signals by a process of modulation (keying of a tone oscillator). This process is initiated at the sending end of
each circuit by operation of the telegraph sending equipment, which originates signals in the form of dc pulses.
Subsequent to modulation, the keyed tone or ac signal conveys the intelligence over the transmission line. At the
distant end, the pulsed ac carrier signals are demodulated (restored to direct current) for actuation of the telegraph
receiving instrument. The ac system usually can be operated over any good telephone line or radio circuit. The
advantage of the ac or keyed tone system over the dc system is the fact that the keyed tone signals can convey
information over long lines without being distorted by line constants. In addition, tone telegraph terminal
equipment is available in either single-channel or multichannel configurations. The terms ac telegraph, tone
telegraph, and VF telegraph are synonymous.
a. On-Off Keying. The on-off method of VF keying is the oldest ac telegraph system devised. It is still
used on short lines and on radio systems having low noise levels. The on-off system keys or modulates a
constant-frequency tone with the telegraph pulses, normally sending tone for mark and no tone for space.
Although the system has a simple equipment configuration, it is hampered in its operation by the noise level
which usually rises during the no-tone interval.
b. Frequency-Shift Keying. The method most generally used for VF teletypewriter operation is to shift the
carrier slightly between two different frequencies, one frequency representing mark and the other frequency
representing space. This is known as the frequency-shift-keying (FSK) method. The receiving equipment
converts these FSK signals to dc signals which, in turn, actuate the receiving teletypewriters. The FSK system is
directly applicable to both radio and wire circuits. Its advantage over the on-off keying method for radio
transmission is that noise buildup is held to a minimum, since a signal is always present. The same advantage
also accrues on long-distance wire circuits, where noise my rise to an appreciable value of signal amplitude.
c. Telegraph-on-Telephone. It is possible to combine both an FSK telegraph signal and a telephone signal
on the same line without causing mutual interference, by using a "speech-plus-duplex" system. This system mates
use of a narrowband filter that removes from the speech channel a 250-Hz band for use by the tone telegraph
channel. There is some degradation of signal quality, but not enough to hinder normal speech communication.
Since both terminals produce the same tone frequencies in the FSK signals, each operator must take turns using
the line facility. This gives rise to the one-way reversible mode of transmission, which is the mode of
transmission most often used in speech plus duplex.
STUDY EXERCISES
In each of the following exercises, select the ONE answer that BEST completes the statement or answers the
question. Indicate your solution by circling the letter opposite the correct answer in the subcourse booklet.
17
Previous Page
