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2018-02-08 21:28:33 -05:00
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+=== AP Decoding
+
+With the QRA64 decoder Nico Palermo, IV3NWV, introduced a technique
+for decoding with the aid of information that naturally accumulates
+during a minimal QSO.  This _a priori_ (AP) information can be
+used to increase the sensitivity of the decoder.
+
+When an operator decides to answer a CQ, he already knows his own
+callsign and that of his potential QSO partner.  He therefore knows
+what to expect for at least 56 of the 72 message bits in a
+standard-format response to his call.  The _WSJT-X_ decoders for QRA64
+and FT8 can use these and similar AP bits to decode messages
+containing them with higher sensitivity than otherwise possible.
+
+We have implemented AP decoding in slightly different ways in QRA64
+and FT8.  To provide some explicit examples for users, we provide here
+a brief description of the FT8 behavior.
+
+AP decoding attempts effectively set the AP bits to the hypothesized
+values, as if they had been received correctly.  The decoder then
+proceeds to determine whether the remaining message and parity bits
+are consistent with the hypothesized AP bits.  If a codeword is found
+that the decoder judges to have high (but not overwhelmingly high)
+probability of being correct, a ? character is appended when the
+decoded message is displayed.  To avoid misleading spots of occasional 
+false decodes, messages so marked are not forwarded to {pskreporter}.
+
+Successful AP decodes are always labeled with an end-of-line indicator
+of the form aP, where P is one of the single-digit AP decoding types
+listed in Table 1.  For example, an `a2` designator says that the
+successful decode used MyCall as hypothetically known information.
+
+[[AP_INFO_TABLE]]
+.AP information types
+[width="35%",cols="h10,<m20",frame=topbot,options="header"]
+|===============================================
+|P | Message components
+|1 | CQ   &#160; &#160;   ?   &#160; &#160;   ? 
+|2 | MyCall &#160; &#160; ?   &#160; &#160;   ? 
+|3 | MyCall DxCall &#160; &#160;  ? 
+|4 | MyCall DxCall RRR
+|5 | MyCall DxCall 73
+|6 | MyCall DxCall RR73
+|===============================================
+
+Table 2 lists the six possible QSO states that are tracked by the
+WSJT-X auto-sequencer, along with the type of AP decoding that would
+be attempted in each state.
+
+[[AP decoding types for each QSO state]]
+[width="35%",cols="h10,<m20",frame=topbot,options="header"]
+|===========================================
+|State        |AP type
+|CALLING      |   1, 2
+|REPLYING     |   2, 3
+|REPORT       |   2, 3
+|ROGER_REPORT |   3, 4, 5, 6
+|ROGERS       |   3, 4, 5, 6
+|SIGNOFF      |   3, 1, 2
+|===========================================
+
+
+=== Decoded Lines
+
+Displayed information accompanying decoded messages generally includes UTC,
+signal-to-noise ratio in dB, time offset DT in seconds, and
+audio frequency in Hz.  Some modes include additional information such
+as frequency offset from nominal (DF), frequency drift (Drift or F1),
+or distance (km or mi).
+
+There may also be some cryptic characters with special meanings
+summarized in the following Table:
+
+[[DECODED_LINES_TABLE]]
+.Notations used on decoded text lines
+[width="50%",cols="h,3*^",frame=topbot,options="header"]
+|===========================================
+|Mode    |Mode character|Sync character|End of line information
+|FT8     | ~            |      | ? &#160; aP
+|JT4     | $            | *, # | f, fN, dNC
+|JT9     | @            |      |
+|JT65    | #            |      |
+|JT65 VHF| #            | *, # | f, fN, dNC
+|QRA64   | :            | *    | R
+|ISCAT   |              | *    | M  N  C  T
+|MSK144  | &            |      | N  H  E
+|===========================================
+Sync character::
+ `*` - Normal sync +
+ `#` - Alternate sync
+
+End of line information::
+ `?` - Decoded with lower confidence +
+ `a` - Decoded with aid of some a priori (AP) information +
+ `C` - Confidence indicator [ISCAT and Deep Search; (0-9,*)] +
+ `d` - Deep Search algorithm +
+ `E` - Size of MSK eye diagram opening - if negative, the eye is closed + 
+ `f` - Franke-Taylor or Fano algorithm +
+ `H` - Number of bit errors corrected +
+ `M` - Message length (characters) +
+ `N` - Number of Rx intervals or frames averaged +
+ `P` - Number indicating type of AP information (Table 1, above) +
+ `R` - Return code from QRA64 decoder +
+ `T` - Length of analyzed region (s)
+