Lesson 2/Learning Event 4
Learning Event 4: DIGITAL SIGNALS
Digital signals constitute a large family of signal types, one type being
teletypewriter (mechanical telegraph).
Digital signals are customarily
associated with data transmission.
The principal difference between
start-stop teletypewriter signals and data signals is the length of the
stop pulse, which is 1.42 times the length of any unit pulse.
Another
difference is the rate of transmission.
Teletypewriter signals are low
speed because of the mechanical limitation of the printer mechanism.
Normal speeds are 60 and 100 wpm. Eventual replacement of mechanical with
electronic reproduction will bring higher speed capability.
a. DC Digital Data Signals. All factors previously stated in regard
to patching DC telegraph signals are applicable in the case of DC digital
data signals. DC digital data signals are more susceptible to distortion
than telegraph signals, principally due to the difference in rate of
transmission and the resulting shorter pulse length.
A circuit suitable
for transmission of low-speed DC teletypewriter signals may not be
suitable for the high-speed signals, because a small amount of circuit
distortion will have little effect on low-speed (long) pulses, while the
same amount of circuit distortion may render the high-speed (short) pulses
useless.
Moreover, electromechanical relays cannot be used on data
signals because they have inherent transition delays.
Such delays may
become significant portions of the short data pulses, causes changes in
their length. The result is distortion of the received signals.
b. VF Digital Signals.
VF digital signals are developed from
modulation of a tone carrier with the DC pulses.
The bandwidth of such
signals depends on the rate of modulation. The higher the rate, the wider
is the bandwidth. Low-speed teletypewriter keying produces a very narrow
band.
Most telegraph terminal equipment presently in use operates on
frequency-division-multiplexing
principles.
Each
channel
in
the
frequency-division arrangement has been designed to accommodate the
narrowband needs of low-speed telegraph keying.
Unfortunately, these
channels cannot handle the wideband keying generated by high-speed data
signals.
When high-speed digital signals modulate a low-speed VF
telegraph channel, the distortion introduced into the signal by the
narrowband filter destroys the signal's value. Time-division-multiplexing
methods being planned for future communication systems will permit higher
rates of digital transmission.
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