a. Neutral Operation. In the neutral telegraph system, transmission is accomplished by sending peak current
over the line for intervals of time corresponding to marking signals, and interrupting the current for intervals
corresponding to spacing signals. Peak current is the normal maximum value of current on which the circuit is
designed to operate.
(1) The neutral telegraph system is used mostly for short lengths of transmission line, because long
transmission lines distort the telegraph pulses. This pulse distortion is caused by reactive line
constants of inductance and capacitance. Although resistance tends to reduce the amplitude of the
pulses, resistance alone does not distort the telegraph pulses.
(2) The neutral system requires little in the way of terminating equipment, and is easy to install and to
operate. It is simple, and generally requires a minimum of system maintenance.
b. Polar Operation. Polar operation differs from neutral operation in that telegraph signals are formed by
reversing the direction of the current flow to a receiving polar relay instead of interrupting the current flow. In
other words, polar operation uses a flow of current in one direction over the line to operate the receiving relay
armature to the marking position, and a flow of current equal in value but opposite in direction to operate the
armature to the spacing condition. It is this sensing of current pulses by relays that complicates the circuitry.
Although the polar system is more complex than the neutral system and requires more equipment for its
operation, its signal is not adversely affected by line constants (resistance, inductance, capacitance) to the same
degree as neutral signals. This results from the fact that signal pulses consisting of oppositely flowing current are
affected similarly by the line's electrical characteristics. Since line constants generally do not distort polar signals
at normal teletypewriter code speeds, their use is not usually limited by line length (within limits). To say this
another way, polar signals can be successfully transmitted and received over much longer lines than is the case
with neutral signals.
c. Modes of Operation. Two modes of operation generally used in teletypewriter systems are half-duplex
and full-duplex.
(1) Half-duplex is the simpler of the two modes. It permits transmission of messages in either direction
on the same line, but not in both directions simultaneously. It also permits the feature of break-in,
whereby the receiving operator can interrupt the transmission of the sending operator.
(2) The full-duplex mode permits transmission of messages in two directions simultaneously. This mode
of operation can handle more traffic than the half-duplex mode because traffic moves continually in
both directions. It normally requires the use of two separate paths, one for each direction of
transmission. Unless specifically engineered into the system, a full-duplex mote of transmission does
not allow one operator to interrupt (break) the other operator.
2-6. AC TELEGRAPHY
In ac or VF telegraph systems, a carrier frequency is provided for each channel. This carrier is made to
conform to the pattern of the telegraph
16
Previous Page
