Electrical Engineering Dictionary - part 85

a point where the power level is at or near the
output power.

high-level vision

the highest stage of vi-

sion, leading to the full extraction of the 3-D
information and to its exploitation for the de-
scription and understanding of the scene.

high-loss resonator

a resonator having a

high value of loss (usually diffraction loss)
per round trip; unstable resonator.

high-pass filter

a filter that has a trans-

fer function, or frequency response, whose
values are small for frequencies lower than
some intermediate frequency. A filter whose
impulse response is a high-pass signal.

high-pass signal

a signal whose Fourier

transform, or power spectral density, is a
small value for all frequency components that
are less than some intermediate frequency
value. As a formal definition, if

X(ω) is

the Fourier transform of the signal

x(t), then

X(ω) = 0 for |ω| < B, for some B > 0.
Compare with

band-pass signal

high-performance metal oxide semicon-
ductor (HMOS) technology

a variation

of the MOS process that scales down devices
to achieve higher speed and thus lower power
consumption in integrated circuits.

high-rate quantization theory

a theory

developed by Bennet, Schutzenberger, Zador
and others. The theory analyzes and predicts
the performance of (infinitely) fine quantiza-
tion (high-rate quantization).

high-resistance grounded system

electrical distribution system in which the
neutral is intentionally grounded through
a high resistance.

The high-resistance

grounded wye system is an alternative to
solidly grounded and ungrounded systems.
High-resistance grounding will limit ground
fault current to a few amperes, thus remov-
ing the potential for arcing damage inherent
in solidly grounded systems.

high-speed carry

in an arithmetic logic

unit, a carry signal that is generated sepa-
rately from the generation of the result and is
therefore faster; a lookahead carry.

high-speed digital subscriber line (HDSL)

a digital subscriber line (DSL) in which two

twisted-pairs provide a rate of 1.544 Mbps
(T1 rate) in both directions (full-duplex).
Each twisted-pair provides a full-duplex con-
nection of 784 kbps. (This is larger than
1.544/2 Mbps due to the duplication of a syn-
chronization bit).

high-speed metal oxide semiconductor
(HMOS) technology

See

high-perfor-

mance metal oxide semiconductor technology

high-stability diode gun Plumbicon

diode gun Plumbicon tube with electrostatic
focus and magnetic deflection, which uses a
high-stability electrode structure evaporated
and bonded to the tube envelope.

high-voltage DC (HVDC) transmission
transmission of electric power (at typically
500–1500 kV) using DC rather than AC. This
can be desirable for several reasons:

1. For economic reasons when a large

amount of power is to be transmitted over
a long distance, i.e., 300–400 miles, or via
underwater cables;

2. For the connection of asynchronous AC

systems; and

3. Improved transient stability and dy-

namic damping of the electrical system os-
cillations.

higher-order mode

mode that spatially

varies faster than the fundamental in a cavity,
or waveguide.

higher-order unit

a neural unit whose

input connections provide not the usual
weighted linear sum of the input variables but
rather a weighted polynomial sum of those
variables.

2000 by CRC Press LLC

highway

See

bus

Hilbert space

an inner product space with

the additional property that is a complete
metric space. An inner product space is a
linear space on which an inner product is de-
fined, while completeness means that there
are no “missing” vectors arbitrarily close to
but not included in the space.

Hilbert transform

a transform that re-

lates the real and imaginary parts of a com-
plex quantity, such as the index of refraction.
For

f , a function of one real variable, the

transform

H(f ) defined as follows:

H(f )(x) = lim

ε→0

|t|≥ε

f (x − t)

dt.

An alternate formula, which coincides with
the previous one, except possibly on a zero-
measure set of points of discontinuity of

f ,

is:

H(f )(x) = lim

ε→0

+∞

−∞

f (x−t)

+ ε

dt.

When

f is square-integrable, the Fourier

transforms

F(f ) and F(H(f )) of f and

H(f ) are related by:

F(H(f ))(u) = −i sign(u)F(f )(u)

almost everywhere; thus for positive frequen-
cies, the Fourier phase is shifted by

−π/2;

in particular,

f and H(f ) have the same L

norm. See

Fourier amplitude

Fourier phase

Fourier transform

histogram

(1) a plot of the frequency of

occurrence of the gray levels in an image.

(2) the frequency distribution of a set of

numbers. In image processing, the distribu-
tion of the gray levels in an image, typically
plotted as the number or percentage of pix-
els at a certain gray level vs. the gray levels.
If the ordinate is the ratio of the number of
pixels at a gray level to the total number of
pixels, the histogram is an approximation of
the probability density function of the gray
levels. See also

probability density function

histogram equalization

a technique for

computing an image gray level transforma-
tion to redistribute the pixel intensity values
and “flatten” the pixel intensity histogram.
Histogram equalization can be used to en-
hance the contrast of the image. Also called
histogram leveling, histogram flattening. See
also

contrast enhancement

histogram

histogram modeling

(1) making the his-

togram of an image have another shape, for
example, the flat line produced by histogram
equalization. This procedure is usually not
automatic, i.e., the shape is usually specified
by a person. Also called histogram modifi-
cation or histogram specification. See also

histogram equalization

(2) the fitting of a function to a histogram

in order to obtain an analytical expression
that approximates the histogram.

histogram sliding

the addition of a con-

stant to all pixels in an image to brighten or
darken the image. A positive constant makes
the histogram slide (translate) to the right; a
negative constant makes it slide to the left.
See also

histogram

histogram stretching

expansion or con-

traction of the gray-level histogram of an im-
age in order to enhance contrast. Usually per-
formed by multiplying or dividing all pixels
by some constant value. See also

contrast

enhancement

histogram

hit

the notion of searching for an address

in a level of the memory hierarchy and finding
the address mapped (and the data present) in
that level of the hierarchy. Often applies to
cache memory hierarchies.

hit rate

the percentage of references to

a cache that find the data word requested in
the cache. Also the probability of finding the
data and given by

number of times required word found in cache

total number references

Also known as hit ratio.

hit ratio

See

hit rate

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hit–miss ratio

in cache memory, the ratio

of memory access requests that are success-
fully fulfilled within the cache to those that
require access to standard memory or to an
auxiliary cache.

hit–miss transform

a class of transforms

in image processing that locate objects larger
than some specific size,

, and smaller than

some size

HL7

a data communications protocol for

interfacing components of a hospital infor-
mation system.

HMIC

See

hybrid microwave integrated

circuit

HMM

See

hidden Markov model

HMOS

See

high-performance metal

oxide semiconductor technology

hold signal

an input signal to a processor

indicating that a DMA device is requesting
the use of the bus. Also called bus request.

hold time

the time required for the data

input(s) to be held stable prior to (or after)
the control input changes to latch, capture, or
store the value indicated by the data inputs.

hold-up time

the time duration that the

output voltage is maintained, within speci-
fication, during an interruption of the input
power. Typical hold-up time is one to three
line cycles.

hole

fictitious positive charge representing

the motion of electrons in the valence band of
a semiconductor; the number of holes equals
the number of unoccupied quantum states in
the valence band.

hole burning

localized reduction in gain

of a laser amplifier due to saturation by an
intense signal; localization may be in space
or frequency.

hole mobility

See

spatial hole burning

spectral hole burning

Hollerith card

a punched card used for

data storage. Developed by Herman Hol-
lerith for use in the 1890 U.S. census, these
cards were widely used for early computer
data storage. Now obsolete.

Hollerith code

an encoding of data

where the data is represented by a pattern of
punched holes in a card.

Hollerith, Herman

(1860–1929) Born:

Buffalo, New York, U.S.A.

Hollerith is best known for his develop-

ment of a tabulating machine using punched
cards for entering data. This machine was
used to tabulate the 1890 U.S. census. The
success of this machine spurred a number of
other countries to adopt this system. The
company Hollerith formed to develop and
market his ideas became one of several com-
panies that merged to form IBM, Interna-
tional Business Machines. Hollerith’s initial
interest in automatic data processing prob-
ably stems from his work as an assistant to
several census preparers at Columbia Univer-
sity. The success of Hollerith’s work gives
him claim to the title of the father of auto-
mated data processing.

hollow beam

an electron beam with hol-

low core.

hollow cathode

a negative electrode

formed using a section of hollow tube; used
in low pressure discharges.

hologram

medium that, when illuminated

optically, provides a three-dimensional im-
age of stored information, sometimes called
holograph.

holograph

See

hologram

holographic data storage

a technique

used to store multiple images, or 2-D ar-
rays of digital information, as multiplexed

2000 by CRC Press LLC

holograms in an optically sensitive mate-
rial.

Multiplexing techniques include an-

gle, frequency, and spatial position. Angle
and frequency multiplexing techniques are
based on Bragg selectivity where angle mul-
tiplexing favors transmission geometries and
frequency multiplexing favors reflection ge-
ometries.

holographic interconnect

free-space in-

terconnect that uses holograms to control op-
tical paths from sources to detectors. The
hologram provides the interconnection by
forming the intentionally distorted image of
the source array on the detector array. A
hologram can be viewed as a combination of
gratings. Each grating directs a light beam
passing through it to a new direction. The
hologram can be displayed by a spatial light
modulator to achieve dynamic reconfigura-
tion.

holography

the science of making and

reading holograms.

homodyne detection

method of measur-

ing the frequency content of an optical signal
in which that signal is combined with itself
by means of a square-law detector; simpler
but sometimes less informative and sensitive
representation of the frequency content of a
signal than heterodyne detection.

homogeneous

having all nodes or ma-

chines in a multiprocessor or multicomputer
be of the same type or class.

homogeneous broadening

spectral broad-

ening of a transition in a laser medium due
to irreversible dephasing processes like spon-
taneous emission, collisions, photon interac-
tions, or spin exchange.

homogeneous coordinate representation

the representation of an

N–component po-

sition vector by an

(N + 1)–component vec-

tor is called homogeneous coordinate rep-
resentation.

For example, a vector

p =

, p

is represented by an aug-

mented vector

, wp

, w

in the

homogeneous coordinate representation. In
the homogeneous transformation matrix it is
assumed that

w = 1.

homogeneous coordinates

a 2-D point

(x, y) with a third coordinate point (x, y, W),
where

W is a constant typically 1. Points

represented in homogeneous coordinates al-
low for translation, rotation and scaling to be
applied as multiplications of transformation
matrices and row vectors. Without homo-
geneous coordinates, translations are treated
as additions while scaling and rotation are
treated as multiplications.

homogeneous linear estimator

an esti-

mator that is a homogeneous linear function
of the data.

homogeneous medium

optical medium

in which none of the properties (e.g., gain,
index of refraction, etc.) are functions of po-
sition.

homogeneous solution

a system of lin-

ear constant-coefficient differential equa-
tions has a complete solution that consists
of the sum of a particular solution and a ho-
mogeneous solution. The homogeneous so-
lution satisfies the original differential equa-
tion with the input set to zero. Analogous
definitions exist for difference equations.

homogeneous transformation matrix

× 4 matrix that maps a position vector ex-

pressed in homogeneous coordinates from
one coordinate system to another coordi-
nate system (reference coordinate frame). A
homogeneous transformation matrix has the
form

T =

×3

×1

×3








0 0 0 1








where

×3

represents the rotation matrix,

which consists of three unit vectors

n (nor-

mal),

o (orientation), a (approach). The

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