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A few more updates tothe User Guide, making it "minimally acceptable" for the GA release of v2.0.
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@ -8,9 +8,9 @@ false decodes.
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For example: when you decide to answer a CQ, you already know your own
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callsign and that of your potential QSO partner. The software
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therefore "`knows`" what to expect for 57 of the 72 message bits (28
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bits for each of two callsigns, 1 bit for message type) in the next
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received message. The decoder's task can thus be reduced to
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therefore "`knows`" what might be expected for at least 57 message
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bits (28 for each of two callsigns, 1 or more for message type) in the
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next received message. The decoder's task can thus be reduced to
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determining the remaining 15 bits of the message and ensuring that the
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resulting solution is reliable.
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@ -2,18 +2,20 @@
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=== Overview
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All QSO modes except ISCAT use structured messages that compress
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user-readable information into fixed-length packets of 72 bits. Each
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message consists of two 28-bit fields normally used for callsigns and
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a 15-bit field for a grid locator, report, acknowledgment, or 73. An
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additional bit flags a message containing arbitrary alphanumeric text,
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up to 13 characters. Special cases allow other information such as
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add-on callsign prefixes (e.g., ZA/K1ABC) or suffixes (e.g., K1ABC/P)
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to be encoded. The basic aim is to compress the most common messages
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used for minimally valid QSOs into a fixed 72-bit length. The
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information payload in FT8 includes 3 additional bits (75 bits total).
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One of the added bits is used to flag special messages used by the
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DXpedition station in FT8 DXpedition Mode. Uses for the remaining two
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bits are yet to be defined.
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user-readable information into fixed-length packets. JT4, JT9, JT65,
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and QRA64 use 72-bit payloads. Standard messages consist of two
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28-bit fields normally used for callsigns and a 15-bit field for a
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grid locator, report, acknowledgment, or 73. An additional bit flags
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a message containing arbitrary free text, up to 13 characters.
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Special cases allow other information such as add-on callsign prefixes
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(e.g., ZA/K1ABC) or suffixes (e.g., K1ABC/P) to be encoded. The basic
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aim is to compress the most common messages used for minimally valid
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QSOs into a fixed 72-bit length.
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The information payload for FT8 and MSK144 contains 77 bits. The 5
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additional bits are used to flag special message types used for FT8
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DXpedition Mode, contesting, nonstandard callsigns, and a few other
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special types.
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A standard amateur callsign consists of a one- or two-character
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prefix, at least one of which must be a letter, followed by a digit
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@ -42,22 +44,16 @@ additional information is sent in place of the grid locator or by
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encoding additional information into some of the 6 million available
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slots mentioned above.
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As a convenience for sending directed CQ messages, the compression
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algorithm supports messages starting with `CQ AA` through `CQ ZZ`.
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These message fragments are encoded internally as if they were the
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callsigns `E9AA` through `E9ZZ`. Upon reception they are converted
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back to the form `CQ AA` through `CQ ZZ`, for display to the user.
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As a convenience for sending directed CQ messages, the 72-bit
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compression algorithm supports messages starting with `CQ AA` through
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`CQ ZZ`. These message fragments are encoded internally as if they
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were the callsigns `E9AA` through `E9ZZ`. Upon reception they are
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converted back to the form `CQ AA` through `CQ ZZ`, for display to the
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user.
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The FT8 and MSK144 modes support a special feature allowing convenient
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transmission and acknowledgment of four-character grid locators, the
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required exchanges in most North American VHF contests. With this
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Contest Mode enabled, _WSJT-X_ supports messages of the form `W9XYZ
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K1ABC R FN42` by converting the grid locator to that of its
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diametrically opposite point on Earth. The receiving program
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recognizes a locator implying a distance greater than 10,000 km, does
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the reverse transformation, and inserts the implied "`R`". Obviously,
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this mode should not be used on the HF bands or under other
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circumstances where world-wide propagation is possible.
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The new FT8 and MSK144 protocols use a different lossless compression
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algorithm with features to generate and recognize the special messages
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used for contesting and the like. (More to come, here ...)
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To be useful on channels with low signal-to-noise ratio, this kind of
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lossless message compression requires use of a strong forward error
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@ -75,9 +71,9 @@ _WSJT-X_ modes have continuous phase and constant envelope.
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==== FT8
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Forward error correction (FEC) in FT8 uses a low-density parity check
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(LDPC) code with 75 information bits, a 12-bit cyclic redundancy check
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(CRC), and 87 parity bits making a 174-bit codeword. It is thus
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called an LDPC (174,87) code. Synchronization uses 7×7 Costas arrays
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(LDPC) code with 77 information bits, a 14-bit cyclic redundancy check
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(CRC), and 83 parity bits making a 174-bit codeword. It is thus
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called an LDPC (174,91) code. Synchronization uses 7×7 Costas arrays
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at the beginning, middle, and end of each transmission. Modulation is
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8-tone frequency-shift keying (8-FSK) at 12000/1920 = 6.25 baud. Each
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transmitted symbol carries three bits, so the total number of channel
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@ -231,7 +227,7 @@ which the probability of decoding is 50% or higher.
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|===============================================================================
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|Mode |FEC Type |(n,k) | Q|Modulation type|Keying rate (Baud)|Bandwidth (Hz)
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|Sync Energy|Tx Duration (s)|S/N Threshold (dB)
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|FT8 |LDPC, r=1/2|(174,87)| 8| 8-FSK| 6.25 | 50.0 | 0.27| 12.6 | -21
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|FT8 |LDPC, r=1/2|(174,91)| 8| 8-FSK| 6.25 | 50.0 | 0.27| 12.6 | -21
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|JT4A |K=32, r=1/2|(206,72)| 2| 4-FSK| 4.375| 17.5 | 0.50| 47.1 | -23
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|JT9A |K=32, r=1/2|(206,72)| 8| 9-FSK| 1.736| 15.6 | 0.19| 49.0 | -27
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|JT65A |Reed Solomon|(63,12) |64|65-FSK| 2.692| 177.6 | 0.50| 46.8 | -25
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@ -329,13 +325,13 @@ For details see Table 4, below.
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==== MSK144
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Standard MSK144 messages are structured in the same way as those in
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the slow modes, with 72 bits of user information. Forward error
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correction is implemented by first augmenting the 72 message bits with
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an 8-bit cyclic redundancy check (CRC) calculated from the message
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bits. The CRC is used to detect and eliminate most false decodes at
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the receiver. The resulting 80-bit augmented message is mapped to a
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128-bit codeword using a (128,80) binary low-density-parity-check
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Standard MSK144 messages are structured in the same way as in FT8,
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with 77 bits of user information. Forward error correction is
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implemented by first augmenting the 77 message bits with a 13-bit
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cyclic redundancy check (CRC) calculated from the message bits. The
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CRC is used to detect and eliminate most false decodes at the
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receiver. The resulting 90-bit augmented message is mapped to a
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128-bit codeword using a (128,90) binary low-density-parity-check
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(LDPC) code designed by K9AN specifically for this purpose. Two 8-bit
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synchronizing sequences are added to make a message frame 144 bits
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long. Modulation is Offset Quadrature Phase-Shift Keying (OQPSK) at
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@ -379,6 +375,6 @@ and your QSO partner ± 200 Hz.
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|JT9F |K=32, r=1/2|(206,72)| 8| 9-FSK| 50.0 | 450 | 0.19| 1.700
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|JT9G |K=32, r=1/2|(206,72)| 8| 9-FSK|100.0 | 900 | 0.19| 0.850
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|JT9H |K=32, r=1/2|(206,72)| 8| 9-FSK|200.0 | 1800 | 0.19| 0.425
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|MSK144 |LDPC |(128,80)| 2| OQPSK| 2000 | 2400 | 0.11| 0.072
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|MSK144 |LDPC |(128,90)| 2| OQPSK| 2000 | 2400 | 0.11| 0.072
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|MSK144 Sh|LDPC |(32,16) | 2| OQPSK| 2000 | 2400 | 0.20| 0.020
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|=====================================================================
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