two-wave mixing, if the two laser beams are
of the same frequency, a stationary interfer-
ence intensity pattern is formed. This leads to
a stationary volume refractive index grating.
Such a kind of two-wave mixing is referred
to as degenerate two-wave mixing.
degradation
situation in which a signal
has been corrupted by noise, blurred by some
point-spread function or distorted in some
other fashion.
degree of membership
the degree to
which a variable’s value belongs in a fuzzy
set. The degree of membership varies from
0 (no membership) to 1 (complete member-
ship).
degree of mobility
each prismatic or rev-
olute joint has one degree of freedom and
provides the mechanical structure with a sin-
gle degree of mobility.
degrees of freedom
the number of in-
dependent position variables that have to be
specified in order to locate all parts of the
mechanism is defined as a number of de-
grees of freedom. Therefore, the degrees of
freedom is defined as the minimal number of
position variables necessary for completely
specifying the configuration of the mecha-
nism.
degree of visual angle
the angle sub-
tended by an object of a given width a given
distance away from the viewer.
delay
(1) the time required for a signal to
propagate along a wire.
(2) the difference in the absolute angles
between a point on a wavefront at the device
output and the corresponding point on the in-
cident input wavefront, expressed in seconds
or degrees. Delay can exceed 360 degrees.
Given by
t
d
, we have
t
d
= θ
out
− θ
in
delay angle
See
firing angle
.
delay locked loop
See
delay-locked loop
.
delay power spectrum
a function charac-
terizing the spread of average received power
as a function of delay. Can be obtained from
the scattering function by integrating over the
Doppler shift variable. See also
scattering
function
,
multipath propagation
.
delay profile
See
delay power spectrum
.
delay range
the difference in arrival times
between the first and last significant compo-
nent of the impulse response of a wideband
communication channel. Also known as the
total excess delay.
delay resolution
the capability, measured
in units of delay (seconds), of a signal used
for channel measurement to resolve received
signal components which arrive with differ-
ent delays. If two signal components arrive at
the receiver with a delay separation less than
the delay resolution, they will be observed
as one signal, superimposed on each other.
The actual value of the delay resolution de-
pends on the criterion by which two signal
components are defined to be resolved. An
approximate measure is given by the inverse
of the channel (or signal) bandwidth. See
also
multipath propagation
.
delay slot
in a pipelined processor, a time
slot following a branch instruction. An in-
struction issued within this slot is executed
regardless of whether the branch condition is
met, so it may appear that the program is ex-
ecuting instructions out of order. Delay slots
can be filled (by compilers) by rearranging
the program steps, but when this is not pos-
sible, they are filled with “no-operation” in-
structions.
delay spread
a measure of the time
through which the duration of a transmitted
signal is extended by dispersion in a wide-
band communication channel. Usually mea-
sured as the RMS delay spread, i.e., the sec-
c
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