Simplex
The simplest way in which electronic communication is conducted is one-way communications, normally referred to as simplex communication. Examples are shown in Fig. 1-3. The most common forms of simplex communication are radio and TV broadcasting. Another example of one-way communication is transmission to a remotely controlled vehicle like a toy car or an unmanned aerial vehicle (UAV or drone).
Figure 1-3 Simplex communication.
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| (a) TV broadcasting |
Full Duplex The bulk of electronic communication is two-way, or duplex communication. Typical duplex applications are shown in Fig. 1-4. For example, people communicating with one another over the telephone can talk and listen simultaneously, as Fig. 1-4(a) illustrates. This is called full duplex communication.
Half Duplex
The form of two-way communication in which only one party transmits at a time is known as half duplex communication [see Fig. 1-4(b)]. The communication is two-way, but the direction alternates: the communicating parties take turns transmitting and receiving. Most radio transmissions, such as those used in the military, i re, police, aircraft, marine, and other services, are half duplex communication. Citizens band (CB), Family Radio, and amateur radio communication are also half duplex.
Analog Signals
An analog signal is a smoothly and continuously varying voltage or current. Some typical analog signals are shown in Fig. 1-5. A sine wave is a single-frequency analog signal. Voice and video voltages are analog signals that vary in accordance with the sound or light variations that are analogous to the information being transmitted.
Digital Signals
Digital signals, in contrast to analog signals, do not vary continuously, but change in steps or in discrete increments. Most digital signals use binary or two-state codes. Some
Figure 1-5 Analog signals. (a) Sine wave “tone.” (b) Voice. (c) Video (TV) signal.
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| (A) |
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| (B) |
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| (C) Figure 1-6 Digital signals. (a) Telegraph (Morse code). (b) Continuous-wave (CW)
code. (c) Serial binary code.  examples are shown in Fig. 1-6. The earliest forms of both wire and radio communication used a type of on/off digital code. The telegraph used Morse code, with its system of
short and long signals (dots and dashes) to designate letters and numbers. See Fig. 1-6(a).
In radio telegraphy, also known as continuous-wave (CW) transmission, a sine wave
signal is turned off and on for short or long durations to represent the dots and dashes.
Refer to Fig. 1-6(b). Data used in computers is also digital. Binary codes representing numbers, letters, and
special symbols are transmitted serially by wire, radio, or optical medium. The most commonly used digital code in communications is the American Standard Code for Information
Interchange (ASCII, pronounced “ask key”). Fig. 1-6(c) shows a serial binary code. Many transmissions are of signals that originate in digital form, e.g., telegraphy
messages or computer data, but that must be converted to analog form to match the
transmission medium. An example is the transmission of digital data over the telephone network, which was designed to handle analog voice signals only. If the digital data is converted to analog signals, such as tones in the audio frequency range, it
can be transmitted over the telephone network. Analog signals can also be transmitted digitally. It is very common today to take
voice or video analog signals and digitize them with an analog-to-digital (A /D) converter.
The data can then be transmitted efi ciently in digital form and processed by computers
and other digital circuits |
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