of transmission. If current is out of phase with voltage, the power factor is poor and transmission losses increase.
Preprocessing Noise reduction, downsampling, cut edit identification and 3:2 pulldown identification are all preprocessing steps needed before compression.
Primary colour Colour emitted by one phosphor in a colour display.
Product code Combination of two one-dimensional error-correcting codes in an array.
Profile A subset of the entire coding repertoire of MPEG.
Program stream Bitstream containing compressed video and audio and timing information.
Progressive scan Scanning proceeds from top to bottom of frame taking in every line in sequence.
Pseudo-random code Number sequence which is sufficiently random for practical purposes but which is repeatable.
Purity Degree to which colour monitor can produce a primary colour.
Q Signal used in NTSC chroma; Q = 0.41(B–Y) + 0.48(R–Y).
QCIF One-quarter resolution (176 × 144 pixels) common intermediate format (see CIF).
QSIF One-quarter resolution source input format. (see SIF).
Quadrature When two signals have 90° relative phase.
Quantizer A device which breaks an analog signal’s voltage range into even intervals and outputs the number of the interval in which the analog input lies.
Quincunx Sampling grid where pixels on alternate rows are shifted to create a pattern resembling the five of dice.
R Video signal representing red component of image.
RLC Run-length coding; coding scheme which counts number of similar bits instead of sending them individually.
RFI Radio frequency interference; interference which is radiated rather than conducted.
Random access Storage device like a disk where contents can be output in any order. Contrasts with serial access.
Randomizing See Scrambling.
Reclocker A combination of a slicer and a phase-locked loop which can remove noise and jitter from a digital signal.
Reconstruction Filtering process which converts a series of samples back to a continuous waveform.
Redundancy (a) In error correction, extra check bits appended to the wanted data. (b) In compression, that part of a signal which can be predicted and so need not be sent.
Reed–Solomon code Error-correction code which is popular because it is as powerful as theory allows.
Requantizing Shortening of sample wordlength.
Router Equivalent of a telephone exchange for video, audio and control signals.
Rushes Film developed urgently after a day’s shooting for confirmation purposes.
Sc-H Subcarrier to horizontal sync phase.
SAV Start of Active Video; see TRS.
SCR System clock reference; clock data carried in program stream pack header.
SDI Serial digital interface; standardized coaxial cable digital video interface
SECAM System essentially contrary to the American method. French colour system.
STC System time clock; common clock used to encode video and audio in the same program.
SDT Service description table; table listing the providers of each service in a transport stream.
SIF Source input format; half-resolution input signal used by MPEG-1.
SMPTE Society of Motion Picture and Television Engineers (USA).
ST Stuffing table.
STB Set-top box (see IRD).
Sampling A process in which some continous variable is measured at discrete (usually uniform) intervals.
Saturation Lack of dilution of a colour by white; e.g. red is saturated, pink is desaturated.
Scalability System where more complex decoder produces better picture but simple decoder only uses part of data.
Scrambling Process in digital transmission which spreads signal spectrum and increases clock content.
Segmentation In VTRs the use of several parallel tracks to record one field.
Seek Process of moving disk drive heads from one track to another.
Serial access Storage system such as tape where data come out in a fixed sequence. Contrasts with random access.
Set-up See Pedestal.
Shuffling Random pixel reordering process used in DVTRs which spreads uncorrected pixels over a large area to aid concealment (q.v) of uncorrectable errors.
Signature analysis Test technique for verifying quality of digital transmission system.
Skew In analog VTRs a timing error caused by incorrect tape tension.
Slicer Electronic circuit which judges an input to be above or below a threshold. Used to clean up binary signals (see Reclocker).
Spline Motion interpolating algorithm used in DVEs.
Split edit Edit in which the image and soundtrack transition at different times.
Square pixel An image-sampling process in which the vertical and horizontal sampling spacing is the same.
Stripview Graphical representation of the time axis through an edit sequence.
Stuffing Meaningless data added to maintain constant bit rate.
Syndrome Initial result of an error-checking calculation. Generally if zero there is assumed to be no error.
TDAC Time domain aliasing cancellation; coding technique used in AC–3 audio compression.
TDT Time and date table; used in DVB-SI.
TRS Timing reference signal; equivalent of sync pulses in digital interfaces.
T-STD Transport stream system target decoder; decoder having a certain amount of buffer memory assumed present by an encoder.
TSO Tape speed override; means of changing speed of VTR to stretch or squeeze duration of a program.
Tally Signal which retraces signal routing path to operate on-air light.
Telecine Machine which drives film past an optical scanning system in order to output a video signal.
Television Literally ‘seeing at a distance’. This implies some long cable or radio transmission between camera and display. If there is no such distance television becomes video.
Terminator Device fitted at the ends of a transmission line to match its characteristic impedance and prevent signal reflections.
Timecode A signal recorded down the side of a video tape allowing individual frames to be located according to the time at which they were shot (see Keycode).
Time compression Process used to squeeze the time taken to send a given quantity of data by raising the data rate.
Track angle In VTRs angle between track and edge of tape.
Transmission line Cable which is long compared to wavelength of signals carried.
Transport stream Multiplex of several program streams which are carried in packets. Demultiplexing is achieved by different packet IDs (PIDs) (see PSI; PAT; PMT; PCR).
Triad Set of three primary phosphor dots in CRT.
Truncation Shortening wordlength of sample or coefficient by removing low-order bits.
Twitter Artifact due to interlace where horizontal picture transitions flicker.
U Scaled component signal used in PAL; U = 0.493(B–Y).
Underflow When a buffer memory consistently receives less data than is being removed from it.
Upconvertor Device which allows standard definition signals to be displayed on HDTV equipment
V Scaled component signal used in PAL; V = 0.877(R–Y).
VAU Video access unit; one compressed picture in program stream.
VBV Video buffer verifier; parameter specifying amount of buffer memory needed to decode a given elementary stream.
VCR Video cassette recorder; generally implying a consumer device.
VLC Variable-length coding; compression technique which allocates short codes to frequent values and long codes to infrequent values.
VOD Video-on-demand; system in which television programs or movies are transmitted to a single consumer only when required.
VSB Vestigial sideband; system used for terrestrial TV transmission in which lower sideband is heavily curtailed.
VTR Video tape recorder.
V-switch Vertical axis switch reversing phase of R–Y component on alternate lines in PAL.
Vector Multi-dimensional quantity. In TV generally a two-dimensional representation of a pair of colour difference signals.
Vertical editing Editing where manipulations take place within the picture. e.g. layering, chroma key. Contrasts with horizontal editing.
Video Literally ‘I see’; an electrical signal which represents a picture.
Wavelet transform Technique for analysing spatial frequencies of an image which does not use blocks. Used in compression.
Weighting Modifying measurements to give better correspondence to human perception.
Winchester disk Disk drive having heads and disks sealed in one unit allowing higher speed and capacity.
Wordlength Number of bits in a sample; typically eight or ten in video.
Working positive A viewable print made from master negative used for making edit decisions. Damage to the working positive is irrelevant as it is only used to create a cutting list (q.v.).
Wrap angle Number of degrees of circumference of VTR drum over which the tape is in contact.
Y Luma signal.
Y/C Recording system used in S-VHS where chroma and luma are kept separate to avoid decoding stage.
Ys Luma signal with sync pulses added.
Zenith angle Angular error which takes a head out of the plane of the tape resulting in poor contact.
Zero-run-length Parameter specifying the number of contiguous DCT coefficients of value zero in a scanning sequence.
Zig-zag scan Method of ordering DCT coefficients so that zero values tend to be at the end of the sequence.