Figure 3-27 is a simplified block diagram of a HALF-DUPLEX (send or receive) uhf, audio-frequency-tone shift system. A half-duplex communications circuit permits two-way communications between stations. Communications can be in either direction but not simultaneously. The term half-duplex is qualified by adding send only, receive only, or send or receive. Let's use the block diagram to trace a signal through the system.
Figure 3-27. - Half-duplex afts teletypewriter system.
SIGNAL FLOW. - On the transmit side, dc signals from the tty set are fed to the communication patching panel. From the panel they are patched to the tone terminal set. The tone terminal set converts 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.
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.
Figure 3-28. - Components of a sine wave.
Figure 3-29, view A, illustrates, the fundamental principle of tdm. Let's look at an example. Assume that a 3,000-hertz tone is applied to each of the six channels in the transmitter. Assume also that the rotating switch turns fast enough to sample, in turn, each of the six channels 2.4 times during each cycle of the 3,000-hertz tone. The speed of rotation of the switch must then be 2.4 X 3,000 or 7,200 rotations per second. This is the optimum sampling for a practical system.
Figure 3-29. - Fundamental principle of time-division multiplexing.
When the transmitter and receiver switches are synchronized, the signals will be fed in the proper sequence to the receiver channels. The samples from transmitter channel one will be fed to receiver channel one. In this way, many channels of audio are combined to form a single output (multiplexed) chain. Time spacing occurs between the components of the separate channels. The chain is transmitted (via wire or radio path) to distant demultiplexing receivers. Each receiving channel functions to select and reconstruct only the information included in the originally transmitted channel.
In most present day applications, electronic switching is used as the sampling component. The main advantage to electronic sampling is the longer life of an electronic switch when compared to an electromechanical switch. We use a mechanical system in our example to make this concept easier for you to see.
Now let's look at figure 3-29, view B, where channel one is shown sampled four times. (This is the output of channel one in our transmitter.) Figure 3-29, view C, shows all six channels being sampled four times during each cycle. (This is the output of the rotating switch in our transmitter.) What you see here is a continuous, time-sharing waveform.
More than six channels (perhaps 24 or more) may be used. As we increase the number of channels, the width of each sample segment must be reduced. The problem with reducing the width of the pulse is that the bandwidth (bw) necessary for transmission is greatly increased. Decreasing the pulse width decreases the minimum required rise time of the sampling pulse and increases the required bandwidth. When you increase the number of channels, you increase the bw. The bw is also affected by the shape of the sampling pulse and the method used to vary the pulse.
Common methods of time-division multiplexing include PULSE AMPLITUDE MODULATION (pam), PULSE WIDTH or PULSE DURATION MODULATION (pwm or pdm), PULSE POSITION MODULATION (ppm), and PULSE CODE MODULATION (pcm). We have been studying an example of pulse amplitude modulation. (These methods of tdm were discussed in NEETS, Module 12, Modulation Principles.)
FREQUENCY DIVISION. - Frequency division multiplexing (fdm), unlike tdm, transmits and receives for the full 360 degrees of a sine wave. Fdm used presently by the Navy may be divided into two categories. One category is used for voice communications and the other for tty communications.
The normal voice speaking range is from 100 to 3,500 hertz. During single channel AM voice communications, the audio frequency amplitude modulates a single rf (carrier frequency). However, in voice fdm, each voice frequency modulates a separate frequency lower than the carrier frequency (subcarrier frequency). If these subcarrier frequencies are separated by 3,500 hertz or more, they may be combined in a composite signal. This signal modulates the carrier frequency without causing excessive interference.
In figure 3-30, the output of channel one is the voice frequency range of 100 to 3,500 hertz. The output of channel two is the combination of a different voice frequency with a subcarrier frequency of 4,000 hertz. The output of channel three is another voice frequency. This voice frequency combined with a subcarrier frequency of 8,000 hertz gives you an output frequency range of 8,100 to 11,500 hertz. The overall bw for the composite modulation package shown is 100 to 15,500 hertz. Each separate channel occupies its own band of frequencies. The composite signal is used to modulate the carrier frequency of the transmitter.
Figure 3-30. - Block diagram of a frequency-division multiplexing system.
Multichannel broadcast and ship/shore terminations use tty fdm. With this system, each channel of the composite tone package of the broadcast is assigned an audio frequency. By multiplexing tty circuits, up to 16 circuits may be carried in any one of the 3,000 hertz multiplexed channels described above. Don't confuse the two types of multiplexing. In the first case, 3,000 hertz audio channels have been combined. In the second case, a number of dc tty circuits are converted to tone keying and combined in a single 3,000-hertz audio channel. Figure 3-31 illustrates a 16-channel, tty-multiplexing system. The output of the dc pulsed circuits is converted to audio keying. Each channel has a separate audio center frequency. Channel frequencies range from 425 hertz for the lowest channel to 2,975 hertz for the highest channel. A mark in an individual tty loop keys an audio tone 42.5 hertz below the center frequency. A space in the input signal keys an audio tone 42.5 hertz above the center frequency. Let's look at an example. The mark and space frequencies for channel one are calculated as 382.5 hertz and 467.5 hertz, respectively (425± 42.5). Combining these keyed tones into a composite signal results in a tone package within a standard 3,000-hertz bandwidth. By occupying no more than 3,000 hertz of the audio spectrum, the output signal is suitable for transmission via radio or landline.
Figure 3-31. - Block diagram of modulator units.