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:
Thor-4 - double spaced mode  173hz wide
Thor-5 - double spaced mode  244 hz wide
Thor-8 - double spaced mode  346hz wide
Thor-11 - single spaced mode  262hz wide
Thor-16, - single spaced mode  355hz wide
Thor-22 - single spaced mode  524 hz wide

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. 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.
THOR is a family of offset incremental multi-frequency shift keyed modes with low symbol rate, closely related to DominoEX. A single carrier of constant amplitude is stepped between 18 tone frequencies in a constant phase manner. As a result, no unwanted sidebands are generated, and no special amplifier linearity requirements are necessary. The tones change according to an offset algorithm which ensures that no sequential tones are the same or adjacent in frequency, considerably enhancing the inter-symbol interference resistance to multi-path and Doppler effects.

A binary varicode with ASCII-256 user interface (same as MFSK16) is used. Lower case characters are sent faster. An ASCII-128 secondary character set extension allows a fixed (typically ID) message to be sent whenever the transmitter is idle. Modulation uses two dibit pairs, symbol synchronous, differential.
The FEC system uses binary convolution to generate two dibits per varicode bit, and halves the corrected data rate compared to the equivalent DominoEX mode. Rate R=1/2, Constraint length K=7, Interleaver L=10 (40 bits).

Mode Menu
Mode Audio Sample
FFT Window Screenshot