95 lines
4.0 KiB
Plaintext
95 lines
4.0 KiB
Plaintext
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=== AP Decoding
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With the QRA64 decoder Nico Palermo, IV3NWV, introduced a technique
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for decoding with the aid of information that naturally accumulates
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during a minimal QSO. This _a priori_ (AP) information can be
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used to increase the sensitivity of the decoder.
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When an operator decides to answer a CQ, he already knows his own
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callsign and that of his potential QSO partner. He therefore knows
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what to expect for at least 56 of the 72 message bits in a
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standard-format response to his call. The _WSJT-X_ decoders for QRA64
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and FT8 can use these AP bits to decode messages containing them with
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higher sensitivity than otherwise possible.
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We have implemented AP decoding in slightly different ways in QRA64
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and FT8. To provide some explicit examples for users, we provide here
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a brief description of the FT8 behavior.
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The FT8 decoder always tries first to decode a signal without using
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any AP information. If this attempt fails, and if *Enable AP* is
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checked on the *Decode* menu, a second attempt hypothesizes that the
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message contains callsigns MyCall and DxCall. If the QSO has
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progressed to the point where signal reports have been exchanged, a
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third attempt hypothesizes that the message contains the known
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callsigns followed by RRR, RR73, or 73.
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AP decoding attempts effectively set the AP bits to the hypothesized
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values, as if they had been received perfectly. The decoder then
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proceeds to determine whether the remaining message and parity bits
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are consistent with the hypothesized AP bits. If a codeword is found
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that the decoder judges to have high (but not overwhelmingly high)
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probability of being correct, a ? character is appended when the
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decoded message is displayed. Decodes thus marked are not sent to
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{pskreporter} to avoid occasional misleading spots of false decodes.
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Successful AP decodes are always labeled with an end-of-line indicator
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of the form aP, where P is one of the single-digit AP decoding types
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listed in Table 1. For example, an a2 designator says that the
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successful decode used MyCall as 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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=== 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
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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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`f` - Franke-Taylor or Fano algorithm +
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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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