3-30
the dc signals into audio tone-shift signals. These signals are then patched to the transmitter section of the
transceiver through the transmitter transfer switchboard. The audio tone-shift signals modulate the rf
carrier generated by the transmitter (xmtr). The rf tone-modulated signals are then radiated by the
antenna.
On the receive side, the rf tone-modulated signals are received at the antenna. You then patch the
signal via the multicoupler to the receiver section of the transceiver. Demodulation takes place at this
point. The resulting audio tone-shift signals are then patched through the receiver transfer switchboard.
The signals now go from the switchboard to the tone terminal set, where they are converted back to dc
signals. The dc signals are then patched through the communication patching panel to the tty for printing.
TONE TERMINAL SET.—In tone modulation transmission, the tty pulses are converted into
corresponding audio tones. These tones amplitude modulate the rf carrier in the transmitter. Conversion to
audio tones is accomplished by an audio oscillator in the tone converter.
An internal relay in the tone converter closes the control line to the transmitter. This keys the
transmitter on the air when the operator begins typing a message. The transmitter remains keyed until
after the message has been transmitted.
On the receive side, the tone converter accepts the mark and space tones coming in from a receiver
and converts them into signals suitable to operate a relay in the converter. The make and break contacts of
the relay are connected in the local tty dc loop circuit. This causes the teletypewriter to print in unison
with the mark and space signals from the distant tty.
Multiplexing Equipment
The number of communications networks in operation throughout any given area is increasing. As a
result, all areas of the rf spectrum have become highly congested.
The maximum number of intelligible transmissions taking place in the radio spectrum is being
increased through the use of MULTIPLEXING. Multiplexing is the simultaneous transmission of a
number of intelligible signals (messages) in either or both directions using only a single rf carrier. You
may use two methods of multiplexing. These are TIME-DIVISION and FREQUENCY-DIVISION.
TIME-DIVISION.—With AM voice and tone communications, we want to transmit and receive for
360 degrees of each sine wave. However, an audio signal may be transmitted and received satisfactorily
by periodically sampling the signal. The sampling process yields a received signal like the one shown in
figure 3-28. There is no limit to the maximum number of samples that may be made, but you must sample
at least twice per cycle of audio to get satisfactory results. In practical systems, 2.4 samples per cycle are
usually taken. This concept of sampling forms the basis for time-division multiplex (tdm) operation.
