The popular weak signal modes developed by Joe Taylor K1JT including JT9, JT65, and WSPR are designed to be received even with very low signal to noise ratios.

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JT-65, for instance can be copied with an SnR of -25dB, that is the signal is 25dB below the measured noise floor in the receiver's bandwidth. Such a signal would be absolutely inaudible to the human ear.

Such modes rely primarily on two things to make this possible:

1. Precise clock synchronization between the transmitter and receiver to enable process gain through the use of a matched receive filter, and

2. A very slow transmission rate.

JT-9 further requires a stable Local Oscillator due to its use of very small (1Hz) frequency shift keying.

Most of the other choices listed are intended for real-time keyboard-to-keyboard communication. FSK441 is a burst mode intended for highly intermittent propagation such as aurora, meteor scatter, or airplane scatter.

http://en.wikipedia.org/wiki/WSJT_(Amateur_radio_software)

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(A). PSK31 is a data transmission method that uses Phase-Shift Keying, with a symbol rate of 31 baud. PSK31 uses characters based on a system called Varicode, which uses shorter character bit sequences for common phases and longer ones for less common phases. As the Varicode name indicates, the number of data bits varies.

For more info see Wikipedia: PSK31

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When forming a packet for transmission the information is organized into a specific order so that the message segments may be properly reassembled and decoded after all parts are received.

Each data packet contain several fields, including the Header, which contains routing and handling information.

The packet also includes an address field indicating where the message is going; a control field, indicating the type of data; a frame number which indicates the section order; the data field; and the frame check or cyclic redundancy check field.

For more info see Wikipedia: Packet Radio

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The Baudot code is a character set used with some radioteletype (RTTY) transmissions. This code assigns each letter of the alphabet with a 5 bit code, with stop and start bits for each character.

Hint: the word "Baudot" has 5 letters plus an additional letter (6 total)

For more info see Wikipedia: Baudot code

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Hint: NACK = Not ACKnowledges so “retransmit”

PACTOR protocol is a modulated packet data mode that uses an ARQ, or Automatic Repeat Request, system for error controls. The receiving station will send a message letting the transmitter know that either ACK, the message received corrrectly and is ACKnowledged, or NAK, that the message has errors or is NOT ACKnowledged. In which case, the receiver is requesting that the packet be re-transmitted.

For more info see Wikipedia: PACTOR

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Hint. Excessive attempts drops the line, like a phone.

It will only try a certain number of times before it terminates the connection. This is called a timeout.

PACTOR combines the bandwidth efficiency of packet radio with the error-correction (CRC) and automatic repeat request (ARQ) of AMTOR.

WINMOR is a radio transmission protocol intended to be used in the Winlink 2000 Global Radio E-mail System by amateur radio operators, marine radio stations, and radio stations in isolated areas.[1] WINMOR will complement the PACTOR modes in the high frequency portion of the Winlink system.

These are internet protocols which have safeguards built in which will drop excessive requests to prevent a DoS situation.

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Hint ARQ = Automatic Repeat Request

When transmitting using the packet system for data transmissions, a mode of operation for automatic error control is ARQ which stands for Automatic Repeat ReQuest.

If the receiving station gets a message containing errors the operator will request that the packet be retransmitted. The packet message will re-transmit automatically until the receiver sends a message confirming that the message was received correctly, or the system times out.

For more info see Wikipedia: Automatic Repeat Request.

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Hint: Remember PSK31 uses Varicode for bauds (31 bauds)

Upper case letters use longer Varicode signals and thus slow down transmission

PSK31 uses Varicode.

The idea behind varicode is that symbols often used for transmission like the character "e" is given a smaller length of data, whereas obscure characters like "^" are assigned longer length codes to encode them.

Upper case letters are given longer codes compared to their lower case companions since lower case letters are used much more often than upper case letters.

Note: This is the same concept as International Morse Code - more common letters like "e" (dit) are shorter than less common letters like "h" (dit-dit-dit-dit).

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Hint: “S” = symbol Phase Shift Keying @31 Baud (symbol rate)

(A). The number 31 in the abbreviation for PSK31 is the approximate transmitted symbol rate.

PSK31 is a type of data mode of "Phase Shift Keying" at "31 baud." The actual baud rate used is 31.25 Hz. Similarly, the less common PSK10 and PSK 05 would correspond to baud rates of approximately 10 baud and 5 baud respectively.

For more info see Wikipedia: PSK31

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Hint: FEC forwards (TRANSMITS) redundant data

(C). Forward error correction allows the receiver to correct errors in received data packets by transmiting redundant information with the data.

This type of correction used in packet data transmissions is useful especially over "noisy" bands or in case of interference which may corrupt the signal.

For more info see Wikipedia: Forward error correction (FEC)

Hint: Look for the word “Data” in the answer.

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Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave.[1] The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information.[2] With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency.

Hint: FSK is "identified" with a "mark".

https://en.wikipedia.org/wiki/Frequency-shift_keying

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Hint: since 31 is a baud CODE, remember VARICODE

Hint 2: Both the question and the answer have the word "code" in them.

Varicode is used sending characters in a PSK31 signal.

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Unlike coding systems such as Baudot (5 bits per symbol) and ASCII (7 bits per symbol) which use a fixed length sequence to represent characters, PSK31 uses a variable-length encoding (1-10 bits) which assigns more commonly used characters to shorter symbols.

Variable-length encoding is generally more time efficient, at least when using the English language, as it requires fewer total symbols to be transmitted for a given message. Improved efficiency is important because of PSK31's very low data rate of 31 symbols per second.

The name chosen for the encoding scheme is Varicode which can be remembered as a shortened form of "Variable Length Coding"

Some other digital modes have adopted varicode including PSK63, PSK128 (both essentially faster version of PSK31) and QPSK31, QPSK128, etc. which transmit two bits per symbol instead of one. The FreeDV digital voice mode uses Varicode to efficiently encode a text subchannel in its datastream.

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See also: http://en.wikipedia.org/wiki/Varicode

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