Page 45 - Electrical Engineering Dictionary

aliasing (1) in signal processing, distor- (3) the process of determining the time or
tion introduced in a digital signal when it is phase shift of a certain signal so that part of
undersampled. it may be matched with another signal. See
In all digital systems the signals should be also image registration.
filtered before they are sampled to eliminate
signal components with frequencies above all-digital synchronization synchroniza-
the Nyquist frequency, tion algorithm, where the analog-to-digital
conversion takes place as early as possible to
ω N = ω s /2 = π/T ,
assist digital implementation of the synchro-
nizer. In most cases, an all-digital synchro-
where T is a sampling time, are eliminated.
nization approach leads to optimal maximum
Ifthisfilteringisnotdone, signalcomponents
likelihood algorithms.
with frequencies
all-optical network an optical communi-
ω> ω N
cations network where the role of electronics
will appear as low-frequency components is reduced to basic supervisory and control
with the frequency functions. All-optical devices are used ex-
clusively between the nodes to re-configure
ω a = |((ω + ω N ) mod ω s ) − ω N | the network which enables the greatest use of
fiber bandwidth.
The prefilters introduced before a sampler are
called anti-aliasing filters (common choices all-optical switch an optically addressed
are second- or fourth-order Butterworth, in- device whose optical transmission can be
tegral time absolute error (ITAE), or Bessel switched between two possible states by
filters). changes in the incident optical power.
(2) in computer graphics, distortion due
to the discrete nature of digital images that all-pass system a system with unit mag-
causes straight lines to appear jagged. nitude and poles and zeroes that are complex
(3) in computer software, a single object conjugate reciprocals of each other. An all-
having two different identities, such as names pass system with a pole at z = a and a zero
in memory space. Aliasing can make it diffi- 1
at z = a ∗ is
cult to determine whether two names (or ac-
cess paths to reach an object) that appear to be
z −1 − a ∗
different really access the identical object; a H a p(z) = −1 .
system designed to find parallelism when two 1 − az
accesses really reach different objects will
have trouble achieving correct (functional)
operation if aliasing is present. alley arm a crossarm meant for use in
an alleyway or other confined area in which
alignment (1) the requirement that a da- poles must be placed close to buildings. See
tum (or block of data) be mapped at an ad- crossarm.
dress with certain characteristics, usually that
the address modulo the size of the datum or allocate to create a block of storage of a
block be zero. For example, the address of a given size in some memory, which is not to
naturally aligned long word is a multiple of be used for any other purpose until expressly
four. freed.
(2) the act of positioning the image of a
specific point on a photomask to a specific allocation the act of allocating. See also
point on the wafer to be printed. allocate.

c
2000 by CRC Press LLC