Active-Matrix
A drive technique used to high brightness and/or contrast ratio in high information content display panels, by placing an active element at each pixel in the array. The most common type of active-matrix display is based on a technology known as thin-film transistor, or TFT. The two terms, active matrix and TFT, are often used interchangeably.
Analog
“Analog” refers to systems which encode variations in one quantity as analogous variations in another; for example, in analog video systems, changes in light intensity (“brightness”) are represented by changes in the voltage level of the video signal. Note that “analog” does not necessarily mean the same thing as “linear” or “continuous”.
Aspect Ratio
The ratio of the physical width to the height of an image or display. Most current displays and standard image formats have a 4:3 aspect ratio; high-definition television uses a 16:9 ratio. Aspect ratio is sometimes stated with respect to unity; i.e., 1.33:1, and 1.78:1 for the above, respectively.
ATSC (Advanced Television Standards Committee)
The ATSC may be considered the successor to the original NTSC; it is an advisory committee established by the US Federal Communication Commission to help define and recommend an advanced digital television standard. The US digital television broadcast standard is often referred to as the “ATSC standard”.
ATV (Advanced Television)
ATV is the generic name used by the Federal Communications Commission and other bodies to refer to television systems, such as the “ATSC” digital television system, which provide performance or features beyond those of the original broadcast television standard.
Bandwidth
In the strict sense, “bandwidth” refers to the range of frequencies which are available for the transmission of information, or which will be passed by a given device (as in the bandwidth of an amplifier). The term “bandwidth” is often misused to refer to the rate at which information is conveyed over any given communications channel or interface, in bits/second. See “bit rate”, “channel capacity”.
Bit
A binary digit; the fundamental unit of information in a binary (two-state) system..
Bit Rate
The rate at which information is conveyed over a communications channel or interface, in terms of bits per second. Often mislabeled “bandwidth”. See “bit” and “bandwidth”.
Brightness
The perceived intensity of light emitted from a source or reflecting from a surface. See “luminance”.
Candela/Candela per square meter
The candela is measure of the intensity of light, equal to one lumen of luminous flux per steradian. The standard unit for luminance is the candela per square meter (cd/m2), sometimes referred to by the obsolete term, “nit”. Another obsolete unit for luminance is the foot-Lambert (ft.-L); one foot-Lambert is equal to 3.426 cd/m2.
Channel Capacity
The maximum rate at which information can be transmitted over a given communications channel or interface, generally given in bits per second. Channel capacity is often referred to as “bandwidth” in casual conversation; technically, channel capacity is determined by several factors including the bandwidth of the channel in question.
Characteristic Impedance
A value, in ohms, which gives the relationship expected between voltage and current on a transmission line in the absence of reflections. While generally expressed as a pure resistance, the characteristic impedance of a line does not describe its loss characteristics, but rather establishes the above relationship and the value of impedance which will properly terminate the line at either the source or load ends, or at any other junction. From another perspective, this is the impedance seen “looking into” a transmission line, and is the impedance of the line itself if the line is either infinitely long or terminated by a lumped network of the same impedance.
COFDM (Coded Orthogonal Frequency Division Multiplexing)
A modulation technique in which transmitted data is distributed among a large number of relatively closely spaced individual carrier frequencies; adjacent carriers are 90° out of phase (“orthogonal”) with respect to each other, to minimize mutual interference. In the DVB terrestrial digital television broadcast standard, COFDM with 2048 or 8192 separate carriers is used; this is one of the major incompatibilities between the DVB system and the competing US “ATSC” system.
Color
The visual perception which distinguishes different wavelengths or frequencies of visible light. Color is often specified through reference to three parameters: hue (e.g., red, yellow, blue, etc.), saturation (e.g., red vs. pink), and value (i.e., the lightness or darkness of a color, as in the distinction between various shades of gray between white and black).
Color Depth
The number of bits used to represent color and/or luminance in storing or displaying an image. Also called “bit depth” or “pixel depth”, color depth determines the number of possible colors that can be displayed by the system. A ”24-bit” system, for example, has a color depth of 2 to the 24th power (about 16.7 million) colors. Note that additional bits of storage may be used for other information not related to the color of the pixel, such as blinking or transparency values, so that the total number of bits provided per pixel by a frame buffer may not be the same as the true color depth.
Color Gamut
The range of colors that can be produced by a given display. In terms of representation within a 2-D color space (such as the CIE xy diagram), this is often indicated by plotting the location of the three primary colors used in the display; the color gamut is then all colors within the triangle thus created.
Color Space
Any of a number of three-dimensional spaces in which colors may be described as a set of values corresponding to the three dimensions or axes. Examples include RGB color systems, the CIE XYZ space, or a hue-saturation-value (HSV) color model. Color spaces may also be distinguished according to whether or not they are perceptually uniform, meaning that equal geometric distances covered within the space correspond to similar changes in perceived color by the normal human observer.
Color Temperature
A means of identifying the color of nominally “white” light sources, by identifying the temperature of a theoretical “black-body” radiator which would emit light of the same color. Color temperature is normally given in degrees Kelvin (K).
CCIR
Comité Consultatif International en Radiodiffusion (in English, the International Consultative Committee on Radio Broadcasting); an international standards group which created many standards in the areas of radio and television broadcasting and related fields. The CCIR was later subsumed into the ITU, as ITU-R.
Contrast Ratio
The ratio of the luminance of a bright (“full white”) pixel or area to a dark (“full black”) pixel or area on a given display or image. Often simply referred to as contrast.
Convergence
In CRT displays, “convergence” refers to the alignment of the three electron beams (red, green, and blue) such that they combine to produce a single point or area of apparently “white” light as seen by the user. A misconverged CRT display will show fringes of color around the edges of “white” objects.
CMY/CMYK
The standard set of subtractive primaries (cyan, magenta, and yellow), also used to refer to systems employing these primaries and thereby a subtractive-color method of color encoding. As such systems generally cannot produce an acceptable “black” color from the three primaries alone, a fourth channel is usually added for black, and the result referred to as CMYK.
Digital
“Digital” refer to systems which generate, store, and/or process data in terms of numeric values, most commonly using a binary (1 or 0) representation. “Digital” should not be confused with “sampled” or “discrete”.
DDWG (Digital Display Working Group)
The group responsible for the Digital Visual Interface (DVI) specification. The seven original “promoter” companies include Compaq Computer Corp., Fujitsu Ltd., Hewlett-Packard Company, Intel Corp., International Business Machines Corp., NEC Corp., and Silicon Image, Inc.. The DDWG also includes an “Implementer’s Forum,” which is a much larger group of companies involved in using and promoting the DVI standard.
DFP
“Digital Flat Panel,” a simple digital-only display interface based on a 20-pin MDR (“micro-delta-ribbon”) connector, and first defined by Compaq Computer Corp.. The DFP specification was later adopted as a standard by the Video Electronics Standards Association (VESA).
DVI (Digital Visual Interface)
The specification produced by the Digital Display Working Group. Similar to the VESA “Plug & Display” standard in that it supports both digital and analog interfaces and employs a very similar physical connector, the DVI specification differs in offering two TMDS channels and no supplemental interfaces such as USB or IEEE-1394.
Driver
An interface circuit that conveys information or signals at higher level (usually in terms of voltage or current) to a receiver or load. An example in many flat-panel display types are those circuits which translate the digital video information to signals required to drive the row or column lines in the panel.
DVD (Digital Versatile Disc or Digital Video Disc)
A digital recording medium, similar in appearance to the audio CD or CD-ROM, but with much greater data capacity. In video applications, DVDs use MPEG-2 compression and provide approximately 135 min of video per side. The video data rate is 1-10 Mbits/s.
EIA
The Electronic Industries Association, a group today comprising several subsidiary organizations, and which has traditionally been one of the major private-sector sources of standards for all forms of electronic equipment and applications, including television and radio broadcasting practices.
Emissive Display
Used to refer to a display technology that emits light, as opposed to one that controls or modulates light provided by a source external to the display elements themselves. The CRT and the OLED display types are both examples of emissive displays.
EVC
The “Enhanced Video Connector” standard defined by the Video Electronics Standards Association (VESA). The EVC standard defined an advanced analog-only interface system, intended as a replacement for the earlier “VGA” industry standard. The EVC definition was never widely adopted, and it is best seen today as the predecessor of the P&D and DVI specifications.
Eye Pattern
A test pattern commonly used to judge the quality of a “digital” signal or transmission; waveforms of similar but opposite-sense transitions are overlaid on an oscilloscope or similar device, and the amount of open area in the resulting “eye” is assumed to correspond to the degree to which the states may be distinguished in time and amplitude.
Field
In display systems, “field” often refers to a portion of the complete image or frame. For example, in an interlaced display, each frame is made up of two fields, one containing the odd-numbered scan lines and the other the even lines. Another relevant example is field-sequential color, in which each complete full-color frame is separated into red, green, and blue fields.
Flat Panel Display (FPD)
An electronic display, typified by a flat screen formed by an array (usually orthogonal) of basic light-controlling or emitting devices, such as electroluminescent devices, light-emitting diodes or liquid crystal cells.
Flicker
The perception of rapid, periodic changes in the brightness of large areas of a display; the perception of flicker is determined by the display luminance, size, viewing distance, refresh rate, and the sensitivity of the individual viewer, along with the specific characteristics of the display device itself.
Foot-Lambert
A unit of luminance equal to 1/π candela per square foot. Use of the foot-Lambert is generally discouraged today, in favor of using cd/m2. See “Candela/candela per square meter.”
Frame
In display systems, “frame” generally refers to the smallest set of information which constitutes one complete, full-color image. It may best be understood by considering motion-picture film; here, each individual picture or image is called a frame. “Frame rate” is an important consideration in imaging systems, but may not always be the same thing as the display’s refresh rate.
Gamma
A quantity used to indicate the non-linearity of a display device, in terms of its output response (luminance) vs. input signal level. In the simplest model, luminance is related to the input signal level by a simple exponential relationship where gamma (γ) is the exponent:
where Y is the output luminance, I is the level of the input signal, and K is a scaling factor. For CRT displays, gamma is usually in the range of 2.0-2.7. A gamma of 1.0 indicates a linear response.
HDTV (High Definition Television)
A generic term used to refer to any television system having a higher “resolution” (larger image format) than the current 525-line or 625-line broadcast standards. A number of different image formats have been referred to as “HDTV”, including 1920 x 1035 and 2048 x 1152 pixels. The US digital television standard provides two formats above “standard definition”, or “SDTV”: these are 1280 x 720 pixels, and 1920 x 1080 pixels. HDTV systems generally provide a 16:9 image aspect ratio.
Hue
In color science, “hue” refers to the perception associated with the dominant wavelength of light in a given source or reflected from an object; in simpler, common language, “hue” is what is being described when something is referred to as “reddish” or “bluish.” Along with saturation and value, hue is one of three parameters which may be used to describe color in an intuitive “color space.”
IEEE-1394 (aka FireWire™)
A serial bus standard that allows for the connection of up to 63 devices and transmission speeds ranging as high as 400 Mbits/s in its original version.
Illuminance
A measure of the light energy reaching a surface, in terms of the amount of energy per unit area. The standard unit of illuminance is the lux, which is one lumen per square meter.
Intensity
The “strength” of a light source, in terms of the amount of luminous flux being emitted over a given solid angle; i.e., two sources could be emitting the same total luminous flux, but have different intensities depending on the spatial distribution of that flux and the location of the point of measurement. Intensity, as opposed to luminance, is not corrected for the characteristics of human vision. The standard unit of intensity is the candela (see definition above).
ISO (International Standards Organization)
An international group of national and other standards bodies. For example, ANSI (American National Standards Institute) is a member of ISO.
ITU
International Telecommunication Union, an organization comprising representatives from numerous national regulatory agencies and related groups, which sets international standards for broadcasting and telecommunications.
Jitter
The instantaneous temporal instability of a given signal, with respect to either a reference point in time for that signal itself, or to a separate reference signal, or the average value over a given period for such instability.
Kelly Chart
A version of any of the various two-dimensional color coordinate diagrams (such as the 1931 CIE xy diagram) in which the area of the chart has been divided into regions with assigned color names; after K. L. Kelly, the color scientist who first presented such a chart.
Light-Emitting Diode (LED)
A semiconductor device that emits light when current is passed through it. An LED operates by having a sufficiently wide band-gap such that the energy emitted when a charge carrier passes through the device falls into the visible spectrum.
Liquid Crystal (LC)
A class of materials, typically organic compounds, which exist in the liquid state at normal temperatures but which also exhibit some ordering of the molecules. In the case of the LC materials used in electronic displays, the molecules are generally in the shape of long rods which are electrically polar, and which exhibit some degree of optical anisotropy (i.e., the effect these molecules have on incident light depends on the orientation of the molecule with respect to the light).
Liquid Crystal Display (LCD)
A large class of display devices employing liquid-crystal materials (see above) to control the transmission or reflection of light. Liquid-crystal displays use a wide variety of LC modes and affects to achieve this.
Lumen
The standard unit of measurement of the rate of emission of light energy. A typical candle produces about 13 lumens; a 100-W bulb generates 1200 (see candela).
Luminance
The intensity of light emitted from a surface, per unit of area, and corrected for the standardized spectral response of human vision (which distinguishes it from the formal use of the term intensity). The standard unit of measurement is the candela per square meter (cd/m2, see definition above).
MacAdam Ellipses
Roughly oval or elliptical areas, as appearing on a two-dimensional color coordinate diagram, which describe the locus of just-noticeable-color-differences (JNCDs), to a given fraction of viewers in the general population, from the center color point. These are often shown larger than the actual JNCD locus for clarity; 10x is a common scaling. Named after D. L. MacAdam, and taken from his work in the 1940s which described typical color-difference sensitivities in the general population of human obsevers.
MicroCross ™
The pseudo-coaxial connector design used in several PC-industry video interfaces; originated by Molex Corp. (“MicroCross” is a trademark of Molex Corp.)
Microdisplay
Most generically, any display device with a diagonal size of under 2.5-5 cm (1-2 inch), in common usage of the term. Microdisplays are commonly used in either direct-view (through magnifying optics) applications, often referred to as “near-eye” applications from the physical location of the display, or in projection applications through the use of high-intensity light sources and projection optics.
MPEG (Motion Picture Expert Group)
Refers to an ISO working group that develops standards for digital video compression.
Nematic
A phase or mode of a liquid crystal material in which the long axes of the molecules are aligned with one another but without further organization. The “twisted-nematic” (TN) mode, in which the molecules’ tendency to so align results in their a helical or “spiral staircase” arrangement (due to other influences built into the device) is the basis for most common liquid-crystal displays.
NTSC (National Television Standards Committee)
The original NTSC was an advisory committee established by the US Federal Communications Commission, which directed the development of broadcast television standards in the 1940s and 1950s. Today, “NTSC” is often used to refer to the US color TV standard itself (although it more properly refers only to the color encoding method used by that standard). The US color TV standard uses a format of 525 lines per frame, transmitted as approx. 60 interlaced fields (of 262.5 lines each) per second. The color encoding technique used is too complex to describe here. This standard is used in North America, parts of South America, and Japan.
Overshoot/Undershoot
The condition in which a waveform exceeds its desired “steady-state” value, following a transition to that value. Overshoot (or undershoot, which is the same thing following a negative-going transition) may be measure in terms of either the absolute amplitude of the excursion beyond the desired value, or as a percentage of that value or of the transition itself.
PAL (Phase Alternating Line)
A color TV standard, originating in Europe, commonly using a format of 625 lines per frame, transmitted as 50 interlaced fields (of 312.5 lines each) per second. The PAL color system is closely related to, but not totally compatible with, the US “NTSC” system. This video standard is used in Europe, Australia, China, and some South American and African countries. A common “digital” version of PAL uses a sampling format of 768 pixels x 576 lines.
Passive Matrix
A common type of flat-panel display in which the pixel array consists of a simple grid of horizontal (row) and vertical (column) electrodes, with the pixels themselves defined by the intersection of these, but without active control or drive elements at these locations. In LC displays, the passive matrix types typically do not have as broad a viewing angle as active-matrix (TFT) displays, and have slower response times.
Pixel
Contraction of “picture element.” Physically, this term is often used to refer to the smallest individually addressable unit of an image that can be rendered or displayed. In strict usage with respect to imaging, a “pixel” is a single point sample of an image, and as such has neither size nor shape.
Pixel Density
The number of picture elements per unit of distance (e.g., pixels per inch or pixels per centimeter). See also “resolution”.
P&D
The “Plug & Display” interface standard, defined by the Video Electronics Standards Association (VESA), including both digital and analog interface specifications. The P&D standard is best viewed now as a predecessor to the Digital Visual Interface (DVI) specification, which it closely resembles.
QAM (Quadrature Amplitude Modulation)
A variation of conventional amplitude modulation in which two versions of the same carrier, 90° out of phase with respect to each other (“in quadrature”) are amplitude-modulated by different signals. This results in a combined signal from which the two original baseband signals may still be recovered separately by the receiver. QAM is used, for example, in modulating the two color signals onto the chroma subcarrier in the “NTSC” color broadcast system. (The original names for these signals, “I” and “Q,” refer to this modulation technique, as they identify the in-phase and quadrature signals, with respect to the original chroma subcarrier signal.)
Resolution
The ability of a display device, image sensor, or viewer to discriminate detail in an image; resolution is most often stated in terms of cycles or lines per visual degree, or similar units indicating the number of intensity or color changes per unit distance in the visual field, which can be discriminated under given conditions. In common usage, “resolution” has also come to be used in reference to what is properly called the “image format” or “addressibil-ity” (i.e., the horizontal and vertical pixel count), as in “a resolution of 1024 x 768”.
RGB (Red, Green, Blue)
The common set of additive primaries, as used in electronic displays such as CRT monitors, or used to refer to systems employing this set and the resulting encoding of color into RGB values. This is in contrast to the reflective or subtractive primary set (cyan, magenta, and yellow, commonly referred to as “CMY” or “CMYK” with the addition of black) as used in printing.
Ringing
The damped oscillation which may occur following an abrupt transition between states in a signal or waveform. Ringing is typically characterized by the degree of overshoot it causes, and the settling time (the amount of time, measured from either a defined point on the transition itself or the first overshoot/undershoot peak, which is required for the signal to settle such that it remains within defined limits from that point on – e.g., “settling to within 5% of full scale”).
Saturation
In color science, “saturation” describes the “purity” of a given light source in terms of the dominant wavelength; the closer the light comes to being purely of a single frequency, the greater the “saturation” of that color is said to be. For example, a light source which emits nothing but light of a wavelength of, say, 505 nm might be said to be a “100% saturated green.” The color commonly called “pink” might also be considered a “low-saturation red.” Along with hue and value, saturation is one of three parameters which may be used to des-ribe color in an intuitive “color space.”
Sequential Color
Any system in which the primary colors or color fields are presented to the viewer as separated in time, rather than in space; in a field-sequential color display, for example, the user sees separate red, green, and blue images presented in rapid succession, to create the illusion of a single full-color image.
SECAM (Sequentiel Couleur avec Memoire)
The color television standard used in France, Eastern Europe and some African and Middle Eastern countries. It most often is transmitted using an image format similar to the PAL systems (625 lines/frame at a 50 Hz field rate), but using a completely incompatible color encoding technique in which the color difference components (R-Y, B-Y) are transmitted on different lines.
Skew
The stable (or nearly so) temporal difference between supposedly aligned portions of two or more signals.
SMPTE
The Society of Motion Picture and Television Engineers, a professional society which has also been extremely active in the establishment of technical standards for the television and motion picture industries.
SDTV (Standard Definition Television)
A term used in the context of the new digital “HDTV” broadcast standards to refer to image formats roughly comparable to the previous analog broadcast systems. In the US digital TV systems, two “SDTV” formats are in normal use: 640 x 480 pixels, and 720 x 480 pixels, the latter of these being displayable as either a 4:3 or a 16:9 image. European systems using the 625/50 format in analog broadcast most commonly use 720 x 576 or 768 x 576 for the “standard definition” digital format.
SPWG (Standard Panels Working Group)
A consortium formed by five companies – Compaq Computer Corp., Dell Computer Corp., Hewlett-Packard Co., IBM, and Toshiba – which produces standards for the mechanical dimensions, mounting, and electrical interfaces for LCD panels (to date, primarily for panels intended for the notebook computer market).
Transflective
A class of displays, most commonly of the liquid-crystal type, which can operate using either reflected or transmitted (i.e., through the device) light. This is most commonly achieved through a design similar to the common reflective LCD, but in which the reflective layer will also pass some amount of light from a source located behind the panel.
Transmissive Display
Any display devices which operates by controlling light passing through the display device proper; in this class of display, the light source and the viewer are on opposite sides of the display device, which then acts as a “switch” or “filter” (or more commonly, an array of switches or filters) to produce the viewed image.
VESA (Video Electronics Standards Association)
An industry standards organization developing display interface, timing, and related standards, primarily for the PC and workstation markets.
Viewing Angle
The largest angle or angular range over which one is able to acceptably view an image, generally defined in terms of minimum acceptable contrast ratio, color variation, or other measure of image quality. Viewing angle may be stated in terms of the total included angle over which the measure of minimum acceptability is met, or similarly as an angular measurement referenced to a line normal to the display surface.
USB (Universal Serial Bus)
A "plug-and-play" serial interface, typically used between a computer and add-on devices (such as keyboards, displays, printers, etc). As originally defined, USB provided a maximum rate of 12 Mbit/s, limiting its use to slow-speed peripherals and limited digital audio. The more recent USB 2.0 specification permits much faster operation, up to 480 Mbit/s.
Value
In color science, “value” refers to the intensity or “lightness” of a light source; for example, a change from black to gray to white, with no change in the “hue” of these shades, is an example of increasing value. Along with hue and saturation, value is one of three parameters which may be used to describe color in an intuitive “color space.”
VSB (Vestigial Side Band)
A modulation method closely related to the conventional amplitude modulation (AM) and single-sideband-suppressed-carrier (SSBSC) types; in a vestigial-sideband AM transmission, one of the two sidebands is reduced in amplitude (as in SSB) but not suppressed altogether; also unlike SSBSC, the full carrier is retained. VSB modulation is used in nearly all analog television broadcast systems, and a variation of this method (8-VSB, with eight discrete amplitude levels) is used in the US digital television broadcast system.
VGA
“Video Graphics Adapter,” originally used to refer to the complete specification for a graphics hardware subsystem introduced by IBM for its personal computer products in 1987. Today, “VGA” is used to refer either to the 640 x 480 image format (also first introduced as part of that IBM definition) or, more commonly, the 15-pin high-density D-subminiature connector used for video output within that system and still a popular de-facto standard within the industry today.
