Thor

Thor is a new forward error correcting incremental frequency shift keyed communications mode.  It was developed specifically to meet the needs of ARQ transfers in the HF spectrum.  It is particularly well suited under conditions of atmospheric static noise.  Thor borrows from two current modem technologies, MFSK and DominoEX.  Fldigi can operate in the following Thor modes:
The sound card sampling rate is 8000 Hz for the 4, 8 and 16 modes.  It is 11025 Hz for the 5, 11 and 22 modes.  This change in sound card sampling rate will be seen in the drop rate on the waterfall.  See: Thor Technical Description.  Thor emits a distinctive double rising tone sequence at the beginning of each transmission.  It is used to flush the receive decoder and also provides a visual and audibal clue to its being used.

The modem code for Thor uses a wide band multiple frequency detector that can lock on and detect the incoming signal even when badly mistuned.  Frequency domain oversampling is used to allow proper tone detection with the need for AFC.  The AFC control does not alter the decoder in any way.


Type in the secondary text.  This text will be sent during periods when your keyboard is inactive (between letters for slow typists).  The default for this text will be your callsign when you have entered that in the "Oper" configuration tab.

Set the BW factor for the decoding prefilter.  2.0 should be adequate unless you are experiencing nearby continuous wave interference (CWI).  You can enable and disable the prefilter with the checkbox.  Please note that the filter requires additional cpu cycles.  Older and slower cpu models might give better decoding with the filter disabled.

Soft decode provides some additional decoder performance at the expense of more cpu cycles dedicated to the decoder.

The decoder can detect and defeat a modest amount of CWI that is within the BW set by the BW factor.  Increasing the CWI threshold increasing the sensitivity for this correction.  The offending tones are punctured thereby rendering them null to the Viterbi decoder.  
The waterfall and digiscope will appear as:



The text displayed in the status area is the secondary text being sent by the transmitting station.  When the keyboard buffer is empty the Thor modem transmits text from the secondary text buffer.  Your secondary text buffer can be edited on the Thor configuration tab.

The digiscope display is similar to the DominoEX display and represents the tone pairs moving through the tone filters.  You can also use an alternate digiscope display (left click on the digiscope display area).



In this display mode the red line represents the center of the multiple tone bins that are in the detector.  The dots will be blurry if the AFC is not locked on and become very distinct when AFC lock has been achieved.  The tone dots will move from bottom to top (opposite the direction of the waterfall).

This is the same signal mistuned:



and with the signal badly mistuned:
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