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.

Figure 3-20A. - Cw compared to an rfcs teletypewriter signal.

Figure 3-20B. - Cw compared to an rfcs teletypewriter signal.

AUDIO FREQUENCY TONE SHIFT. - Tone-modulated (afts) systems use amplitude modulation to change dc mark and space impulses into audio electrical impulses.

A basic tone-modulated system is shown in figure 3-21.Conversion to audio tones is accomplished by an audio oscillator in the tone converter. Rapid varying of the tone, according to the characters transmitted from the teletypewriter equipment, amplitude modulates the carrier wave in the transmitter. The receiver receives the modulated signal and separates the audio signal from the carrier. This process of separating the modulated signal is known as detection or demodulation.

Figure 3-21. - Basic tone modulated (afts) system.

RADIO-FREQUENCY-CARRIER SHIFT. - For frequency-shift (fsk) systems, the transmitter provides a source of radio-frequency excitation. Figure 3-22 illustrates a basic frequency-shift keyed system. In modern systems, the keyer is built into the transmitter. The keyer shifts the signal box below or above the assigned frequency to correspond with the mark or space required to transmit tty characters. Normally the keyer is adjusted for an 850-hertz spread, 425 hertz above and 425 hertz below the assigned frequency. A spacing impulse will be 425 hertz above the operating frequency, and a marking impulse will appear 425 hertz below.

Figure 3-22. - Basic radio-frequency-carrier shift system (rfcs).

In both the tone-modulated system and the carrier-frequency shift system, all tty signals pass through the tty panel that controls the looping current in all the circuits. Looping current is the current supplied by the tty battery. The tty panel integrates the tone-modulated and the carrier-frequency shift systems. It provides every possible interconnection of available tty equipment. With this configuration maximum operational flexibility is achieved with the least amount of circuitry and equipment.

Q.19 What is the function of a keyer?
Q.20 What is the function of a converter?
Q.21 Basically describe an afts system.
Q.22 Basically describe an rfcs system.