Page 92 - The Illustrated Dictionary of Electronics

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black box • bleeder resistor 77

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