Electrical Engineering Dictionary - part 145

random access memory (RAM)

direct-

access read/write storage in which each ad-
dressable unit has a unique hardwired ad-
dressing mechanism. The time to access a
randomly selected location is constant and
not dependent on its position or on any previ-
ous accesses. The RAM has a set of k address
lines (

m = 2

), n bidirectional data lines, and

a set of additional lines to control the direc-
tion of the access (read or write), operation
and timing of the device.

RAM is commonly used for the main

memory of a computer and is said to be static
if power has to be constantly maintained in
order to store data and dynamic if periodic
absences of power do not cause a loss of
data. RAM is usually volatile. See also

nonvolatile random-access

memory

random behavior

response without a

spectral or amplitude pattern or relationship
to the excitation. The excitation may be in-
ternal, and may be thermal in nature.

random coding

coding technique in

which codewords are chosen at random ac-
cording to some distribution on the codeword
symbols. Commonly a tool used in the devel-
opment of information theoretic expressions.

random logic

a digital system con-

structed with logic gates and flip-flops
and other basic logic components intercon-
nected in an non-specific manner. See also

microprogramming

random process

a mathematical pro-

cedure for generating random numbers to
a specific rule called a process,

x which

is defined on continuous

x(t), t ∈ R

or discrete

x(k), k ∈ Z

space / time.

The value of the process at each point in
space or time is a random vector. See also

random variable

random vector

See

also

random replacement algorithm

in a

cache or a paging system, an algorithm that
chooses the line or page in a random man-
ner. A pseudo-random number generators
may be used to make the selection, or other
approximate method. The algorithm is not
very commonly used, though it was used in
the translation buffers of the VAX11/780 and
the Intel i860 RISC processor.

random signal

a signal

X(t) that is either

noise

N(t), an interfering signal s(t), or a

sum of these:

X(t) = s

(t)+· · ·+s

(t)+N

(t)+· · ·+N

(t)

random testing

the process of testing us-

ing a set of pseudo-randomly generated pat-
terns.

random variable

a continuous or discrete

valued variable that maps the set of all out-
comes of an experiment into the real line (or
complex plane). Because the outcomes of
an experiment are inherently random, the fi-
nal value of the variable cannot be predeter-
mined.

random vector

a vector (typically a col-

umn vector) of random variables. See also

random variable

random process

randomized decision rule

a hypothesis

decision/classification rule that is not deter-
ministic (that is, the measurement or obser-
vation does not uniquely determine the de-
cision). Although typically not useful given
continuous observations, a randomized rule
can be necessary given discrete observations.
See also

receiver

operating characteristics

curve

range filter

an edge detection filter that

finds edges by taking the difference between
the maximum and minimum values in a lo-
cal region of the image. The range filter also
accepts a weight mask the size of the local
image region that controls pixel values be-
fore they enter the minimum and maximum

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calculations. The weight mask allows edges
in certain directions to be searched for.

range image

an image in which the in-

tensity of point x is a function of the distance
between x and the corresponding point in the
scene (object) projected on the image plane.

range of Jacobian

denoted

J , it is defined

as a subspace

R(J ) in R

of the end-effector-

velocities that can be generated by the joint
velocities, in the given manipulator posture.
See

full rank Jacobian

rank filter

an image transform used in

mathematical morphology. Assume that to
every pixel

p one associates a window W(p)

containing it. Let

k be an integer > 1 which

is less than or equal to the size of each win-
dow

W(p). The rank filter with rank k and

windows

W(p) transforms an image I into

a filtered image

whose gray-level

(p)

at pixel

p is defined as the k-th least value

among all initial gray-levels

I (q) for q in the

window

W(p). In a dual version, the k-th

greatest value is selected. When each

W(p)

is the translate by

p of a structuring element

W of size n, three particular cases are note-
worthy:

k = 1: the rank filter is the erosion by W.

k = n: the rank filter is the dilation by e

the symmetric of

n is odd and k = (n + 1)/2: the rank

filter is a median filter. See

rare gas

one of the rare gases specified in

the periodic table, He, Ne, Ar, Kr, Xe.

rare gas halides

excimer molecule

formed by a reaction of a rare gas atom and
a halogen atom: e.g., XeF, ArF, KrF.

rare gas molecule

excimer molecule

formed by a reaction of an excited and a neu-
tral rare gas atom, usually with a third atom
present. For example, Ar

, Kr

rare gas oxides

an excimer molecule

formed by the reaction of a rare gas atom
and an oxygen atom.

rare-earth magnet

a magnet that has any

of the rare-earth elements in its composition.
Typically stronger than other magnet materi-
als, these include neodymium iron boron and
samarium cobalt.

rare-earth permanent magnet

magnet

made of compounds of iron, nickel, and
cobalt with one or more of the rare-earth el-
ements such as samarium. These materials
combine the high residual flux density of the
alnico-type materials with greater coercivity
than ferrites.

raster

a predetermined pattern of scan-

ning lines used to provide uniform coverage
of the area used for displaying a television
picture.

raster coordinates

coordinates in a dis-

play system that specifically identify a phys-
ical location on the display surface.

raster graphics

a computer graphics sys-

tem that scans and displays an image peri-
odically in a raster, or left-to-right, top-to-
bottom fashion.

raster image

See

bitmapped image

raster width

(1) physical distance be-

tween raster lines on a display surface and
between distinguishable points in the same
raster line; the two distances are frequently
different.

(2) the physical distance between raster

lines on a display surface and between dis-
tinguishable points in the same raster line.
The two distances are frequently different.

rate distortion function

the minimum

rate at which a source is represented by one
of a set of discrete points.

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rate distortion theory

a theory aimed

at quantifying the optimum performance of
source coding systems. Using information
theory, for several source models and distor-
tion measures, rate distortion theory provides
the optimum distortion function and the op-
timum rate function. The distortion function
is optimum in that the distortion for a given
rate is the theoretical minimum value of dis-
tortion for encoding the source at the given or
lower rate. The rate function is optimum in
that the rate at a given distortion is the min-
imum possible rate for coding the source at
the given or lower distortion.

rate equation approximation

assump-

tion, in a semiclassical model for the inter-
action of light with atoms, that all fields and
populations change negligibly within the co-
herence time of the wave functions, loses all
information about the phase of the fields and
wave functions.

rate equation model

model for the in-

teraction of light with atoms in which the
atoms are represented only by their popula-
tions or population densities and the electro-
magnetic field is represented only by its in-
tensity, power, energy, or photon density.

rate equations

coupled ordinary nonlin-

ear differential equations governing the in-
teraction of an electromagnetic field (repre-
sented by an intensity, energy density, or pho-
ton density) with an atomic or molecular laser
medium (represented by populations of the
energy states); phase information relating to
the fields and wavefunctions is absent from
these equations.

rate split multiple access

coding tech-

nique for the multiple-access channel in
which each user splits their information
stream into two or more streams, which
are independently encoded. These encoded
streams are multiplexed according to some
rule prior to transmission. Used to show that
time-sharing is not required to achieve cer-

tain points in the capacity region of multiple
access channels.

rate-adaptive

digital

subscriber

line

(RDSL)

a digital subscriber line (DSL)

in which the rates in each direction are ad-
justed according to the quality of the channel.
In general, longer loops are associated with
lower rates.

rate-compatible punctured convolutional
(RCPC) code

one of a family of punc-

tured convolutional codes derived from one
low-rate convolutional parent code by suc-
cessively increasing the number of punctured
symbols, given that the previously punc-
tured symbols should still be punctured (rate-
compatibility). These codes have applica-
tions in, for example, variable error protec-
tion systems and in hybrid automatic repeat
request schemes using additional transmitted
redundancy to be able to correctly decode a
packet. Also called RCPC code.

rate-distortion theory

Claude Shannon’s

theory for source coding with respect to a
fidelity criterion, developed during the late
1940s and the 1950s.

Can be viewed as

a generalization of Shannon’s earlier theory
(late 1940s) for channel coding and infor-
mation transmission. The theory applies to
the important methods for vector quantiza-
tion and predicts the theoretically achievable
optimum performance.

rated voltage

the voltage at which a power

line or electrical equipment is designed to op-
erate.

ratio detector

a circuit for recovering

(demodulating) baseband information (usu-
ally audio) from a frequency modulated (FM)
wave.

rational function

a function that is the

ratio of two polynomials.

Rational func-

tions often arise in the solution of differential
equations by Laplace transforms. See also

Laplace transform

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ray

one of a family of lines (rays) used to

represent the propagation of an electromag-
netic wave; most useful in real media when
the wave amplitude varies slowly compared
to the wavelength (see geometrical optics).

ray equation

set of second-order differ-

ential equations governing the trajectory of a
light ray propagating along an arbitrary path.

ray optics

approximate representation of

electromagnetic wave propagation in terms
of light rays, most useful in real media when
the wave amplitude varies slowly compared
to the wavelength.

ray tracing

(1) a high-frequency elec-

tromagnetic analysis technique in which the
propagation path is modeled by flux lines or
“rays.” The ray density is proportional to
the power density, and frequently, bundles of
these rays are called ray tubes.

(2) a rendering technique in which the

paths of light rays reaching the viewpoint are
computed to obtain realistic images. Given
a 3-D description of a scene as a collection
of surfaces characterized by different optical
properties, rays are traced backward from the
viewpoint through the image plane until they
hit one of the surfaces or go off to infinity.

ray transfer matrix

real two-by-two ma-

trix governing the transformation of the ray
displacement and slope with respect to a fixed
axis.

Rayleigh criterion

a method of distin-

guishing between rough and smooth surfaces
in order to determine whether specular re-
flection will occur.

A surface is consid-

ered smooth if the phase difference between
waves reflected from the surface is less than
ninety degrees.

Rayleigh distribution

the probability dis-

tribution of the magnitude of a complex quan-
tity whose real and imaginary parts are inde-
pendent Gaussian random variables with zero
mean. Frequently used to approximate multi-

path fading statistics in non-line-of-sight mo-
bile radio systems.

Rayleigh length

distance over which the

spot size of a Gaussian beam increases from
its value at the beam waist to a value 2.5
larger; a measure of the waist size of a Gaus-
sian beam,

π times waist spot size squared

divided by wavelength; half of the confocal
parameter.

Rayleigh noise

the envelope of a zero

mean, wide-sense stationary, narrowband
Gaussian noise process. The probability den-
sity function of a sinusoid in narrowband
noise is a generalized Rayleigh distribution,

p(z) =

−

σ 2

z ≥ 0. Also known as a

Rician distribution.

Rayleigh scattering

(1) theory for the in-

teraction between light and a medium com-
posed of particles whose size is much smaller
than the wavelength. According to it, the
scattering cross section is proportional to the
fourth power of the wavelength of the scat-
tered light. This explains both the red and
blue colors of the sky.

(2) an intrinsic effect of glass that con-

tributes to attenuation of the guided optical
wave. The effect is due to random localized
variations in the molecular structure of the
glass which acts as scattering centers.

Rayleigh–Ritz procedure

a procedure

for solving functional equations. See also

moment method

Rayleigh-wing scattering of light

the

scattering of light with no change in central
frequency, and with moderate (of the order
of 10

1 Hz) broadening of spectrum of the

light. Rayleigh-wing scattering occurs when
light scatters from anisotropic molecules.

RBC

See

radiation boundary condition

RC time constant

the time needed for

signal traveling from an end to the other end
of a wire is constant when the wire and the

2000 by CRC Press LLC

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