Below 30 megahertz, DTMF with FM signal would take a large amount of bandwidth as well as spread spectrum. The only other options left are Pulse modulation, and FSK.
Commonly, Frequency shift keying is used on these bands, which modulates the frequency a bit with data.
Silly memory aid: The question asks about modulation below 30 MHz. FSK is the shortest/smallest answer.
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Forward Error Correction is a means to prevent transmitted data that has been corrupted by noise or signal loss from being misinterpreted at the receiving device. It involves transmitting redundant data bits that the receiving device can use to verify the accuracy of the transmission. A simple form of forward error correction would be transmitting each bit of data three times (a triplet). The receiver uses an algorithm to interpret this data stream averaging the bits in each triplet to correct for noise. It then "decides" what the correct interpretation of the data should be based on that computation. Forward Error Correction does require more bandwidth than Backward Error Correction due to the additional data being sent as well as more processor time. But FEC is preferred for long range, single data transmissions that are subject to noise, as can occur with amateur radio transmissions.
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JT65 is an exceptionally efficient protocol that can detect very weak signals. One way that the protocol has been optimized is to assume that stations have a reliable time source (e.g. synchronized from the internet, NIST, WWV, or other means). This allows stations to listen to an entire transmission even when they're not sure if someone is transmitting and then perform extensive post-processing and error correction to pull the signal out of the noise.
The protocol used is that one station transmits beginning on odd minutes (e.g. 8:01, 8:03, 8:05) and the other station in a QSO transmits on the even minutes (8:00, 8:02, 8:04).
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Here is an explanation on how the crossed-ellipse display (or station monitor) works with RTTY:
The principle should be the same.
Silly hint: suddenly disappear = selective fading
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Winlink is a store and forward type of messaging system offering worldwide radio email service that uses radio pathways where the internet is not present. Winlink users email with attachments, position reporting, weather and information bulletins, and used in emergency and disaster relief communications. winlink.org
For more on winlink, see Winlink network on Wikipedia.*
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This data rate allows HF packet transmissions to stay within the bandwidth used by other transmission modes such as SSB speech. This is important on the crowded HF bands.
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MFSK16 (Multi Frequency Shift Keying) is a way of transmitting digital data using multiple tones. In MFSK16 the number of tones is 16. Relatively high data throughput is attainable at unusually narrow bandwidth.
For example, a MFSK16 transmission at 62.5 bits/second (80 words/minute) has a bandwidth of 316 Hz. In practice, this text throughput figure is reduced by the need for forward error correction (FEC) characters.
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PACTOR is a radio modulation mode used by amateur radio operators, marine radio stations, and radio stations in isolated areas to send and receive digital information via radio. Of the choices presented it is the best mode to transfer binary files. Not to say it cannot be done in other modes.
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Similar to the way Morse Code has different lengths for different characters (E is one dot) PSK31 uses variable length coding where the most used characters are given shorter lengths to save bandwidth.
It was originally called varicode by its inventor, an amateur radio operator.
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PSK31 operates with a very narrow bandwidth of only 31Hz, with contacts spaced as close as 100Hz apart. As many as 20 conversations can be realistically squeezed into the bandwidth of a single 2.5KHz SSB transmission. [PSK31 Spectrum efficiency on Wikipedia]
MFSK16 uses about 316 Hz bandwidth.
(PS)K Pretty skinny
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Amateur transceivers use two different methods to modulate a signal using FSK: direct FSK and audio FSK. The difference between direct FSK and audio FSK is that direct FSK applies the data signal to the transmitter VFO. (E2E11) When using audio FSK, audio, typically from a computer sound card, is used to shift the frequency of the transmitted signal.
Note that only one answer addresses both Direct FSK and AFSK as presented in the question. -KE0NUI
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Automatic Link Establishment, commonly known as ALE, is the worldwide de facto standard for digitally initiating and sustaining HF radio communications. ALE is a feature in an HF radio transceiver system, that enables the station to make contact, or initiate a circuit, between itself and another HF radio station or network of stations automatically. The purpose is to provide a reliable, rapid method of calling and connecting during constantly changing HF ionospheric propagation, reception interference, and busy or congested HF channels. -w3mit -KE0IPR
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If you are transmitting on a frequency your intended recipient is not listening on, naturally they will not receive it. Additionally if you are using a different digital protocol than the intended recipient, they will not understand much the same way speaking in a foreign language would cause confusion. Finally, there may be a station from another direction that they can hear and you can't; this station's signal might be more powerful than yours and would either overpower yours or simply make both of them unintelligible.
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