2-9Point-to-Point.—Most point-to-point, long-haul circuits between naval communications stationsquickly use up the available frequency spectrum that ssb provides. Independent sideband (isb)transmission is normally used to compensate for the deficiency. Isb is used extensively in navalcommunications to expand our traffic capabilities. You will find there is a similarity between ssb and isb.Isb uses outputs from two sideband generators; it suppresses both carriers and then filters out an uppersideband from one and a lower sideband from the other. We then combine the two remaining sidebandsand transmit an envelope with upper and lower sidebands that contain different intelligence. Isb can beused with MULTIPLEXING (a method for simultaneous transmission of two or more signals over acommon carrier wave) to transmit a lot of intelligence on one circuit. Independent sideband andmultiplexing will be discussed in more detail in chapter 3.Ship-to-Shore.—Many ships handle enough message traffic to justify ship-to-shore teletypewritercircuits. Depending on traffic load, these circuits may contain from one to four (minimum) teletypewritercircuits on one sideband circuit. If the traffic load warrants more than one teletypewriter circuit, weusually use time division multiplex or frequency division multiplex (mux) equipment. This equipment iscapable of handling many incoming and outgoing circuits. One circuit normally is used as anORDERWIRE CIRCUIT for operator-to-operator service messages and for making frequency changeswhen necessary. The remaining circuits are available for handling official message traffic.Q8.Why are frequency multipliers used?Q9.What are two advantages of ssb transmission?Q10.What is the purpose of an order-wire circuit?Ship-to-Ship.—Ship-to-ship ssb teletypewriter circuits are in wide use today. Their main applicationis with task force or task group networks or with several ships in company. By using this type of network,ships can send their outgoing messages to another ship that relays traffic ashore. You can see thisprocedure saves manpower and circuit time, prevents individual ships from overcrowding ship-to-shorecircuits, and conserves the frequency spectrum. Depending on the number and types of ships in company,the guard can be shifted to other ships from time to time. A major advantage of these circuits is thatelectronic cryptographic devices can be used to send classified messages without need for manualencryption. These circuits are used for incoming as well as outgoing traffic, and they can use either hf oruhf communications equipment.RECEIVER FUNDAMENTALSAn AM receiver processes amplitude-modulated signals received by its antenna. It delivers an outputthat is a reproduction of the signal that originally modulated the rf carrier at the transmitter. The signalcan then be applied to some reproducing device, such as a loudspeaker, or to a terminal device, such as ateletypewriter. Actual AM receivers vary widely in complexity. Some are very simple; others contain alarge number of complex circuits.FUNCTIONSWhatever its degree of sophistication, a receiver must perform certain basic functions to be useful.These functions, in order of their performance, are reception, selection, detection, and reproduction.
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