18WuD2x

132 commutation • compass
commutation 1. In a direct-current (dc) generator,
periodic reversal of the current in the armature
coils as the coils alternately pass the north and
south poles of the magnetic field. When the ends
of each coil are connected to opposite bars of the
commutator, the electrical polarity at the commutator
brushes remains constant. 2. In a thyratron
or silicon-controlled rectifier (SCR) circuit,
momentarily reversing the polarity to cut the device
off.
commutator 1. In a direct-current (dc) motor,
generator, or rotating selector, an arrangement
of parallel metal bars or strips on a rotating
drum. As the drum turns, the bars contact one
or more brushes that are in sliding contact with
the commutator. 2. An electronic circuit that
switches a single input sequentially to a series of
output terminals, or switches a number of inputs
sequentially to a single pair of output terminals.
commutator ripple The pulsating voltage superimposed
on the direct-current (dc) voltage delivered
by an unfiltered dc generator.
compact disc Abbreviation, CD. A digital, highdensity
optical disc, used in high-fidelity stereo
sound systems. Also used to store computer data.
The information is encoded as tiny pits on the
surface of the disc, and is recovered by a laser, a
sensor and a digital-to-analog (D/A) converter.
These disks have largely superseded magnetic
tapes, and have rendered long-playing vinyl disks
and turntables obsolete. See also COMPACT-
DISK READ-ONLY MEMORY.
compact-disk read-only memory Abbreviation,
CD-ROM. A digital COMPACT DISC used for the
long-term storage of computer data and/or software
programs. Usually the same size as a highfidelity
stereo disk, it can hold over 600
megabytes of data. Although data can be read
from the disk, it cannot be overwritten.
compander Term for compressor/expander. In the
transmission and reception of audio-frequency
(AF) intelligence, a system that uses an amplitude
compressor at the transmitter and an amplitude
expander at the receiver. The compressor reduces
the dynamic range before transmission, and the
expander restores it after reception. Provides improved
signal-to-noise ratio under marginal communications
conditions. Also increases the ratio
of average power to peak power. See COMPAND-
ING.
companding A process in which a signal is compressed
at the transmitting end of a circuit and
expanded at the receiving end, yielding a signal
like the original at the receiver output. Signals
are more efficiently transmitted when they are
compressed because the average power increases,
relative to the peak power. This improves
the average signal-to-noise ratio for weak signals.
See COMPANDER.
companding law The mathematical function used
for companding. It is an output-amplitude versus
input-amplitude function for the compression at
the transmitter, and the inverse of this function
for the expansion at the receiver.
companion keyboard An auxiliary keyboard connected
to a regular keyboard and operated remotely.
companionship machine A computer or robot
with sufficient machine intelligence to provide entertainment
and mental stimulation for humans.
comparator 1. An integrated circuit (IC) with two
inputs, called A and B. The device compares the
voltages that appear at these inputs. If the input
voltage at A is significantly greater than the input
voltage at B, the output is about +5 V. If the input
voltage at A is not greater than the input
voltage at B, the output voltage is about +2 V.
These ICs are used to actuate, or trigger, other
devices such as relays and electronic switching
circuits. 2. In general, any circuit that compares
some characteristic of two input signals and produces
an output that depends on the relationship
between the inputs. 3. An instrument for
checking the condition of a component by comparing
it directly with an identical component of
known quality has a scale reading in percentage
deviation, or simply “GO/NO-GO.” Examples: capacitor
comparator, resistor comparator, coil
comparator.
compare In computer operations, a relational test
performed on two quantities to determine their
relative magnitude, including an indication of the
test result and, sometimes, the taking of action.
Example: the process and acton resulting from
execution of the statement “IF A > B THEN GO TO
LINE 250.”
comparison 1. An expression of the relationship between
two voltages, currents, phase angles, component
values, or other quantities in an electrical
or electronic circuit or system. 2. An examination
of different data bits or items, which results in a
conclusion about some aspect of their relationship.
comparison bridge A bridge designed especially
for the quick comparison of components (e.g., the
comparison of resistors with a standard resistor,
inductors with a standard inductor, and capacitors
with a standard capacitor).
comparison measurement A measurement in
which a quantity or component is compared with
a known, similar quantity or component value,
rather than having the measurement displayed
directly by a meter. Examples: bridge measurements,
potentiometric measurements, and frequency
matching.
compass 1. Any of several instruments for determining
direction on the earth’s surface [e.g., magnetic
(mariner’s) compass and gyrocompass]. 2. A
radio direction finder. 3. An instrument for drawing
circles.