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149 lines
5.3 KiB
Plaintext
149 lines
5.3 KiB
Plaintext
// Status: edited
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=== AP Decoding
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The _WSJT-X_ decoders for FST4, FT4, FT8, JT65, and Q65 include
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procedures that use naturally accumulating information during a
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minimal QSO. This _a priori_ (AP) information increases sensitivity
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of the decoder by up to 4 dB, at the cost of a slightly higher rate of
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false decodes. AP is optional in FT8 and JT65, but is always enabled
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for Q65 and for FT4 and FST4 when decode depth is Normal or Deep.
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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 might be expected for at least 57 message
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bits (28 for each of two callsigns, one or more for message type) in the
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next received message. The decoder's task is thus 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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AP decoding starts by setting AP bits to the hypothesized values, as
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if they had been received correctly. We then determine whether the
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remaining message and parity bits are consistent with the hypothesized
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AP bits, with a specified level of confidence. Successful AP decodes
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are labeled with an end-of-line indicator of the form `aP`, where `P`
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is one of the single-digit AP decoding types listed in Table 1. For
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example, `a2` indicates that the successful decode used MyCall as
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hypothetically known information.
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[[FT8_AP_INFO_TABLE]]
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.FST4, FT4, and FT8 AP information types
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[width="35%",cols="1h,<10m",frame=topbot,options="header"]
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|===
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|aP|Message components
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|a1|CQ     ?     ?
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|a2|MyCall     ?     ?
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|a3|MyCall DxCall     ?
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|a4|MyCall DxCall RRR
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|a5|MyCall DxCall 73
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|a6|MyCall DxCall RR73
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|===
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If a codeword is found that is judged to have high (but not
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overwhelmingly high) probability of being correct, a `?` character is
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appended when the decoded message is displayed. To avoid misleading
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spots of occasional false decodes, messages so marked are not
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forwarded to {pskreporter}.
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Table 2 lists the six possible QSO states that are tracked by the
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_WSJT-X_ auto-sequencer, along with the type of AP decoding that would
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be attempted in each state in FT4 or FT8. The FST4 table (not shown)
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is the same except that it omits the decoding attempts for AP types
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4 and 5 to save time.
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[[FT8_AP_DECODING_TYPES_TABLE]]
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.FT4 and FT8 AP decoding types for each QSO state
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[width="35%",cols="10h,<20m",frame=topbot,options="header"]
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|===
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|State |AP type
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|CALLING STN | 2, 3
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|REPORT | 2, 3
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|ROGER_REPORT | 3, 4, 5, 6
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|ROGERS | 3, 4, 5, 6
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|SIGNOFF | 3, 1, 2
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|CALLING CQ | 1, 2
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|===
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Decoding with _a priori_ information behaves slightly differently
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in JT65. Some details are provided in Tables 3 and 4.
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[[JT65_AP_INFO_TABLE]]
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.JT65 AP information types
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[width="35%",cols="1h,<10m",frame=topbot,options="header"]
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|===
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|aP | Message components
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|a1 | CQ     ?     ?
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|a2 | MyCall     ?     ?
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|a3 | MyCall DxCall     ?
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|a4 | MyCall DxCall RRR
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|a5 | MyCall DxCall 73
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|a6 | MyCall DxCall DxGrid
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|a7 | CQ DxCall DxGrid
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|===
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[[JT65_AP_DECODING_TYPES_TABLE]]
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.JT65 AP decoding types for each QSO state
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[width="35%",cols="10h,<20m",frame=topbot,options="header"]
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|===
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|State |AP type
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|CALLING STN |2, 3, 6, 7
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|REPORT |2, 3
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|ROGER_REPORT |3, 4, 5
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|ROGERS |3, 4, 5
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|SIGNOFF |2, 3, 4, 5
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|CALLING CQ |1, 2, 6
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|===
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=== Decoded Lines
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Displayed information accompanying decoded messages generally includes UTC,
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signal-to-noise ratio in dB, time offset DT in seconds, and
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audio frequency in Hz. Some modes include additional information such
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as frequency offset from nominal (DF), frequency drift (Drift or F1),
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or distance (km or mi).
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There may also be some cryptic characters with special meanings
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summarized in the following Table:
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[[DECODED_LINES_TABLE]]
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.Notations used on decoded text lines
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[width="50%",cols="h,3*^",frame=topbot,options="header"]
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|===
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|Mode |Mode character|Sync character|End of line information
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|FST4 | ` | | ?   aP
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|FT4 | + | | ?   aP
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|FT8 | ~ | | ?   aP
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|JT4 | $ | *, # | f, fN, dCN
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|JT9 | @ | |
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|JT65 | # | |
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|JT65 VHF| # | *, # | f, fN, dCN
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|Q65 | : | | qP
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|MSK144 | & | |
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|===
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Sync character::
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`*` - Normal sync +
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`#` - Alternate sync
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End of line information::
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`?` - Decoded with lower confidence +
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`a` - Decoded with aid of some _a priori_ (AP) information +
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`C` - Confidence indicator [Deep Search; (0-9,*)] +
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`d` - Deep Search algorithm +
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`f` - Franke-Taylor or Fano algorithm +
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`N` - Number of Rx intervals or frames averaged +
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`P` - Number indicating type of AP information (Table 1 or Table 6) +
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[[Q65_AP_INFO_TABLE]]
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.Q65 end-of-line codes
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[width="50%",cols="1h,<10m",frame=topbot,options="header"]
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|===
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|qP|Message components
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|q0|?     ?     ?
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|q1|CQ     ?     ?
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|q2|MyCall     ?     ?
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|q3|MyCall DxCall     ?
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|q4|MyCall DxCall     [<blank> \| RRR \| RR73 \| 73]
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|q5|MyCall DxCall     ?         (*Max Drift* = 50)
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|===
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