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105 lines
4.1 KiB
Plaintext
105 lines
4.1 KiB
Plaintext
=== AP Decoding
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The _WSJT-X_ decoders for QRA64 and FT8 include optional procedures
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that use naturally accumulating information during a minimal QSO.
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This _a priori_ (AP) information increases sensitivity of the decoder
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by up to 4 dB, at the cost of a slightly higher rate of 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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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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[[AP_INFO_TABLE]]
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.AP information types
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[width="35%",cols="h10,<m20",frame=topbot,options="header"]
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|===============================================
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|P | Message components
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|1 | CQ     ?     ?
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|2 | MyCall     ?     ?
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|3 | MyCall DxCall     ?
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|4 | MyCall DxCall RRR
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|5 | MyCall DxCall 73
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|6 | 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.
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[[AP_DECODING_TYPES_TABLE]]
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.AP decoding types for each QSO state
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[width="35%",cols="h10,<m20",frame=topbot,options="header"]
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|===========================================
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|State |AP type
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|CALLING | 1, 2
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|REPLYING | 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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|===========================================
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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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|FT8 | ~ | | ?   aP
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|JT4 | $ | *, # | f, fN, dNC
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|JT9 | @ | |
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|JT65 | # | |
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|JT65 VHF| # | *, # | f, fN, dNC
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|QRA64 | : | * | R
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|ISCAT | | * | M N C T
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|MSK144 | & | | N H E
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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 [ISCAT and Deep Search; (0-9,*)] +
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`d` - Deep Search algorithm +
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`E` - Size of MSK eye diagram opening - if negative, the eye is closed +
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`f` - Franke-Taylor or Fano algorithm +
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`H` - Number of bit errors corrected +
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`M` - Message length (characters) +
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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, above) +
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`R` - Return code from QRA64 decoder +
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`T` - Length of analyzed region (s)
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