18WuD2x

218 duct • dump check
Channel 1
Channel 2
dual trace
duct 1. A narrow propagation path, sometimes
traveled by microwaves, created by unusual atmospheric
conditions. 2. A pipe or channel for cables
and wires.
dubbing The adding of sound to a recorded magnetic
tape, record disk, or film (e.g., replacing the
sound track of a film in one language with that of
another language).
dubnium Symbol, Db. Also clalled unnilpentium
(Unp) and hahnium (Ha). Atomic number, 105.
The most common isotope has atomic weight
262. Classified as a transition metal. It has a
half-life on the order of a few seconds to a few
tenths of a second (depending on the isotope), is
human-made, and is not known to occur in nature.
duct 1. A narrow propagation path, sometimes
traveled by microwaves, created by unusual atmospheric
conditions. 2. A pipe or channel for cables
and wires. 3. An opening, vent, or other
airway used for various purposes, such as cooling
and acoustic wave transmission.
ducted port An opening in a speaker cabinet that
has an airway (duct) extending several inches
into the cabinet. It improves the quality of sound
from a speaker system by equalizing the air pressure
inside and outside the cabinet. Also provides
resonant audio effects at frequencies that depend
on the dimensions of the duct.
duct effect see TROPOSPHERIC DUCTING.
ductilimeter An instrument used for measuring
the ductility of metals.
ducting The confinement of a radio wave to a duct
(see DUCT, 1) between two layers of the atmosphere
or between an atmospheric layer and the
earth.
Duddell arc A carbon copper arc circuit that produces
audible continuous waves. Consists of a
series inductance-capacitance (LC) circuit shunting
an electric arc.
Duerdoth’s multiple feedback system In an amplifier,
feedback through several paths to improve
response over that afforded by single-path feedback.
In a simple application of multiple feedback,
a single external loop is augmented with
unbypassed emitter resistors in the amplifier
stages.
Duerdoth’s stability margin A feedback-amplifier
stability margin equal to a 6-dB increase in gain
at low and high frequencies over beta values between
0.3 and somewhat less than 2. For higher
beta values, Duerdoth adopts an angular margin
(for example, 15°); below β = 0.3, no danger of instability
is present.
dummy 1. A nonoperative model of a piece of
equipment, usually assembled with dummy components
(see DUMMY COMPONENT, 1) for the
purpose of developing a layout. 2. DUMMY AN-
TENNA, DUMMY COMPONENT, or DUMMY
LOAD. 3. Part of a computer program that, rather
than being useful for the problem at hand, only
serves to satisfy some other format or logic requirement.
dummy antenna 1. A nonradiating device that
serves as a load for a transmitter (i.e., it takes the
place of the regular antenna during tests and adjustments
of the transmitter). 2. A device
containing a network of discrete inductive,
capacitive, and resistive elements, inserted between
a radio-frequency signal generator and receiver
to simulate a standard antenna.
dummy component 1. A nonoperative component
used in developing a layout or package. 2.
A nonoperative component fraudulently included
in a piece of equipment (e.g., an unwired
transistor in a receiver circuit, a common occurrence
during the early days of the transistor,
when a 10-transistor radio brought more money
than an 8-transistor radio, without regard to the
circuit itself).
dummy instruction In a computer, a command
that serves no operational purpose, other than to
fill a format requirement.
dummy load 1. A load device, usually consisting of
resistance without reactance, used to terminate a
power generator or power amplifier during adjustments
and tests. The load resistance is equal
to the output impedance of the generator or amplifier.
2. See DUMMY ANTENNA.
dummy resistor A power-type resistor used as a
dummy load.
dump 1. In digital-computer operations, to transfer,
completely or partially, the contents of memory
into a peripheral. 2. To switch off all power to
a computer, deliberately or accidentally, thereby
losing what is in the volatile memory.
dump and restart During a halt in a computer
program run, to backtrack to the last dump point
and use the data there to resume the run. Also
see DUMP POINT.
dump check In digital-computer operations, the
checking of all digits being transferred (see
DUMP, 1) to prevent errors when they are retransferred.