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lack box • bleeder resistor 77
a system in development or maintenance operations.
3. Any subcircuit or stage that can be specified
in total as required in a system, in terms of
its known or prescribed performance, but whose
internal structure need not be known.
black-box concept A technique for development of
equivalent circuits and of considering their operation.
The “box” has a pair of input terminals and
a pair of output terminals; one input terminal is
often common to one output terminal. The contents
of the box need not be known, but from the
input and output current and voltage relationships,
its nature can be determined. Moreover,
from the available input signal and desired output
signal, the internal circuit of the box can be
specified. Integrated circuits (ICs) are often
treated as black boxes by engineers designing
complex electronic equipment.
black compression Attenuation of the level of dark
areas in a television picture.
blacker than black The video-signal amplitude region
above the level that just darkens the screen.
Signal information (such as control pulses) in
this region are therefore not seen.
black light 1. Ultraviolet radiation—especially
when used to cause visible fluorescence in certain
materials. 2. A lamp that produces a principal
portion of its radiation in the ultraviolet
region, causing visible fluorescence of certain
substances. Such lamps are used in some scientific
experiments, and also for creating special effects
at presentations or parties. It is hazardous
to look directly at the output of such a lamp with
unprotected eyes.
blackout
1. A complete interruption of ac utility
power to numerous customers at the same time.
2. A complete cessation of ionospheric radio-wave
propagation, such as might be caused by a solar
flare. 3. Complete blanking of the screen of an oscilloscope
or picture tube.
black reference In a television signal, the blanking
level of pulses, beyond which the sync pulse is in
the blacker-than-black region.
black reference level In a television signal, the
voltage threshold of the BLACK REFERENCE
(i.e., its level above zero volts).
black transmission A system of picture or facsimile
transmission in which the maximum copy
darkness corresponds to the greatest amplitude
(in an amplitude-modulated transmitter) or the
lowest instantaneous frequency (in a frequencymodulated
transmitter). Compare WHITE
TRANSMISSION.
blank
1. A piezoelectric plate cut from a quartz
crystal, but not yet finished to operate at a desired
frequency. 2. To obscure or interrupt a signal
or electron beam (usually momentarily), as in
z-axis blanking in an oscilloscope. 3. A silicon
wafer cut from a large slab, containing dopants
only. 4. A magnetic diskette or tape on which
nothing is recorded. 5. An optical diskette on
which nothing is recorded. 6. A location (such as
a symbol or space) that is used to verify proper
data character grouping and values.
blanketing A form of radio interference accompanied
by severe degradation of reception, virtually
unaffected by tuning, over a wide range of frequencies.
An example is ac line noise caused by
an arcing power transformer or electrical appliance
in the vicinity of a receiving antenna. It
tends to occur most often at low, medium, and
high frequencies.
blanking Obscuring or momentary elimination of a
signal (see BLANK, 2).
blanking interval The short period during which
the electron beam of a cathode-ray tube is cut off
so that the beam can return to its start position
without creating a trace on the screen.
blanking level The discrete, predetermined level
(usually a threshold voltage) at which BLANKING
occurs.
blanking pedestal In the horizontal pulse of a television
signal, the lower portion between zero volts
and the blanking level.
blanking pulse A pulse that produces momentary
blanking (see BLANK, 2).
blanking time The time interval during which the
electron beam of a cathode-ray tube is interrupted
by a blanking signal.
blank tape 1. Magnetic tape that has never been
subjected to the recording process and that is
substantially free from noise. 2. Magnetic tape
from which all preexisting information has been
erased.
blasting 1. Severe overloading of a sound system,
usually caused by setting the volume control at
or near maximum and then applying a significant
input signal to the amplifier. Accompanied by distortion,
in its worst form, it can cause damage to
speakers and/or headsets. 2. In a communications
receiver, the result of a strong signal coming
in unexpectedly when the automatic gain control
(AGC) has been switched off, and the audiofrequency
(AF) and radio-frequency (RF) gain controls
are set high for reception of weak signals.
bleeder
A resistor or group of resistors, used permanently
to drain current from charged capacitors.
It establishes the predetermined initial load
level for a power supply or signal source, and it
serves as a safety device in high-voltage power
supplies.
bleeder current The current normally flowing
through a bleeder.
bleeder divider A network of resistors, seriesstrung
across the output of a power supply or its
regulator. As a load resistor, the bleeder improves
regulation and protects against no-load voltage
surges. The resistor junctions allow various voltages
to be drawn from the supply.
bleeder power
bleeder.
bleeder resistor
Power dissipated as heat in a
See BLEEDER.