it achieves less compression than arithmetic
coding but is easier to implement.
Huffman encoding
See
Huffman coding
.
hum bars
horizontal black and white bars
that extend over the entire TV picture and
usually drift slowly through it. Hum bars are
caused by an interfering power line frequency
or one of its harmonics.
human visual system (HVS)
the collec-
tion of mechanisms in humans which process
and interpret the visual world. These mech-
anisms include the eye, the retina, the optic
nerve, the visual cortex, and other parts of the
brain.
hunting
a mechanical oscillation in the
speed of a synchronous machine due to
changes in the load. Damper windings are
used to reduce the hunting by providing a
torque that opposes the change in speed.
Hurter–Driffield (H-D) curve
the stan-
dard form of the H-D or contrast curve is
a plot of the relative thickness of resist re-
maining after exposure and development of a
large clear area as a function of log-exposure
energy. The theoretical H-D curve is a plot
of log-development rate versus log-exposure
energy. (Hurter and Driffield are the two sci-
entists who first used a related curve in 1890.)
Hurwitz matrix
a square matrix with real
elements and whose all eigenvalues have neg-
ative real parts.
Hurwitz polynomial
a polynomial
p(s) =
a
n
s
n
+a
n−1
s
n−1
+a
n−2
s
n−2
+· · ·+a
0
, where
all of the
a
i
’s are either real or complex num-
bers, is said to be Hurwitz if all of its roots
have negative real part.
Huygen’s principle
a principle stating
that each point of a wave front can be consid-
ered to be a source of a secondary spherical
wave that combines with the other secondary
spherical waves to form a new wave front.
Huygen’s principle is often used as an ap-
proximate technique for solving diffraction
and antenna radiation problems. See also
equivalence theorem
,
equivalent source
.
HVC color coding
technique whereby the
HVS contrast sensitivity functions for differ-
ent wavelengths are taken into account for
compressing the color planes independently
with different sets of parameters.
HVDC transmission
See
high-voltage
DC transmission
.
hybrid bipolar MOS thyristor (HBMT)
like its transistor cousin, it is an advanced de-
vice designed to take advantage of the unique
characteristics of both bipolar devices (high
current capability with low forward volt-
age drop) and MOS device (high switching
speeds with low gate drive requirements).
hybrid circuit
a circuit based on at least
two different technologies. For instance, a
circuit built by using solid state circuits and
tubes.
hybrid computer
a computer based on
at least two different technologies. For in-
stance, a computer presenting both digital
and analog circuits and signals.
hybrid coupler
generally a four port cir-
cuit, which has an electrical response such
that a signal applied to port one is divided
equally in amplitude between ports three and
four. The phase of the signals at ports three
and four are either 0, 90, or 180 degrees apart,
dependent on the type of hybrid coupler. Port
two is completely isolated, and no signal ap-
pears at this port.
hybrid magnetic bearing
an active mag-
netic bearing that also incorporates perma-
nent magnets, thus reducing energy con-
sumption and improving performance.
hybrid
microwave
integrated
circuit
(HMIC)
a planar assembly that com-
c
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