Page 73 - Electrical Engineering Dictionary

with no target present. The background software control, loading data to RAM when
includes the collective unwanted power re- it is needed and returning it to the backing
ceived from sources other than the desired store when it has been unused for a while.
target under test such as positioners, foam
columns, fixtures necessary to support a tar- backing storage See backing memory.
get, and the room or ground environment.
The background level is vectorially sub- backoff a technique used in amplifiers
tracted from the received level with the target when operated near saturation that reduces
present to obtain the raw data set for a partic- intermodulation products for multiple carri-
ular target. ers. In its implementation, the drive signal
(2) any unwanted signal. The background is reduced or backed off. Input backoff is
is a lower limit on the detection of small sig- the difference in decibels between the input
nals when devices are used to make a mea- power required for saturation and that em-
surementinanexperimentalsetup. Themea- ployed. Output backoff refers to the reduc-
surementisasuperpositionofeventsfromthe tion in output power relative to saturation.
experiment itself and events from all other
sources including the background. backplane See backplane bus.

backgroundnoise thenoisethat typically backplane bus a special data bus espe-
affects a system but is produced independent cially designed for easy access by users and
of the system. This noise is typically due to allowing the connection of user devices to
thermal effects in materials, interpreted as the the computer. It is usually a row of sockets,
random motion of electrons, and the intensity each presenting all the signals of the bus, and
depends on the temperature of the material. each with appropriate guides so that printed
In radio channels, background noise is typi- circuit cards can be inserted. A backplane
cally due to radiation that is inherent to the differs from a motherboard in that a back-
universe and due mainly to radiation from plane normally contains no significant logic
astronomical bodies. There is a fundamen- circuitry and a motherboard contains a sig-
tal lower bound on the intensity of such noise nificant amount of circuitry, for example, the
which is solely dependent on the universe and processor and the main memory.
independent of antenna and receiver design.
See also thermal noise, noise temperature, backplane optical interconnect See
noise figure. board-to-board optical interconnect.

background subtraction for images, the backprojection an operator associated
removal of stationary parts of a scene by sub- with the Radon transform
tracting two images taken at different times.
Z +inf ty Z +inf ty
For 1-D functions, the subtraction of a con-
g(s, θ) = f (x, y)
stant or slowly varying component of the −inf ty −inf ty
function to better reveal rapid changes. δ(x cos θ + y sin θ − s) dx dy.

backing memory the largest and slowest The backprojection operator is defined as
level of a hierarchical or virtual memory, usu-
Z π
ally a disk. It is used to store bulky programs
b(x, y) = g(x cos θ + y sin θ, θ) dθ.
or data (or parts thereof) not needed imme-
0
diately, and need not be placed in the faster
but more expensive main memory or RAM. b(x, y) is called the backprojection of
Migration of data between RAM and back- g(s, θ). b(x, y) is the sum of all rays that
ing memory is under combined hardware and pass through the point (x, y).

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2000 by CRC Press LLC