question: How do radios (or other communications) send different messages on very similar frequencies?
comment: Imagine all the frequencies in the air are like a bucket of mixed coins. You throw the coins through one of those coin sorters and then just look at the nickels for one channel, and the quarters for another channel. That coin sorter is called a band-pass filter.
passages: Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal
  
  This raw signal is called an interference pattern. The receiver then extracts an intelligible signal for any known sender by combining the sender's code with the interference pattern. The following table explains how this works and shows that the signals do not interfere with one another:
   For each message, the first line is provided by the intercept operator, giving call signs, frequency, time, and reference number. The rest of the message is a transcript of the Morse code transmission.
  
  At the beginning of the intercepted message there is a header which consists of 8 four-digit groups. Initially, the meaning of the numeric header is not known; the meanings of various components of this header (such as a serial number assigned by the transmitting organization's message center) can be worked out through traffic analysis.
   The most common test signal is a series of "RYRYRY" characters, as these form an alternating tone pattern exercising all bits and are easily recognized. Pangrams are also transmitted on RTTY circuits as test messages, the most common one being "The quick brown fox jumps over the lazy dog", and in French circuits, "Voyez le brick géant que j'examine près du wharf"
  
  Section::::Technical specification.
   There is one second of blank audio between each section, and before and after each message. For those used to packet communications systems where each packet has a checksum, note that there is no checksum used in the message format. Each message is supposed to be transmitted 3 times, and the receiver is obliged to implement columnar parity correction.
  
  The combined tones date back to 1976 when they were made part of the Emergency Broadcast System, the EAS' predecessor.
  
  Section::::Event codes.
   For cooperative decoding, the destination node combines two signals received from the source and the relay nodes which results in the diversity advantage. The whole received signal vector at the destination node can be modeled as:
  
  where formula_16 and formula_17 are the signals received at the destination node from the source and relay nodes, respectively. As a linear decoding technique, the destination combines elements of the received signal vector as follows:
   A number of alternative possibilities exist as to how this frequency domain spreading can take place, such as by using a long PN code and multiplying each data symbol, d, on a subcarrier by a chip from the PN code, c, or by using short PN codes and spreading each data symbol by an individual PN code — i.e. d is multiplied by each c and the resulting vector is placed on N subcarriers, where N is the PN code length.
   The truth in both cases is the articulation of the message and the package as one. The construction of the message from social and historical context is the seed as is the pre-existing message is for the transmission model. Therefore, any look into communication theory should include the possibilities drafted by such great scholars as Richard A. Lanham and Goffman that style and performance is the whole process. lun
   BULLET::::- Direct digital synthesis
  
  Section::::Determining the frequency.:Frequency multiplication.
   This is a one-way model to communicate with others. It consists of the sender encoding a message and channeling it to the receiver in the presence of noise. In this model there is no feedback or response which may allow for a continuous 
  
  exchange of information (F.N.S. Palma, 1993).
   Section::::Technical Details.:Sharing the channel.
  
  A number of data "conversations" are possible on a single radio channel over a finite period.
  
  Section::::Technical Details.:Station configuration.
   During World War II, Golden Age of Hollywood actress Hedy Lamarr and avant-garde composer George Antheil developed an intended jamming-resistant radio guidance system for use in Allied torpedoes, patenting the device under US Patent 2,292,387 "Secret Communications System" on August 11, 1942. Their approach was unique in that frequency coordination was done with paper player piano rolls - a novel approach which was never put into practice.
  
  Section::::Spread-spectrum clock signal generation.
   This radiotelegraph message format (also "radio teletype message format", "teletypewriter message format", and "radiotelephone message format") and transmission procedures have been documented in numerous military standards, going back to at least World War II-era U.S. Army manuals.
  
  Section::::Historical Development.
  
  BULLET::::- U.S. Army Field Manual FM 24-5 Basic Field Manual, Signal Communication
  
  BULLET::::- U.S. Army Field Manual FM 24-17 Tactical Communications Center Operations
  
  BULLET::::- U.S. Army Field Manual FM-24-19 Radio Operator's Handbook (Chapter 5: Operating Procedures)
  
  BULLET::::- Messages were formerly prepared on DD Form 173/4, Joint Message Form
   Daniel Chandler criticizes the transmission model in the following terms:
  
  BULLET::::- It assumes that communicators are isolated individuals.
  
  BULLET::::- It makes no allowance for differing purposes.
  
  BULLET::::- It makes no allowance for differing interpretations.
  
  BULLET::::- It makes no allowance for unequal power relationships.
  
  Section::::Berlo.
  
  In 1960, David Berlo expanded Shannon and Weaver's 1949 linear model of communication and created the Sender-Message-Channel-Receiver (SMCR) Model of Communication. The SMCR Model of Communication separated the model into clear parts and has been expanded upon by other scholars.
   Many other types of modulation are also used. In some types a carrier wave is not transmitted but just one or both modulation sidebands. The modulated carrier is amplified in the transmitter, and applied to a transmitting antenna which radiates the energy as radio waves. The radio waves carry the information to the receiver location.
   For example, the publication "Communication Instructions, 1929", from the U.S. Navy Department, includes:
  
  BULLET::::- One procedure for messages transmitted "in naval form over nonnaval systems" (Part II: Radio, Chapter 15)
  
  BULLET::::- One procedure for exchanging messages with commercial radio stations (Part II: Radio, Chapter 16, pages 36–37 for examples; see also Part I: Chapter 7)
  
  BULLET::::- One procedure for messages transmitted within the Navy (Part IV: Procedure and Examples, Chapter 32, especially pages 21 & 22 for the format)
   Transmissions of messages for merchant ships via the Zone, Area or Coastal stations are arranged by certain designated shore-based "Arranging Authorities" (AA). These stations read the BAMS "message header" and route the BAMS messages, ensuring it is transmitted by correct Zone and Area stations assigned to them, and by Coastal stations if required. The Arranging Authority also ensures that Coastal stations under their control, transmit messages that are considered necessary, even if the originator does not specify such a transmission.
  
  Section::::Shore to Ship Communication.:Originators responsibility.
   The concept of the standard message format originated in the wired telegraph services. Each telegraph company likely had its own format, but soon after radio telegraph services began, some elements of the message exchange format were codified in international conventions (such as the International Radiotelegraph Convention, Washington, 1927), and these were then often duplicated in domestic radio communications regulations (such as the FCC in the U.S.) and in military procedure documentation.
  
  Military organizations independently developed their own procedures, and in addition to differing from the international procedures, they sometimes differed between different branches of the military within the same country.
   In the mid-twentieth century, the High Frequency radio bands were used by numerous stations sending seemingly random Morse code, usually in five-letter groups. As more advanced communications methods, such as teleprinter and satellite, took over, the number of such stations diminished, but another type appeared that transmitted spoken and also seemingly random number and letter groups, the latter usually using words from a radio alphabet such as ICAO/NATO alphabet.
   Section::::Radio and SFX.
   BULLET::::- Thiele, A. Neville (1960). "Techniques of Delay Equalisation," "Proceedings of the Institute of Radio Engineers, Australia", 21(4), pp. 225–241.
  
  BULLET::::- Thiele, A. Neville (1961). "A Hybrid Network for Mixing and Splitting Signals," "Proceedings of the Institute of Radio Engineers, Australia", 22(6), pp. 383–387.
   We consider a wireless relay system that consists of source, relay and destination nodes. It is assumed that the channel is in a half-duplex, orthogonal and amplify-and-forward relaying mode. Differently to the conventional direct transmission system, we exploit a time division relaying function where this system can deliver information with two temporal phases.
  
  On the first phase, the source node broadcasts information formula_1 toward both the destination and the relay nodes. The received signal at the destination and the relay nodes are respectively written as:
   For the case of MC-DS-CDMA where OFDM is used as the modulation scheme, the data symbols on the individual subcarriers are spread in time by multiplying the chips on a PN code by the data symbol on the subcarrier. For example, assume the PN code chips consist of {1, -1} and the data symbol on the subcarrier is -"j". The symbol being modulated onto that carrier, for symbols 0 and 1, will be -"j" for symbol 0 and +"j" for symbol 1.
   BULLET::::- Demodulation: After the radio signal is filtered and amplified, the receiver must extract the information-bearing modulation signal from the modulated radio frequency carrier wave. This is done by a circuit called a demodulator (detector). Each type of modulation requires a different type of demodulator
   Alan Turing in 1940 used similar ideas as part of the statistical analysis of the breaking of the German second world war Enigma ciphers.
   Although each message will be received by all installed teleswitches, the unique user and group-codes carried by the message ensure that only teleswitches carrying the same combination of codes will act on it.
  
  Section::::History.
  