ter several more telegraph assignments Ayr-
ton traveled to Tokyo, where he established
the first electrical engineering teaching lab-
oratory at the Imperial Engineering College.
Among his many inventions he is credited
with the ammeter and an improved voltmeter.
His wife Bertha was also an active researcher
and became the first woman to be admitted
to the Institute of Electrical Engineers.

azimuth recording

a recording scheme

whereby the data is recorded at an acute angle
from the direction of movement of the record-
ing medium. Used in the recording scheme
of video information, FM radio, and audio in
VCRs.

2000 by CRC Press LLC

B coefficient

See

loss coefficient

B-ISDN

See

broadband integrated

services digital network

B-mode display

returned ultrasound

echoes displayed as brightness or gray-scale
levels corresponding to the amplitude versus
depth into the body.

B-site

in a ferroelectric material with the

chemical formula ABO

, the crystalline lo-

cation of the B atom.

B-spline

the shortest cubic spline consist-

ing of different three-degree polynomial on
four intervals; it can be obtained by convolv-
ing four box functions.

Babbage, Charles

(1792–1871) Born:

Totnes, England

Babbage is best known for his ideas on

mechanical computation. Babbage is said to
have been disgusted with the very inaccurate
logarithm tables of his day, as well as ap-
palled by the amount of time and people it
took to compute them. Babbage attempted
to solve the problem by building mechan-
ical computing engines. The government-
funded Difference Engine was beyond the
technology of the craftsman who attempted
to build it. Undeterred, Babbage followed
this failure with the larger and more complex
Analytical Engine (also unfinished).

The

ideas behind the Analytical Engine formed
the basis for Howard Aiken’s 1944 Mark
I computer. Babbage’s assistant, Ada Au-
gusta, the Countess of Lovelace and the poet
Lord Byron’s daughter, is honored as the first
programmer for her work and because her
meticulous notes preserved the descriptions
of Babbage’s machines.

Babinet principle

principle in optics that

states that the diffraction patterns produced
by complementary screens are the same ex-
cept for the central spot. It can be rigorously
proved both for acoustic and electromagnetic
waves. The Babinet’s principle for scalar
fields is the following: let

p be the resul-

tant field in

z > 0 due to the incident field p

from

z < 0 and let p

be the total field when

the same incident wave falls on the comple-
mentary screen. Then, in

z > 0,

p + p

= p

back

in a motor, the end that supports the

major coupling or driving pulley.

back EMF

See

counter-EMF

back end

that portion of the nuclear fuel

cycle which commences with the removal of
spent fuel from the reactor.

back porch

a 4.7 microsecond region in

the horizontal blanking interval of the NTSC
composite video signal that contains a burst
of eight to ten cycles of the 3.579545 MHz
(3.58 MHz) color subcarrier. The back porch
occupies 7% of the total horizontal line time;
starting at the end of the horizontal line sync
signal and ending with the start of the video.

backbone

wiring that runs within and be-

tween floors of a building and connects local-
area network segments together.

backfeed

in power distribution work,

power which flows from the secondary lines
into the primary lines through the distribution
transformer, e.g.,from an emergency genera-
tor connected to customer load.

backflash

an arc which forms along a

tower during a lightning strike due to high
tower or footing impedance.

background

(1) refers to the received

vector power level of an electromagnetic
measurement (usually radar cross section)

2000 by CRC Press LLC

with no target present.

The background

includes the collective unwanted power re-
ceived from sources other than the desired
target under test such as positioners, foam
columns, fixtures necessary to support a tar-
get, and the room or ground environment.
The background level is vectorially sub-
tracted from the received level with the target
present to obtain the raw data set for a partic-
ular target.

(2) any unwanted signal. The background

is a lower limit on the detection of small sig-
nals when devices are used to make a mea-
surement in an experimental set up. The mea-
surement is a superposition of events from the
experiment itself and events from all other
sources including the background.

background noise

the noise that typically

affects a system but is produced independent
of the system. This noise is typically due to
thermal effects in materials, interpreted as the
random motion of electrons, and the intensity
depends on the temperature of the material.
In radio channels, background noise is typi-
cally due to radiation that is inherent to the
universe and due mainly to radiation from
astronomical bodies. There is a fundamen-
tal lower bound on the intensity of such noise
which is solely dependent on the universe and
independent of antenna and receiver design.
See also

thermal noise

noise temperature

noise figure

background subtraction

for images, the

removal of stationary parts of a scene by sub-
tracting two images taken at different times.
For 1-D functions, the subtraction of a con-
stant or slowly varying component of the
function to better reveal rapid changes.

backing memory

the largest and slowest

level of a hierarchical or virtual memory, usu-
ally a disk. It is used to store bulky programs
or data (or parts thereof) not needed imme-
diately, and need not be placed in the faster
but more expensive main memory or RAM.
Migration of data between RAM and back-
ing memory is under combined hardware and

software control, loading data to RAM when
it is needed and returning it to the backing
store when it has been unused for a while.

backing storage

See

backing memory

backoff

a technique used in amplifiers

when operated near saturation that reduces
intermodulation products for multiple carri-
ers. In its implementation, the drive signal
is reduced or backed off. Input backoff is
the difference in decibels between the input
power required for saturation and that em-
ployed. Output backoff refers to the reduc-
tion in output power relative to saturation.

backplane

See

backplane bus

backplane bus

a special data bus espe-

cially designed for easy access by users and
allowing the connection of user devices to
the computer. It is usually a row of sockets,
each presenting all the signals of the bus, and
each with appropriate guides so that printed
circuit cards can be inserted. A backplane
differs from a motherboard in that a back-
plane normally contains no significant logic
circuitry and a motherboard contains a sig-
nificant amount of circuitry, for example, the
processor and the main memory.

backplane optical interconnect

See

board-to-board optical interconnect

backprojection

an operator associated

with the Radon transform

g(s, θ) =

+inf ty

−inf ty

+inf ty

−inf ty

f (x, y)

δ(x cos θ + y sin θ − s) dx dy.

The backprojection operator is defined as

b(x, y) =

g(x cos θ + y sin θ, θ) dθ.

b(x, y) is called the backprojection of

g(s, θ). b(x, y) is the sum of all rays that
pass through the point

(x, y).

2000 by CRC Press LLC

backpropagation

the way in which error

terms are propagated in a multilayer neural
network. In a single layer feedforward net-
work, the weights are changed if there are dif-
ferences between the computed outputs and
the training patterns. For multiple layer net-
works, there are no training patterns for the
outputs of intermediate (‘hidden’) layer neu-
rons. Hence the errors between the outputs
and the training patterns are propagated to
the nodes of the intermediate neurons. The
amount of error that is propagated is propor-
tional to the strength of the connection.

backpropagation algorithm

a super-

vised learning algorithm that uses a form
of steepest descent to assign changes to the
weights in a feedforward network so as to re-
duce the network error for a particular input
or set of inputs. Calculation of the modifica-
tions to be made to the weights in the output
layer allows calculation of the required mod-
ifications in the preceding layer, and modi-
fications to any further preceding layers are
made a layer at a time proceeding backwards
toward the input layer; hence the name of the
algorithm.

backscatter

energy from a reflected elec-

tromagnetic wave. In optics, the optical en-
ergy that is scattered in the reverse direction
from the transmitted optical energy in an op-
tical fiber transmission link or network. The
backscattered energy comes from impurities
in the fiber; mechanical or environmental ef-
fects that cause changes in the attenuation
in the fiber; connectors, splices, couplers,
and other components inserted into the op-
tical fiber network; and faults or breaks in
the optical fiber.

backscattering

the reflection of a por-

tion of an electromagnetic wave back in
the direction of the wave source. See also

backscatter

backside bus

a term for a separate bus

from the processor to the second level cache

(as opposed to the frontside bus connecting
to the main memory).

backward error recovery

a technique of

error recovery (also called rollback) in which
the system operation is resumed from a point,
prior to error occurrence, for which the pro-
cessing was backed up.

backward wave interaction

interaction

between backward propagating microwave
electric fields against an electron stream and
the electron in the electron beam. The di-
rection of propagating microwaves and the
direction of motion of electrons in the beam
are opposite each other.

backward wave oscillator (BWO)

a mi-

crowave oscillator tube that is based on a
backward wave interaction.

balanced

See

balanced line

balanced amplifier

an amplifier in which

two single-ended amplifiers are operated in
parallel with 90 degree hybrid. Balanced am-
plifiers feature a low voltage standing wave
ratio because of an absorption of reflected
power at the terminating resistor of the hy-
brids.

balanced code

a binary line code that

ensures an equal number of logic ones and
logic zeros in the encoded bit sequence. Also
called a DC-free code because the continuous
component of the power spectral density of
a balanced encoded sequence falls to zero at
zero frequency.

balanced line

symmetric multiconduc-

tor transmission line in which the voltage on
each conductor along the transmission line
has the same magnitude, but the phases are
such that the voltage would sum to zero. In
a two conductor transmission line, the volt-
ages would be equal and 180 degrees out of
phase.

This is the equivalent of a virtual

ground plane or zero E-field plane at the ge-
ometric center plane of the transmission line

2000 by CRC Press LLC

Content .. 14 15 16 17 ..