3-20
BASIC SYSTEMS
When two ttys are connected by communications wire or cable (over short or long distances), the
exchange of information between them is direct. When the teletypewriters are not physically joined,
exchange of information is more involved. Direct-current mark and space intervals cannot be sent through
the air. The gap between the machines must be bridged by radio using a radio transmitter and receiver.
The transmitter produces a radio frequency carrier wave to carry the mark and space intelligence. A
KEYER is needed to change the dc pulses from the tty into corresponding mark and space modulation for
the carrier wave in the transmitter. The radio receiver and a CONVERTER are required to change the
radio frequency signal back to dc pulses.
Radio Teletypewriter Systems
The Navy uses two basic radio teletypewriter (ratt) systems. These are the TONE-MODULATED
SYSTEM, referred to as audio-frequency tone shift (afts), and the CARRIER-FREQUENCY SHIFT
SYSTEM, referred to as radio-frequency-carrier shift (rfcs). The rfcs system is also called frequency-shift
keying (fsk).
Figure 3-19 shows a modulated carrier wave with audio tone impulses impressed on the radio-
frequency carrier wave. These correspond to dc mark and space signals.
Figure 3-19.—Modulated carrier wave with audio tone for mark and space.
We can best explain the rfcs signal by comparing it to the on-off cw signal. Cw signals are
essentially a constant frequency with no variations along the frequency axis. Figure 3-20, view A, is an
example. The complete intelligence is carried as variations in the signal amplitude. Figure 3-20, view B,
shows the same signal as a shift in frequency between the mark and space.
