18WuD2x

338 heterodyne wave analyzer • high-efficiency linear amplifier
sharp bandpass filter, whose output actuates an
alternating-current (ac) voltmeter. The internal
oscillator is tuned slowly so that different components
of the balanced-modulator output sideband
pass successively through the filter. The
amplitude-versus-frequency function of the input
signal is determined by noting the meter readings
as the internal oscillator is tuned.
heterodyne wavemeter See HETERODYNE FRE-
QUENCY METER.
heterodyne whistle See HETERODYNE, 2.
heterogeneous Pertaining to a group of objects or
devices that have differing characteristics.
heterogeneous radiation Any broadband form of
radiation. In particular, broadbanded radio
waves, infrared radiation, visible light, ultraviolet,
X rays, or gamma rays.
heterolysis The HYDROLYSIS of a compound into
two oppositely charged ions.
heteropolar generator An electric generator in
which the active conductors move through magnetic
fields, first in one direction, then in the
other direction. This is done by means of rotation
in a nonuniform magnetic field. Most generators
in common use are of this kind.
heuristic knowledge In artificial intelligence (AI), a
form of machine knowledge in which a computer
learns from its mistakes. As a complex program is
repeatedly run over a period of time, the number
of errors per run approaches zero.
heuristic program In artificial intelligence (AI), a
program with which the computer solves a problem
by trial and error, often learning in the process
so that mistakes will not be repeated on
subsequent runs.
Heusler’s alloys Ferromagnetic alloys that contain
one or more non-magnetic metals (such as aluminum,
copper, or manganese).
hexadecimal number system An alphanumeric,
base-16 system of number notation used in some
computers. The system uses the usual digits plus
the letters A through F to represent the numbers
10 through 15 (each place can only hold one symbol).
hex inverter A collection of six digital inverters, or
NOT gates, contained within one package, usually
an integrated circuit.
Hf Symbol for HAFNIUM.
HF Abbreviation of HIGH FREQUENCY.
Hg Symbol for MERCURY.
HH beacon In radionavigation, a nondirectional
homing beacon.
hi 1. Contraction of HIGH. 2. Radiotelegraph symbol
for a laugh, often verbalized by radio amateurs.
HIC Abbreviation of HYBRID INTEGRATED CIR-
CUIT.
HIDM Abbreviation of HIGH-INFORMATION
DELTA MODULATION.
hierarchical password protection Also called
multilevel password protection. A security feature
that prevents unauthorized use of a computer,
network, or database. The password levels allow
users various degrees of control over the host machine.
hi-fi 1. Contraction of HIGH-FIDELITY. 2. In video
recording, the addition of sound having high fidelity.
high 1. Pertaining to a circuit point or condition at
some potential above ground. 2. The logical digit
1. 3. The condition of having relatively large magnitude
(e.g., HIGH FREQUENCY and HIGH VOLT-
AGE). 4. Pertaining to the upper portion of a
range, as in HIGH BAND or HIGH FREQUENCY.
5. Characterized by greater-than-normal response
or performance, as in HIGH Q or HIGH FI-
DELITY.
high band 1. The higher or highest frequency band
used in communications, testing, or processing,
when several bands are available. 2. The veryhigh-frequency
(VHF) television channels 7
through 13. 3. The communications frequency
range from about 144 MHz to about 170 MHz.
high boost In sound recording and reproduction,
the emphasis of high frequencies in an operating
spectrum. Also called HIGH-FREQUENCY COM-
PENSATION.
high-C circuit A tuned circuit having high capacitance
and low inductance at a given frequency.
Such a circuit is characterized by high selectivity
and low voltage. Compare HIGH-L CIRCUIT. Also
see LC RATIO.
high contrast In an image, a limited range of gray
values between black and white, or a similar condition
in a color image (overbright whites, little
shadow detail). Also see CONTRAST.
high definition In facsimile or television, a condition
of minute detail so that the original scene is
faithfully reproduced.
high-definition television Abbreviation, HDTV. A
method of getting enhanced detail into a television
(TV) picture and for obtaining better audio
quality, compared with standard analog TV.
There are several different schemes. The most noticeable
feature is the crispness of the picture.
This is vividly apparent in big-screen installations,
which have traditionally suffered from image
blurring. A standard TV picture has 525 lines
per frame, but HDTV systems can have more
than twice that number. Another important difference
is that HDTV is digital; this minimizes
susceptibility to interference. Interlacing is used
in some systems.
high-density bipolar-3 code A communications or
digital code in which two logic highs (ones) can
occur in sequence, without the need for an intervening
logic low (zero) to separate them.
high-efficiency linear amplifier A LINEAR AM-
PLIFIER with higher operating efficiency than is
obtainable with conventional class-B linear amplifiers.
Efficiencies on the order of 60% at 100%
modulation are possible.