a posteriori probability See posterior statistics.

a priori probability See prior statistics.

A-mode display returned ultrasound echoes displayed as amplitude versus depth into the body.

A-site in a ferroelectric material with the chemical formula ABO3, the crystalline location of the A atom.

A/D See analog-to-digital converter.

AAL See ATM adaptation layer.

ABC See absorbing boundary condition.

ABCD propagation of an optical ray through a system can be described by a simple 2_2 matrix. In ray optics, the characteristic of a system is given by the corresponding ray matrix relating the ray’s position from the axis and slope at the input to those at the output.

ABCD formalism analytic method using two-by-two ABCD matrices for propagating Gaussian beams and light rays in a wide variety of optical systems.

ABCD law analytic formula for transforming a Gaussian beam parameter from one reference plane to another in paraxial optics, sometimes called the Kogelnik transformation.

ABCD refers to the ABCD matrix.

ABCD matrix the matrix containing

ABCD parameters. See ABCD parameters.

ABCD parameters a convenient mathematical form that can be used to characterize two-port networks. Sometimes referred to as chain parameters. ABCD parameters are widely used to model cascaded connections of two-port microwave networks, in which case the ABCD matrix is defined for each two-port network. ABCD parameters can also be used in analytic formalisms for propagating Gaussian beams and light rays. Ray matrices and beam matrices are similar but are often regarded as distinct. ABC parameters have a particularly useful property in circuit analysis where the composite ABCD parameters of two cascaded networks are the matrix products of the ABCD parameters of the two individual circuits. ABCD parameters are defined as _v1i1_D_A BC D__v2i2_ where v1 and v2 are the voltages on ports one and two, and i1 and i2 are the branch currents into ports one and two.

aberration an imperfection of an optical system that leads to a blurred or a distorted image.

abnormal event any external or programgenerated event that makes further normal program execution impossible or undesirable, resulting in a system interrupt. Examples of abnormal events include system detection of power failure; attempt to divide by 0; attempt to execute privileged instruction without privileged status; memory parity error.

abort (1) in computer systems, to terminate the attempt to complete the transaction, usually because there is a deadlock or because completing the transaction would result in a system state that is not compatible with “correct” behavior, as defined by a consistency model, such as sequential consistency. (2) in an accelerator, terminating the acceleration process prematurely, either by inhibiting the injection mechanism or by removing circulating beam to some sort of dump. This is generally done to prevent injury to some personnel or damage to accelerator components.

ABR See available bit rate.

absolute address an address within an instruction that directly indicates a location in the program’s address space. Compare with relative addressing.

absolute addressing an addressing mode where the address of the instruction operand in memory is a part of the instruction so that no calculation of an effective address by the CPU is necessary. For example, in the Motorola M68000 architecture instructionADD5000,D1, a 16-bit word operand, stored in memory at the word address 5000, is added to the lower word in register D1. The address “5000” is an example of using the absolute addressing mode.

See also addressing mode.

absolute encoder an optical device mounted to the shaft of a motor consisting of a disc with a pattern and light sources and detectors. The combination of light detectors receiving light depends on the position of the rotor and the pattern employed (typically the Gray code). Thus, absolute position information is obtained. The higher the resolution required, the larger the number of detectors needed. See also encoder.

absolute moment The pth order absolute moment _p of a random variable X is the expectation of the absolute value of X raised to the pth power:_ D ETjXjUp: See also central moment, central absolute moment. See also expectation.

absolute pressure units to measure gas pressure in a vacuum chamber with zero being a perfect vacuum. Normally referred to as psia (pounds per square inch absolute).

absolute sensitivity denoted S.y; x/, is simply the partial derivative of y with respect to x, i.e., S.y; x/ D @y=@x, and is used to establish the relationships between absolute changes. See sensitivity, sensitivity measure, relative sensitivity, semi-relative sensitivity.

absolute stability occurs when the network function H.s/ has only left half-plane poles.

absorber generic term used to describe material used to absorb electromagnetic energy. Generally made of polyurethane foam and impregnated with carbon (and fireretardant salts), it is most frequently used to line the walls, floors and ceilings of anechoic chambers to reduce or eliminate reflections from these surfaces.

absorbing boundary condition (ABC) a fictitious boundary introduced in differential equation methods to truncate the computational space at a finite distance without, in principle, creating any reflections.

absorption (1) process that dissipates energy and causes a decrease in the amplitude and intensity of a propagating wave between an input and output reference plane. (2) reduction in the number of photons of a specific wavelength or energy incident upon a material. Energy transferred to the material may result in a change in the electronic structure, or in the relative movement of atoms in the material (vibration or rotation). (3) process by which atoms or molecules stick to a surface. If a bond is formed, it is termed chemisorption, while the normal case is physisorption. The absorption process proceeds due to, and is supported by, the fact that this is a lower energy state.

absorption coefficient (1) in a passive device, the negative ratio of the power absorbed (pabsorbed D pin−pout) ratioed to the power in (pin D pincident −preflected) per unit length (l), usually expressed in units of 1/wavelength or 1/meter.

(2) factor describing the fractional attenuation of light with distance traversed in a medium, generally expressed as an exponential factor, such as k in the function e −kx, with units of (length)-1. Also called attenuation coefficient.

absorption cross section energy absorbed by the scattering medium, normalized to the wavenumber. It has dimensions of area.

absorption edge the optical wavelength or photon energy corresponding to the separation of valence and conduction bands in solids; at shorter wavelengths, or higher photon energies than the absorption edge, the absorption increases strongly.

absorption grating (1) a diffraction grating where alternate grating periods are opaque. (2) an optical grating characterized by spatially periodic variation in the absorption of light. Absorption gratings are generally less efficient than phase gratings.

absorption optical fiber the amount of optical power in an optical fiber captured by defect and impurity centers in the energy bandgap of the fiber material and lost in the form of longwave infrared radiation.

AC See alternating current.

AC bridge one of a wide group of bridge circuits used for measurements of resistances, inductances, and capacitances, and to provideACsignal in the bridge transducers including resistors, inductors, and capacitors. The Wheatstone bridge can be used with a sinusoidal power supply, and with an AC detector (headphones, oscilloscope), one can use essentially the same procedure for measurement of resistors as in DC applications. Only a small number of other AC bridges are used in modern electric and electronic equipment. A strong selection factor was the fact

that in a standard capacitor the electrical parameter are closest to the parameters of an ideal capacitor. Hence, not only a capacitance is measured in terms of capacitance (in resistive ratio arms bridges), but the inductance as well is measured in terms of capacitance (Hay and Owen bridges). The AC bridges with ratio arms that are tightly coupled inductances allow measurement of a very small difference between currents in these inductances, and this fact is used in very sensitive capacitance transducers.

ACcircuit electrical network in which the voltage polarity and directions of current flow change continuously, and often periodically. Thus, such networks contain alternating currents as opposed to direct currents, thereby giving rise to the term.

AC coupling a method of connecting two circuits that allows displacement current to flow while preventing conductive currents. Reactive impedance devices (e.g., capacitors and inductive transformers) are used to provide continuity of alternating current flow between two circuits while simultaneously blocking the flow of direct current.

AC motor an electromechanical system that converts alternating current electrical power into mechanical power.

AC plasma display a display that employs an internal capacitive dielectric layer to limit the gas discharge current.

AC steady-state power the average power delivered by a sinusoidal source to a network, expressed as P Dj V j _ j I j cos._/ where p 2_ j V j and p 2_ j I j are the peak values, respectively, of the AC steady-state voltage and current at the terminals. _ represents the phase angle by which the voltage leads the current.

AC/AC converter a power electronics device in which an AC input voltage of some magnitude, frequency, and number of phases is changed to an AC output with changes to any of the previously mentioned parameters.

AC/AC converters usually rectify the input source to a DC voltage and then invert the DC voltage to the desired AC voltage.

AC/DC converter See rectifier.

AC-DC integrated system a power system containing bothACandDCtransmission lines.

ACARS aircraft communications addressing and reporting. A digital communications link using the VHF spectrum for two-way transmission of data between an aircraft and ground. It is used primarily in civil aviation applications.

ACC See automatic chroma control.

accelerated testing tests conducted at higher stress levels than normal operation but in a shorter period of time for the specific purpose to induce failure faster.

accelerating power the excess electric power at a synchronous machine unit which cannot be transmitted to the load because of a short circuit near its terminals. This energy gives rise to increasing rotor angle.

acceleration error the final steady difference between a parabolic setpoint and the process output in a unity feedback control system. Thus it is the asymptotic error in position that arises in a closed loop system that is commanded tomove with constant acceleration. See also position error, velocity error.

acceleration error constant a gain Ka from which acceleration error ea is readily determined. The acceleration error constant is a concept that is useful in the design of unity feedback control systems, since it transforms a constraint on the final acceleration error to a constraint on the gain of the open loop system. The relevant equations

are ea D 1  Ka and Ka D lims!inf tys2q.s/, where q.s/ is the transfer function model of the open loop system, including the controller and the process in cascade, and s is the Laplace variable. See also position error constant, velocity error constant.

accelerator (1) a positive electrode in a vacuum tube to accelerate emitted electrons from its cathode by coulomb force in a desired direction. (2) a machine used to impart large kinetic energies to charged particles such as electrons, protons, and atomic nuclei. The accelerated particles are used to probe nuclear or subnuclear phenomena in industrial and medical applications.

acceptable delay the voice signal delay that results in inconvenience in the voice communication. A typically quoted value is 300 ms.

acceptance in an accelerator, it defines how “large” a beam will fit without scraping into the limiting aperture of a transport line. The acceptance is the phase-space volume within which the beam must lie to be transmitted through an optical system without losses. From an experimenters point of view acceptance is the phase-space volume intercepted by an experimenter’s detector system.

acceptor (1) an impurity in a semiconductor that donates a free hole to the valence band. (2) a dopant species that traps electrons, especially with regard to semiconductors.

access channel a channel in a communications network that is typically allocated for the purpose of setting up calls or communication sessions. Typically the users share the access channel using some multiple access algorithm such as ALOHA or CSMA.

access control a means of allowing access to an object based on the type of access sought, the accessor’s privileges, and the owner’s policy.

access control list a list of items associated with a file or other object; the list contains the identities of users that are permitted access to the associated file. There is information (usually in the form of a set of bits) about the types of access (such as read, write, or delete) permitted to the user.

access control matrix a tabular representation of the modes of access permitted from active entities (programs or processes) to passive entities (objects, files, or devices). A typical format associates a row with an active entity or subject and a column with an object; the modes of access permitted from that active entity to the associated passive entity are listed in the table entry.

access line a communication line that connects a user’s terminal equipment to a switching node.

access mechanism a circuit board or an integrated chip that allows a given part of a  computer system to access another part. This is typically performed by using a specific access protocol.

access protocol a set of rules that establishes communication among different parts. These can involve both hardware and software specifications.

access right permission to perform an operation on an object, usually specified as the type of operation that is permitted, such as read, write, or delete. Access rights can be included in access control lists, capability lists, or in an overall access control matrix.

access time the total time needed to retrieve data from memory. For a disk drive this is the sum of the time to position the read/write head over the desired track and the time until the desired data rotates under the head. (LW)

accidental rate the rate of false coincidences in the electronic counter experiment produced by products of the reactions of more than one beam particle within the time resolution of the apparatus.

accumulation (1) an increase in the majority carrier concentration of a region of semiconductor due to an externally applied electric field.

accumulator (1) a register in the CPU (processor) that stores one of the operands prior to the execution of an operation, and into which the result of the operation is stored. An accumulator serves as an implicit source and destination of many of the processor instructions. For example, register A of the Intel 8085 is an accumulator. See also CPU . (2) the storage ring in which successive pulses of particles are collected to create a particle beam of reasonable intensity for colliding beams.

achievable rate region for a multiple terminal communications system, a set of rate-vectors for which there exist codes such that the probability of making a decoding error can be made arbitrarily small. See also capacity region, multiple access channel.

achromatic the quality of a transport line or optical system where particle momentum has no effect on its trajectory through the system. In an achromatic device or system, the output beam displacement or divergence (or both) is independent of the input beam’s momentum. If a system of lenses is achromatic, all particles of the same momentum will have equal path lengths through the system.

ACI See adjacent channel interference.

acknowledge (1) a signal which indicates that some operation, such as a data transfer, has successfully been completed. (2) to detect the successful completion of an operation and produce a signal indicating the success.

acoustic attenuation the degree of amplitude suppression suffered by the acoustic wave traveling along the acousto-optic medium.

acoustic laser a laser (or maser) in which the amplified field consists of soundwaves or phonons rather than electromagnetic waves; phonon laser or phaser.

acoustic memory a form of circulating memory in which information is encoded in acousticwaves, typically propagated through a trough of mercury. Now obsolete.

acoustic velocity the velocity of the acoustic signal traveling along the acoustooptic medium.

acoustic wave a propagating periodic pressure wave with amplitude representing either longitudinal or shear particle displacement within the wave medium; shear waves are prohibited in gaseous and liquid media.

acousto-optic cell a device consisting of a photo-elastic medium in which a propagating acoustic wave causes refractive-index changes, proportional to acoustic wave amplitude, that act as a phase grating for diffraction of light. See also Bragg cell.

acousto-optic channelized radiometer See acousto-optic instantaneous spectrum analyzer in Bragg mode.

acousto-optic correlator an optical system that consists of at least one acoustooptic cell, imaging optics between cells and fixed masks, and photodetectors whose outputs correspond to the correlation function of the acoustic wave signal within one cell with another signal in a second cell, or with fixed signals on a mask.

acousto-optic deflector device device where acousto-optic interaction deflects the incident beam linearly as a function of the input frequency of the RF signal driving the device.

acousto-optic device descriptor of acousto-optic cells of any design; generally describes a cell plus its transducer structure( s), and may encompass either bulk, guided-wave, or fiber-optic devices.

acousto-optic effect the interaction of light with sound waves and in particular the modification of the properties of a light wave by its interactions with an electrically controllable sound wave. See also Brillouin scattering.

acousto-optic frequency excisor similar to an acousto-optic spectrum analyzer where the RF temporal spectrum is spatially and selectively blocked to filter the RF signal feeding the Bragg cell.

acousto-optic instantaneous spectrum analyzer in Braggmode device in which the temporal spectrum of a radio frequency signal is instantaneously and spatially resolved in the optical domain using a Fourier transform lens and a RF signal-fed Bragg cell.

acousto-optic modulator a device that modifies the amplitude or phase of a light wave by means of the acousto-optic effect.

acousto-optic processor an optical system that incorporates acousto-optic cells configured to perform any of a number of mathematical functions such as Fourier transform, ambiguity transforms, and other timefrequency

transforms.

acousto-optic scanner a device that uses an acoustic wave in a photoelastic medium to deflect light to different angular positions based on the frequency of the acoustic wave.

acousto-optic space integrating convolver device that is the same as an acousto-optic space integrating convolver except that it implements the convolution operation.

acousto-optic space integrating correlator an acousto-optic implementation of the correlation function where two RF signals are spatially impressed on two diffracted beams from Bragg cells, and a Fourier transform lens spatially integrates these beams onto a point sensor that generates a photo current representing the correlation function. acousto-optic spectrum analyzer an acousto-optic processor that produces at a photodetector output array the Fourier decomposition of the electrical drive signal of an acousto-optic device.

acousto-optic time integrating convolver same as the acousto-optic time integrating correlator, except implements the signal convolution operation. See acousto-optic time integrating correlator.

acousto-optic time integrating correlator an acousto-optic implementation of the correlation function where two RF signals are spatially impressed on two diffracted beams from Bragg cells, and a time integrating sensor generates the spatially distributed correlation results.

acousto-optic triple product processor signal processor that implements a triple integration operation using generally both space and time dimensions.

acousto-optic tunable filter (AOTF) an acousto-optic device that selects specific optical frequencies from a broadband optical beam, depending on the number and frequencies of acoustic waves generated in the device.

acousto-optics the area of study of interaction of light and sound in media, and  its utilization in applications such as signal processing and filtering.

ACP See adjacent channel power.

acquisition (1) in digital communications systems, the process of acquiring synchronism with the received signal.  There are several levels of acquisitions, and for a given communication system several of them have to be performed in the process of setting up a communication link: frequency, phase, spreading code, symbol, frame, etc. (2) in analog communications systems, the process of initially estimating signal parameters (for example carrier frequency offset, phase offset) required in order to begin demodulation of the received signal. (3) in vision processing, the process by which a scene (physical phenomenon) is converted into a suitable format that allows for its storage or retrieval. See also synchronization.

across the line starter a motor starter that applies full line voltage to the motor to start. This is also referred to as “hard starting” because it causes high starting currents. Larger motors require reduced voltage or “soft starting.”

ACRR See adjacent channel reuse ratio.

ACSR aluminum cable, steel-reinforced.

A kind of overhead electric power conductor made up of a central stranded steel cable overlaid with strands of aluminum.

ACT See anticomet tail.

action potential a propagating change in the conductivity and potential across a nerve cell’s membrane; a nerve impulse in common parlance.

activation function in an artificial neural network, a function that maps the net output of a neuron to a smaller set of values. This set is usually T0; 1U. Typical functions are the sigmoid function or singularity functions like the step or ramp.

active contour a deformable template matching method that, by minimizing the energy function associated with a specific model (i.e., a specific characterization of the shape of an object), deforms the model in conformation to salient image features.

active device a device that can convert energy from a DC bias source to a signal at an RF frequency. Active devices are required in oscillators and amplifiers.

active filter (1) a filter that has an energy gain greater than one, that is, a filter that outputs more energy than it absorbs. (2) a form of power electronic converter designed to effectively cancel harmonic currents by injecting currents that are equal and opposite to, or 180_ out of phase with, the target harmonics. Active filters allow the output current to be controlled and provide stable operation against AC source impedance variations without interfering with the system impedance. The main type of active filter is the series type in which a voltage is added in series with an existing bus voltage. The other type is the parallel type in which a current is injected into the bus and cancels the line current harmonics.

active impedance the impedance at the input of a single antenna element of an array with all the other elements of the array excited.

active layer See active region.

active learning a form of machine learning where the learning system is able to interact with its environment so as to affect the generation of training data.

active load a transistor connected so as to replace a function that would conventionally be performed by a passive component such as a resistor, capacitor, or inductor.

active load-pull measurement a measurement method where transfer characteristics of a device can be measured by electrically changing the load impedance seen from the device. In an active load-pull measurement, the load impedance is defined by using an output signal from the device and an injected signal from the output of the device.

active logic a digital logic that operates all of the time in the active, dissipative region of the electronic amplifiers from which it is constructed. The output of such a gate is determined primarily by the gate and not by the load.

active magnetic bearing a magnetic bearing that requires input energy for stable support during operation.  Generally implemented with one or more electromagnets and controllers.

active mixer a mixer that uses three terminal devices such as FET rather than diodes as nonlinear element. One advantage of active mixers is that they can provide conversion gain.

active network an electrical network that contains some solid state devices such as bipolar junction transistors (BJTs) or metaloxide-silicon field effect transistors (FETs) operating in their active region of the voltagevs. current characteristic. To ensure that these devices are operating in the active region, they must be supplied with proper DC

biasing.

active neuron a neuron with a non-zero output. Most neurons have an activation threshold. The output of such a neuron has zero output until this threshold is reached.

active power See real power.

active power line conditioner a device which senses disturbances on a power line and injects compensating voltages or currents to restore the line’s proper waveform.

active RC filter an electronic circuit made up of resistors, capacitors, and operational amplifiers that provide well-controlled linear frequency-dependent functions, e.g., low-, high-, and bandpass filters.

active redundancy a circuit redundancy technique that assures fault-tolerance by detecting the existence of faults and performing some action to remove the faulty hardware, e.g., by standby sparing.

active region semiconductor material doped such that electrons and/or holes are free to move when the material is biased. In the final fabricated device, the active regions are usually confined to very small portions of the wafer material.

active-high (1) a logic signal having its asserted state as the logic ONE state. (2) a logic signal having the logic ONE state as the higher voltage of the two states.

active-low (1) a logic signal having its asserted state as the logic ZERO state. (2) a logic signal having its logic ONE

state as the lower voltage of the two states; inverted logic.

actuator (1) a transducer that converts electrical, hydraulic, or pneumatic energy to effective motion. For example in robots, actuators set the manipulator in motion through actuation of the joints. Industrial robots are equipped with motors that are typically electric, hydraulic, or pneumatic. See also industrial robot. (2) in computers, a device, usually mechanical in nature, that is controlled by a computer, e.g., a printer paper mechanism or a disk drive head positioning mechanism.

ACTV See advanced compatible television.

acuity sharpness. The ability of the eye to discern between two small objects closely spaced, as on a display.

adaptability the capability of a system to change to suit the prevailing conditions, especially by automatic adjustment of parameters through some initialization procedure or by training.

adaptation layer control layer of a multilayer controller, situated above the direct control layer and—usually—also above the optimizing control layer, required to introduce changes into the decision mechanisms of the layer (or layers) below this adaptation layer; for example adaptation layer of the industrial controller may be responsible for adjusting the model used by the optimizing control and the decision rules used by the direct (regulation) control mechanisms.

adapter a typical term from personal computers. A circuit board containing the interface toward an additional peripheral device. For example, a graphic adapter (interface boards like EGA, VGA, CGA), a game controller, a SCSI controller, a PCMCI interface, etc.

adaptive algorithm (1) a method for adjusting the parameters of a filter to satisfy an objective (e.g., minimize a cost function). (2) an algorithm whose properties are adjusted continuously during execution with the objective of optimizing some criterion.

adaptive antenna antenna, or array of antennas, whose performance characteristics can be adapted by some means; e.g., the pattern of an array can be changed when the phasing of each of the array elements is changed.

adaptive array an array that adapts itself to maximize the reception of a desired signal and null all interfering or jamming signals. This is achieved by finding the correct weights (input excitations) to the elements comprising the array.

adaptive coding a coding scheme that adapts itself in some fashion to its input or output.

adaptive coding of transform coefficients coding technique that is carried out by threshold sampling and exploiting masking effects by variable quantization for different blocks. High detail blocks are coded with more quantization error than low detail blocks. This is done to take into account masking and boundary distortion effects. Transform coding becomes more attractive compared with DPCM when adaptive coding is used. The main drawback of adaptive transform coding is its sensitivity to transmission bit errors due to synchronization problems at the decoder. See also DPCM.

adaptive control a control methodology in which control parameters are continuously and automatically adjusted in response to be measured/estimated process variables to achieve near-optimum system performance.

adaptive critic learning technique where the system learns to evaluate the actions of a system (usually a controller) so as to provide a reinforcement signal that is an estimate of the future value of the system’s current action.

adaptive differential pulse code modulation (ADPCM) a modulation scheme in which only the difference between successive signal samples is encoded for transmission, and the quantization of the coding is adapted to the characteristics of the signal source.

adaptive filtering a filtering strategy in which filter coefficients or governing parameters evolve over time according to some updating strategy to optimize some criterion.

adaptive FIR filter a finite impulse response structure filter with adjustable coefficients. The adjustment is controlled by an adaptation algorithm such as the least mean square (LMS) algorithm. They are used extensively in adaptive echo cancellers and equalizers in communication systems.

adaptive fuzzy system fuzzy inference system that can be trained on a data set through the same learning techniques used for neural networks. Adaptive fuzzy systems are able to incorporate domain knowledge about the target system given from human experts in the form of fuzzy rules and numerical data in the form of input–output data sets of the system to be modeled. See also neural network, fuzzy inference system.

adaptive intrafield predictors a technique used for picture signal prediction based on local properties of the signal or side information if portions of local properties have not been transmitted. Intrafield methods require correlation with local information for prediction purposes. A common technique is to use a measure of the directional orrelation based on local pixels that have already been transmitted. A predictor is chosen from a set to give minimum prediction error. For example, the previous line or previous pixel can be used for prediction, and the switching can then be done as follows: \X D predictor for element X D _ A ifkB − Ck < kA – Bk C otherwise An extension of this concept is called contour prediction where the direction of pixelA is determined by searching among E, B, C, or G.

adaptive logic network tree-structured network whose leaves are the inputs and whose root is the output. The first hidden layer consists of linear threshold units and the remaining layers are elementary logic gates, usually AND and OR gates. Each linear threshold unit is trained to fit input data in those regions of the input space where it is active (i.e., where it contributes to the overall network function).

adaptive manipulator controller a controller that uses an adaptation process which, based on observation of the manipulator position and velocity, readjusts the parameters in the nonlinear model until the errors disappear. An adaptive manipulator controller is depicted in the figure below. Such a system would learn its own dynamic properties. The adaptive manipulator control scheme Adaptive manipulator control scheme. presented in the figure belongs to the joint space control schemes. See also joint space control.

adaptive predictor a digital filter whose coefficients can be varied, according to some error minimization algorithm, such that it can predict the value of a signal say N sampling time intervals into the future. The adaptive

predictor is useful in many interference cancellation applications.

adaptive resonance theory (ART) network A clustering network developed to allow the learning of new information without destroying what has already been learnt. Each cluster is represented by a prototype and learning

is achieved by comparing a new input pattern with each prototype. If a prototype is found that is acceptably close to that input, the new pattern is added to that prototype’s cluster and the prototype is adjusted so as to move closer to the new input. If no prototype is acceptable, the pattern becomes a new prototype around which a new cluster may develop.

adaptive vector quantization term that refers to methods for vector quantization that are designed to adaptively rack changes in the input signal.

ADC See analog-to-digital converter.

ADCPM See adaptive differential pulse

code modulation.

add instruction a machine instruction that causes two numeric operands to be added together. The operands may be from machine registers, memory, or from the instruction itself, and the result may be placed in a machine register or in memory.

adder a logic circuit used for adding binary numbers.

additive acousto-optic processing acousto-optic signal processing where the summation of acousto-optic modulated light waves is used to implement the signal processing operation.

additive polarity polarity designation of a transformer in which terminals of the same polarity on the low- and high-voltage coils are physically adjacent to each other on the transformer casing. With additive polarity, a short between two adjacent terminals results in the sum of the two coil voltages appearing between the remaining terminals. Additive polarity is generally used for transformers up to 500kVA and 34.5kV. Larger units use subtractive

polarity. See the diagram below. See also subtractive polarity.

additive white Gaussian noise (AWGN) the simplest form of channel degradation in a communication system in which the source Transformer with additive polarity. of errors in the channel can be modeled as the addition of random noise with a Gaussian distribution and a constant (white) power spectrum. See also thermal noise.

address a unique identifier for the place where information is stored (as opposed to the contents actually stored there). Most storage devices may be regarded by the user as a linear array, such as bytes or words in RAM or sectors on a disk. The address is then just an ordinal number of the physical or logical position. In some disks, the address may be compound, consisting of the cylinder or track and the sector within that cylinder. In more complex systems, the address may be a “name” that is more relevant to the user but must be translated by the underlying software or hardware.

address aliasing See cache aliasing.

address bus the set of wires or tracks on a backplane, printed circuit board, or integrated circuit to carry binary address signals between different parts of a computer. The number of bits of address bus (the width of the bus) determines the maximum size of memory that can be addressed. Modern microchips have 32 address lines, thus 4 gigabytes of main memory can be accessed.

address decoder logic that decodes an address. 1. A partial decoder responds to a small range of addresses and is used when recognizing particular device addresses on an I/O address bus, or when recognizing that addresses

belong to a particular memory module. 2. A full decoder takes N bits and asserts one of 2N outputs, and is used  ithin memories (often within RAM chips themselves).

address error an exception (error interrupt) caused by a program’s attempt to access unaligned words or long words on a processor that does not accommodate such requests. The address error is detected within the CPU. This ontrasts with problems that arise in accessing the memory itself, where a logic circuit external to theCPUitself must detect and signal the error to cause the CPU to process the exception. Such external problems are called bus errors.

address field the portion of a program instruction word that holds an address.

address generation interlock (AGI) a mechanism to stall the pipeline for one cycle when an address used in one machine cycle is being calculated or loaded in the previous cycle. Address generation interlocks cause the CPU to be delayed for a cycle. (AGIs on the Pentium are even more important to remove, since two execution time slots are

lost).

address locking a mechanism to protect a specific memory address so that it can be accessed exclusively by a single processor.

address map a table that associates a base address in main memory with an object (or page) number.

address mapping the translation of virtual address into real (i.e., physical) addresses for memory access. See also virtual memory.

address register a register used primarily to hold the address of a location in memory. The location can contain an operand or an executable instruction.

address size prefix a part of a machine instruction that provides information as to the length or size of the address fields in the instruction.

address space an area of memory seen or used by a program and generally managed as a continuous range of addresses. Many computers use separate address spaces for code and data; some have other address spaces for system. An address space is usually subject to protection, with references to a space checked for valid addresses and access (such as read only). The physical address space of a computer (232 bytes, and up to 264 bytes) is often larger

than the installed memory. Some parts of the address range (often at extreme addresses) may be reserved for input–output device addresses. See also byte, memory, memory mapped I/O.

address translation See address mapping.

addressing (1) in processors: a mechanism to refer to a device or storage location by an identifying number, character, or group of characters. That may contain a piece of data or a program step. (2) in networks, the process of identifying a network component, for instance, the unique address of a node on a local area network.

addressing fault an error that halts the mapper when it cannot locate a referenced object in main memory.

addressing mode a form of specifying the address (location) of an operand in an instruction. Some of the addressing modes found in most processors are direct or register direct, where the operand is in aCPUregister; register indirect (or simply indirect), where a CPU register contains the address of the operand in memory; immediate, where the

operand is a part of the instruction. See also central processing unit.

addressing range numbers that define the number of memory locations addressable by the CPU. For a processor with one address space, the range is determined by the number of signal lines on the address bus of the CPU.

adequate service in terms of the blocking probability, term associated with a fixed blocking. A typically quoted value may be 2. See also blocking.

adiabatic a system that has no heat transfer with the environment.

adiabatic cooling a process where the temperature of a system is reduced without any heat being exchanged between the system and its surroundings. In particle beam acceleration this term is used to describe the process in the particle source storage ring where beam emittances are reduced without affecting beam energy.

adiabatic following an approximation made when some states in a quantum mechanical system respond to perturbations more quickly than the other states. In this approximation the rapidly responding states are assumed to depend only on the instantaneous values of the other states and are said to “follow” those states.

adiabatic passage a technique for the creation of a long-lived coherence in a quantum mechanical system by manipulating electromagnetic field intensities so that the system always remains in an eigenstate. In practice, this involves changing field strengths on a time scale slower than the inverse of the energy spacing between relevant eigenstates of the system. For example, consider a lambda system in which only one field is present initially and all population starts out in the uncoupled ground state. If a field is gradually turned on to couple this initial state to the excited state, the system can remain transparent by evolving in such a way that it is always mathematically equivalent to the dark state that would be produced by coherent population trapping. Adiabatic passage is often

used for selective transfer of population between two long-lived states of a multistate system, especially in cases where the twostep process of absorption followed by spontaneous decay (optical pumping) would tend to populate many other states.

adjacency graph a graph in which each node represents an object, component, or feature in an image. An edge between two nodes indicates two components that are touching or connected in the image.

adjacent channel interference (ACI) the interference caused by an adjacent frequency band, e.g., in a system with frequency division duplex (FDD). Classified as either inband or out-of-band adjacent channel interference (ACI). The in-band ACI occurs when the center frequency of interfering signal falls within the band of the desired signal. The out-of-band ACI occurs when the center frequency of interfering signal falls outside the bandwidth of the desired signal.

adjacent channel leakage power See adjacent channel power.

adjacent channel power (ACP) a power of distortion components generated in adjacent channel, which is caused by a nonlinearity of high-power amplifier amplifying a digitally modulated signal such as QPSK, QAM, etc. Adjacent channel power is defined as a ratio of signal power in channel and leakage power in adjacent channel.

adjacent channel reuse ratio (ACRR) the reuse ratio between radio communication cells using adjacent radio channels. See also reuse ratio.

adjacent channels radio chanels occupying radio frequency allocations n and n_1.

adjoint network a network with an identical structure to the original one, but with possibly different elements. As an example, for a network described by the nodal admittance  matrix, its adjoint network is represented by the transposed admittance matrix of the original network. The adjoint network is a basic tool in the computer-aided sensitivity analysis of electronic and microwave circuits.

adjustable-speed drive See variable speed DC drive, variable speed AC drive.

admissible matrix a matrixM − that can be obtained by fixing the free parameters of the matrix M at some particular values. M − is said to be admissible with respect to M.

admittance the reciprocal of the impedance of an electric circuit.

admittance inverter an idealized device or set of matrix parameters that functions electrically like a quarter-wave lossless transmission line of characteristic impedance J at each frequency, thus transforming the load admittance (YLOAD) by C90 degrees and modifying the magnitude, resulting in an input admittance (Yin). Yin D J 2 Yload

admittance matrix the inverse of the impedance matrix in the method of moments.

ADP Seeammoniumdihydrogen phosphate.

ADPCM See adaptive differential pulse code modulation.

ADSL See asymmetric digital subscriber line.

adsorbent the material of an adsorber, for example, silica gel, alumina, and charcoal. Adsorbent materials are characterized by high surface to volume ratio.

adsorber (1) condensation of a gas on the solid material. (2) material that attracts and holds (byVan der Waal forces) molecular layers of dense gases (i.e., very near condensation temperatures) on porous high surface/volume ratio materials.

ADTV See advanced digital television.

advanced compatible television (ACTV) an extended definition television system that can operate with existing bandwidths on existing receivers and is compatible with the NTSC broadcasting system. The ACTV system was proposed by the Advanced Television Research Consortium and was the first high definition television (HDTV) system. HDTV system was tested by the FCC July 17, 1992. The additional picture information needed to increase the picture width and to increase the resolution to the HDTV format is transmitted in an augmented channel as an alternative to simulcast transmission. See Advanced Television Research Consortium.

advanced digital television (ADTV) a high definition television (HDTV) digital transmission television system was proposed to the Federal Communications Commission by the Advanced Television Research Consortium.

The ADTV system introduced a layered system to separately describe the digital transmission system, the video compression system, and the data packet transport system. The video compression method uses a MPEGCC standard that provides for compatibility with multimedia computing. See Advanced Television Research Consortium.

advanced mobile phone system (AMPS) a standard for a cellular radio communications network originally developed in the 1970s by AT&T and later adopted as an industry standard by the U.S.-based Telecommunications

Industries Association (TIA). It is the first cellular standard widely deployed in North America. It is also referred to as the analog cellular system. Frequency modulation with 30 kHz channels is used.

Advanced Television Research Consortium an organization consisting of David Sarnoff Research Center, Thompson Consumer Electronics, North American Philips Corporation, NBC, and Compression Laboratories.

aeolian vibration a high-frequency mechanical vibration of electric power lines caused by wind.

aerial cable any fully-insulated electric power cable which is carried overhead upon poles, as opposed to the use of the more usual overhead bare conductors.

aerodynamic head See disk head.

AFC See automatic frequency control.

affine transform a geometric image transformation including one or more translations, rotations, scales, and shears that is represented by a 4_4 matrix allowing multiple geometric transformations in one transform step. Affine transformations are purely linear and do not include perspective orwarping transformations.

AFM See atomic force microscope.

AFT See automatic fine tuning.

AFV See audio follow-video switcher.

AGC See automatic gain control or

automatic generation control.

agent a computational entity that acts on behalf of other entities in an autonomous fashion.

agent-based system an application whose component are agents. See also agent.

aggregation an operation performed on system variables whose purpose is to collect them in a way enabling order and/or uncertainty reduction. For linear systems both continuous-time and discrete-time state aggregation is obtained by linear transformation of the original state represented by an aggregation matrix G endowed with the following properties: GA D A _ GIGB D B _ICG 0 D C _I where A;B;C are original system matrices (respectively state, input, and output ones) and A _ ;B _ ;C _ are aggregated system matrices. The aggregation is an eigenvaluespreservation

approach and it provides order reduction by neglecting some of the system modes. For uncertainties, the aggregation defines some deterministic measures for a set of uncertain variables. For stochastic model of uncertainty the aggregation may be given by mean value, higher stochastic models or other statistical characteristics, while set membership uncertainties could be aggregated by their maximal or minimal values, mass center of the set or higher inertial moments.

AGI See address generation interlock.

Aiken, Howard Hathaway (1900–1973) Born: Hoboken, New Jersey, U.S.A. Aiken is best known as the inventor of the Mark I and Mark II computers. While not commercially successful, these machines were significant in the development of the modern computer. The Mark I was essentially a mechanical computer. The Mark II was an electronic computer. Unlike UNIVAC ( See Eckert, John Presper) these machines had a stored memory. Aiken was a professor of mathematics at Harvard. He was given the assignment to develop these computers by the Navy department. Among his colleagues in this project were three IBM scientists and Grace Hopper. Itwas whileworking

on the Mark I that Grace Hopper pulled the first “bug” from a computer.

air bridge a bridge made of metal strip suspended in air that can connect components on an integrated circuit in such a way as to cross over another strip. Air bridges are also used to suspend metalization in spiral inductors off of the semi-conducting substrate in a way that can lead to improved performance in some cases.

air capacitor a fixed or variable capacitor in which air is the dielectric material between the capacitor’s plates.

air circuit breaker a power circuit breaker where the power contacts operate in air. Some versions employ an air blast to extend and clear the arc on contact opening, while others employ arc chutes with magnetic or thermal assists.

air core transformer two or more coils placed so that they are linked by the same flux with an air core. With an air core the flux is not confined.

air gap See magnetic recording air gap.

air ionization chamber a device used to monitor neutron flux.

air line a coaxial transmission line in which the volume between the inner and outer conductors are air-filled.

air terminal a lightning rod; any device which extends upward into the air from a structure for purposes of lightning protection.

air-blast circuit breaker a circuit breaker in which the arc which forms between the contacts on opening is extinguished with a blast of high-pressure air.

air-gap line the line that is obtained by continuing the linear portion of the saturation curve of a synchronous machine or a DC machine. The figure shows a plot of generated voltage vs. field current at constant machine

speed. Initially, an increase in field current yields a linear increase in the generated voltage, but as the iron becomes saturated, the voltage rolls off. The air-gap line gives the  Plot of generated voltage vs. field current at constant machine speed. voltage that would be obtained without saturation.

air-gap voltage the internal voltage of a synchronous machine that is generated by the air gap flux. Also referred to as the voltage behind leakage reactance.

airline a precision coaxial transmission line with air dielectric used in a variety of calibration techniques and measurements as an impedance standard and to establish a reference plane.

airy disk the central portion of the farfield optical diffraction pattern.

AlAs aluminum arsenide.

albedo the ratio between the total scattered intensity and the whole extracted from the incident light by scattering and absorption.

ALC See automatic level control.

AlGaAs symbol for aluminum gallium arsenide.

algebraic reconstruction the process of reconstructing an image x from a noisecorrupted and blurred image y. An arbitrary image is selected as the initial condition of an iterative algorithm for solving a set of linear equations. A set of linear constraints is specified. In each iteration one constraint is applied to a linear equation. The constraints are repeated in a cyclic fashion until convergence is reached. The linear constraints are vectors in a vector space with specified basis images for the type of problem to be solved.

algorithm (1) a systematic and precise, step-by-step procedure (such as a recipe, a program, or set of programs) for solving a certain kind of problem or accomplishing a task, for instance converting a particular kind of input data to a particular kind of output data, or controlling a machine tool. An algorithm can be executed by a machine. (2) in image processing, algorithms can be either sequential, parallel, or ordered. In sequential algorithms, pixels are scanned and processed in a particular raster-scan order. As a given pixel is processed, all previously scanned pixels have updated (processed) values, while all pixels not yet scanned have old (unprocessed) values. The algorithm’s result will in general depend on the order of scanning. In a parallel algorithm, each pixel is processed independently of any changes in the others, and its new value is written in a new image, such that the algorithm’s result does not depend on the order of pixel processing. In an ordered algorithm, pixels are put in an ordered queue, where priority depends on some value attached to each pixel. At each time step, the first pixel in the queue is taken out of it and processed, leading to a possible modification of priority of pixels in the queue. By default, an algorithm is usually considered as parallel, unless stated otherwise.

algorithmic state machine (ASM) a sequential logic circuit whose design is directly specified by the algorithm for the task the machine is to accomplish.

aliasing (1) in signal processing, distortion introduced in a digital signal when it is undersampled. In all digital systems the signals should be filtered before they are sampled to eliminate signal components with frequencies above the Nyquist frequency, !N D !s=2 D _=T ; where T is a sampling time, are eliminated. If this filtering is not done, signal components with frequencies  ! > !N will appear as low-frequency components with the frequency

!a D j..! C !N/ mod !s / − !Nj The prefilters introduced before a sampler are called anti-aliasing filters (common choices are second- or fourth-order Butterworth, integral time absolute error (ITAE), or Bessel filters). (2) in computer graphics, distortion due to the discrete nature of digital images that causes straight lines to appear jagged.

(3) in computer software, a single object having two different identities, such as names in memory space. Aliasing can make it difficult to determine whether two names (or access paths to reach an object) that appear to be different really access the identical object; a system designed to find parallelism when two accesses really reach different objects will have trouble achieving correct (functional) operation if aliasing is present.

alignment (1) the requirement that a datum (or block of data) be mapped at an address with certain characteristics, usually that the address modulo the size of the datum or block be zero. For example, the address of a naturally aligned long word is a multiple of four. (2) the act of positioning the image of a specific point on a photomask to a specific point on the wafer to be printed. (3) the process of determining the time or phase shift of a certain signal so that part of it may be matched with another signal. See also image registration.

all-digital synchronization synchronization algorithm, where the analog-to-digital conversion takes place as early as possible to assist digital implementation of the synchronizer. In most cases, an all-digital synchronization approach leads to optimal maximum likelihood algorithms.

all-optical network an optical communications network where the role of electronics is reduced to basic supervisory and control functions. All-optical devices are used exclusively between the nodes to re-configure the network which enables the greatest use of fiber bandwidth.

all-optical switch an optically addressed device whose optical transmission can be switched between two possible states by changes in the incident optical power.

all-pass system a system with unit magnitude and poles and zeroes that are complex conjugate reciprocals of each other. An allpass system with a pole at z D a and a zero at z D 1 a _ is Hap.z/ D z −1 – a _ 1 − az−1 :

alley arm a crossarm meant for use in an alleyway or other confined area in which poles must be placed close to buildings. See crossarm.

allocate to create a block of storage of a given size in some memory, which is not to be used for any other purpose until expressly freed.

allocation the act of allocating. See also allocate.

allocation of authority process by which the authority (scope of competence) is allocated to various decision units; this allocation may result form the natural reasons or be a product of system partitioning.

almost sure convergence for a stochastic process, the property of the sample values converging to a random variable with probability one (for almost all sample paths).

alnico a permanent magnet material consisting mainly of aluminum, nickel, cobalt, and iron, which has a relatively low-energy product and high residual flux density. An alnico is most suitable for high-temperature applications.

ALOHA a random access, multiple access protocol, originally developed by Norman  Abramson at the University of Hawaii in 1970. Agiven user transmits a message when the message is generated without regard for coordination with the other users sharing the channel. Messages involved in collisions are retransmitted according to some retransmission algorithm. Literally, “aloha” is a greeting in the Hawaiian native language.

alpha channel a grayscale image associated with the color channels of an image that dictates the opacity/transparency of the corresponding color channel pixels. If the color channels are multiplied by the alpha channel when stored, the image is referred to as premultiplied; otherwise, it is known as unpremultiplied.

alpha particle a subatomic particle emitted by ceramic packaging materials that causes soft errors in memory integrated circuits.

alpha particle noise this type of noise occurs exclusively in small semiconductor capacitors, when an energetic alpha particle, either from cosmic rays or from the packaging or substrate itself, traverses the capacitor, discharging it, thereby creating an error in the stored charge. Such an accumulation of errors in a digital system has the effect of creating a noise signal.

alpha-cut the set of all crisp, or nonfuzzy, elements whose membership function in A is greater than or equal to a given value, _.

alphanumeric mode relates to alphabetic characters, digits, and other characters such as punctuation marks. Alphanumeric is a mode of operation of a graphic terminal or other input/output device. The graphics terminal should toggle between graphic and alphanumeric data.

alternate channel power a measure of the linearity of a digitally modulated system. The amount of energy from a digitally transmitted RF signal that is transferred from the intended channel to one which is two channels away. It is the ratio (in decibels) of the power measured in the alternate channel to the total transmitted power.

alternating current (AC) a periodic current the average value of which over a period is zero.

alternating current machine an electromechanical system that either converts alternating current electrical power into mechanical power (AC motor), or converts mechanical power into alternating current electrical power (AC generator, or alternator). Some AC machines are designed to perform either of these functions, depending on the energy source to the dynamo.

alternator-rectifier exciter a source of field current of a synchronous machine derived from the rectified output voltage of an alternator. The components of the exciter consist of the alternator and the power rectifier (including possible gate circuitry), exclusive of all input control elements. The rectifier circuits may be stationary, or rotate

with the alternator, which may be driven by a motor, prime mover, or by the shaft of the synchronous machine.

ALU See arithmetic and logic unit.

AM See amplitude modulation.

AM to PM conversion phase variations of an output signal, due to passing through an active device, where the phase of the output signal varies in response with the amplitude of the input signal.

AMvideo the amplitude modulated video carrier wave is produced by an amplitude modulated video transmitter where the amplitude of the wave form varies in step with the video signal similar to that shown in the figure.

amateur radio The practice and study of electronic communications as an avocation; most often referring to those persons possessing a license earned by examination (in the U.S., the Federal Communications Commission

grants such licenses).

ambient field the background magnetic field level existing in the environment, without contribution from specific magnetic field sources.

ambient temperature the temperature of the air or liquid surrounding any electrical part or device. Usually refers to the effect of such temperature in aiding or retarding removal of heat by radiation and convection from the part or device in question.

ambiguity in artificial intelligence, the presence of more than one meaning or possibility.

Amdahl’s law states that the speedup factor of a multiprocessor system is given by S.n/ D n 1 C .n − 1/f where there are n processors and f is the fraction of computational that must be performed sequentially (by one processor alone). The remaining part of the computation is assumed to be divided into n equal parts each executed by a separate processor but simultaneously. The speedup factor tends to 1=f as n!1, which demonstrates that under the assumptions given, the maximum speedup is constrained by the serial fraction.

American National Standards Institute (ANSI) The U.S. organization that recommends standards for metrology, drawing symbology and numerous other facets for products and industries.

American standard code for information interchange (ASCII) a binary code comprised of seven digits, originally used to transmit telegraph signal information.

ammeter an instrument for measuring electric current in amperes.

ammonia maser first maser, invented by Charles H. Townes. Such a maser operates at microwave frequencies.

ammonium dihydrogen phosphate (ADP) a strong linear electro-optic material. Its chemical formula is NH4H2PO4. See also potassium dihydrogen phosphate (KDP).

amorphous alloy a ferromagnetic material with very low coercive force (i.e., a narrow hysteresis loop). The material is formed as a very thin ribbon, by freezing the molting alloy before it can crystallize, thus providing a random molecular orientation.

amortisseur winding See damper winding.

ampacity the maximum current which can be safely carried by a conductor under specified conditions.

ampere interrupting rating the interrupting rating of a device expressed in amps (often rms symmetrical amps). See also MVA interrupting rating.

Ampere’s Law a fundamental relationship in electromagnetic theory. In a fairly general form it is expressed by one of Maxwell’s equations, r _H.r; t/ D @D.r; t/ @t C J.r; t/ where t is the time, r is the coordinate vector, and the other vectors are defined as D.r; t/ electric displacement;H.r; t/, magnetic field strength; J.r; t/, electric current density. Ampere, Andre Marie (1775–1836) Born: Lyon, France Ampere is best known for his pioneering work in the field of Electrodynamics. During his emotionally troubled life, he held several professorships: at Bourg, Lyon, and at the Ecole Polytechnic in Paris. While Ampere worked in several sciences, the work of the Danish physicist Hans Christian Oerstad on the electric deflection of a compass needle, as demonstrated to him by Dominique Arago,

caused Ampere’s great interest in electromagnetism. His seminal work, Notes on the Theory of Electrodynamic Phenomena Deduced Solely from Experiment, established the mathematical formulations for electromagnetics

including what is now known as Ampere’s Law. It can be said that Ampere founded the field of electromagnetics. He is honored for this by the naming of the unit of electric current as the ampere.

amperometric sensor an electrochemical sensor that determines the amount of a substance by means of an oxidation– reduction reaction involving that substance. Electrons are transferred as a part of the reaction, so that the electrical current through the sensor is related to the amount of the substance seen by the sensor.

amplidyne a special generator that acts like a DC power amplifier by using compensation coils and a short circuit across its brushes to precisely and fastly control high powers with low level control signals.

amplified spontaneous emission spontaneous emission that has been enhanced in amplitude and perhaps modified in spectrum by propagation through an amplifying medium, usually the medium in which it was first generated.

amplifier a circuit element that has a linear input-output signal relationship, with gain in voltage, current, and/or power. See also balanced amplifier, feedback amplifier, feedforward amplifier, laser amplifier, maser amplifier, optical amplifier, single-ended amplifier.

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