system. You will find that the keyer is part of the
transmitter in modern systems. The keyer shifts the
signal below or above the assigned frequency,
corresponding to the mark or space required for tty
transmission. Usually, the keyer is adjusted for 850 Hz
(425 Hz above and 425 Hz below the assigned
frequency). A space will be 425 Hz above and a mark
will be 425 Hz below the operating frequency.
AFTS systems use amplitude modulation to
change dc mark and space pulses into audio. A basic
tone-modulated system is shown in figure 2-16. The
audio conversion is done by an audio oscillator in the
tone convertor. Varying the tone according to the
characters transmitted from the tty equipment
amplitude-modulates the transmitter carrier wave. The
receiver demodulates this signal, separating the audio
signal from the carrier.
In both the RFCS and AFTS systems, the tty
signals pass through the panel that controls looping
current. Looping current is the current supplied by the
tty solid-state power supply. The tty panel integrates
the tone-modulated and carrier-frequency shift
systems. By allowing the tty equipment to be set up in
any configuration desired, this panel gives the
operators maximum operational flexibility with the
least amount of circuitry and equipment.
Figure 2-16.—Basic tone-modulated (AFTS) system.
2-18
