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2. Procedural codes
3. Codes and ciphers
Presented for what it may be worth, with no claim for technical expertise, and not the slightest warranty!
Context is critical, and usage is not at all uniform. Here is a start:
Field telephone systems (usually military):
Think of two manual switchboards connected by a two-wire line. Now think of a transformer at each switchboard. The circuit from the switchboard goes to one coil of the transformer, and the other coil is connected to the line between switchboards. The varying voice signal is transmitted without the direct current from the batteries at the switchboard. (Oversimplified, to be sure.)
Next, think of the 'line' side of each transformer coil having a center tap. Then, at each switchboard, connect a line from the center tap to a telephone (or a telegraph set) that provides one wire to each telephone, etc. Connect the other telephone wire to ground. Then the current for one side of the circuit flows through the ground. The current for the other side divides (theoretically evenly) at the center tap of the transformer, flowing in the same direction through both wires to the other switchboard, where it combines at the center tap and goes to the other telephone, etc. Because the currents in the two halves of the transformer are equal and opposite, the signal does not appear on the switchboard side of the transformer. That is (or was) a simplex circuit. The center-tapped transformer is a repeating coil. (There were also loading coils that could be used to add inductance to a line with excess capacitance, and there were different repeating coils with other uses).
If have two phone lines (A and B) between the same two switchboards, with two repeating coils at each switchboard, you can connect telephones just as for a simplex circuit, except that the other wires from the telephones are connected to the center taps of the other repeating coils instead of to ground. That is a phantom circuit, with the advantage that the wire circuit usually has lower resistance than the ground path. It was also possible to make a simplexed phantom circuit.
The theoretically even split of voice current at the repeating coil does not always occur in practice, so both these circuits are more susceptible to noise, interference, and cross-talk than the 'plain' connection.
Time-share computer systems:
In the old days (about 1970) when computers were scarce and expensive, it was possible for many users, typically connected through telephone lines, to share a computer. The term 'simplex', to the extent it was used at all, referred to a one-way transmission, such as a temperature measurement sent to the computer every 10 minutes.
'Duplex' was divided into 'half duplex' and 'full duplex'. A half duplex connection sent each typed character to your teletype printer and also to the telephone line to the computer. A full duplex connection sent each typed character through the line to the computer, which then echoed it back to your printer. (I say teletype printer because that is what was available in those days.)
This deals with two-way communications, not broadcasts, and the specific meaning is particularly sensitive to context. Note that although I speak of two operators, the messages can be heard by anyone listening.
A simplex radio link is two-way, but only one way at a time, using a single radio frequency. Operator A sends a message to Operator B. In that process, A's transmitter is on, and his (or her) receiver is off, so B can only listen. When A finishes, typically with the word "over", releasing the push-to-talk switch, then B can send a reply.
There are at least two kinds of duplex radio link.
In one kind, Operator A and B each (usually) have two transmitter-receiver combinations (transceivers). One transceiver is set to Frequency 1 and the other to Frequency 2. A transmits on Frequency 1 and listens on 2, while B transmits on 2 and listens on 1. Both users can talk at the same time (as with cell phones).
The other kind of duplex also uses two frequencies, but it gets more complicated. Radio communication at very high and ultra-high frequencies (above about 30 MHz) is limited in range (distance) because the signals are mostly "line of sight". Two operators can communicate in a simplex link only if they can (approximately) see each other. Because the range is short anyway, these transceivers often operate on batteries at low power and can be small and convenient. Because the line of sight is much longer from an elevated location, high ground (or a very tall antenna tower) is desirable, but few individual users have that option. However, a group of users can install a jointly used receiver-transmitter combination at a high point. That system is a repeater. Then Operator A transmits to the repeater on its input frequency, Frequency 1. The repeater converts the incoming message to its output frequency, Frequency 2 and transmits that as the message is coming in on Frequency 1. Operator B, listening on Frequency 2, hears the message even when he is too far away, or in a poor position, so that he cannot hear A directly. Then Operator 2 replies back to the repeater on Frequency 1, which the repeater converts to Frequency 2, which in turn is heard by Operator 1. Unlike the other kind of duplex operation, only one operator should transmit at a time.
Frequency 1 and Frequency 2 are typically in the same band, a segment of the radio spectrum dedicated to a specific use. There are also cross-band repeaters which work between bands. For example, a highway patrol officer, out of the patrol car, might carry a small short-range radio operating at something like 800 MHz. That would communicate with the car radio, set up as a cross-band repeater, which would retransmit at something like 50 MHz, and with longer range and more power, to a dispatcher. The system works in both directions, of course.