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keyboard keyer • keying speed 391
from about 5 words per minute (wpm) to 60 or 70
wpm, although some keyboard keyers are programmed
for speeds over 100 wpm.
keyboard lockout A keyboard interlock in a data
transmission circuit that prevents data from being
transmitted while the transmitter of another
station on the same circuit is operating.
keyboard send-receive unit A teletypewriter lacking
an automatic input device.
key cabinet In a telephone system, a facility that
shows a subscriber which lines are busy and
which lines are open.
key chirp
A chirping sound in a received signal, resulting
from slight frequency shift when a radiotelegraph
transmitter is keyed. It does not
occur with a well-designed transmitter.
key-click filter An inductance-capacitance (LC) or
resistance-capacitance (RC) filter for smoothing a
keying wave to eliminate KEY CLICKS. It functions
by optimizing the rise and decay times of
the keyed waveform.
key clicks
Excessive bandwidth of a radiotelegraph
signal that can result when a keyed signal
has rise and decay times that are too rapid. Produces
characteristic clicking or popping sounds,
with resulting interference, in receivers tuned to
frequencies near that of the transmitted signal. A
KEY-CLICK FILTER can eliminate this.
Amplitude
No click
produced
key clicks
Click
produced
Time
keyed AGC A controlled automatic gain control
(AGC) system in a television receiver circuit. The
AGC acts when the horizontal sync pulse appears;
it is inactive between pulses. This prevents
unwanted actuation of the AGC by noise transients
and picture-signal elements.
keyed clamp A clamping circuit that uses a control
signal to determine the clamping time.
keyed interval In a transmission system that is
keyed periodically, an interval beginning with a
change in state and having a duration of the
shortest time between changes in state.
keyed rainbow generator For testing of colortelevision
receivers, a signal generator that
produces a rainbow color pattern on the screen
(i.e., a set of 10 vertical color bars representing
the spectrum, with blank bars in between). The
pattern results from gating the 3.56-MHz oscillator
in the receiver at a frequency of 189 kHz.
keyer An automatic device for keying a radiotelegraph
transmitter or wire telegraph circuit. The
keyer can operate from perforated tape, an embossed
disk, magnetic tape, or other similar
recording.
keyer adaptor A modulated-signal detector that
produces a direct-current signal having an amplitude
sympathetic with the modulation; it provides
the keying signal for a frequency-shift
exciter in radio facsimile transmission.
key escrow A controversial system in which the
government is provided with certain components
of decryption keys to all communications ciphers,
according to laws that would allow the government
to eavesdrop on private communications or
transactions after getting a court order. The key
components are held in a secure place, that is, in
“escrow,” unless and until the necessary court order
is obtained. See DECRYPTION KEY.
keying 1. The modulation of a carrier by switching
it on and off. It is commonly used in radiotelegraphy.
2. The modulation of a carrier by switching
its frequency between two defined values. It is
also called FREQUENCY-SHIFT KEYING (FSK)
and it is used in data transmission. 3. The modulation
of a carrier with an audio tone that is
switched on and off. The carrier can be modulated
via amplitude modulation (AM), frequency
modulation (FM), pulse modulation, or any other
form that will convey the audio tone. It is occasionally
used in radiotelegraphy at very-high frequencies
(VHF) and above. Also called audio
keying. 4. The modulation of a carrier with an audio
tone whose frequency is switched between
two defined values. The carrier can be modulated
via amplitude modulation (AM), frequency modulation
(FM), pulse modulation, or any other form
that will convey the audio tone. It is commonly
used in data transmission at VHF and above.
Also called AUDIO FREQUENCY-SHIFT KEYING
(AFSK).
keying chirp A rapid change in the frequency of a
continuous-wave signal, occurring at the beginning
of each code element. In the receiver, the resulting
sound is a chirp.
keying error rate In data transmission, the ratio
of incorrectly keyed signals to the total number of
signals keyed.
keying filter See KEY-CLICK FILTER.
keying frequency 1. In audio-keyed radiotelegraphy,
the audio frequency (tone) of the dot and
dash signals (as opposed to the carrier frequency).
2. In radiotelegraphy, the transmission
speed (see KEYING SPEED). 3. The number of
times per second that a black-line signal occurs
while an object is scanned in a facsimile system.
keying monitor A simple detector used by an operator
to listen to the keying of a radiotelegraph
transmitter.
keying speed The speed (in words per minute) of a
telegraph or radio-telegraph transmission.