The following is an excerpt from the web page "Sights and Sounds of
Digital Signals", http://www.w1hkj.com/FldigiHelp/Modes/index.htm.
THOR Modes
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General Description
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.
The mode has full-time Forward Error Correction, so is extremely robust.
The default speed (11 baud) was designed for NVIS conditions (80m at
night), and other speeds suit weak signal LF, and high speed HF use. The
use of incremental keying gives the mode complete immunity to
transmitter-receiver frequency offset, drift and excellent rejection of
propagation induced Doppler.
Protocol
These are unconnected, manually controlled message asynchronous simplex
chat modes, using binary convolutional Forward Error Correction. The
default calling mode is THOR11.
Coding and Character Set
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).
*Operating Parameters* *Mode* *Symbol Rate* *Typing Speed^1 * *Duty
Cycle^2 * *Bandwidth^3 * *ITU Designation^4 *
THOR4^5 3.90625 baud 14 wpm 100% 173 Hz 173HF1B
THOR5^5 5.3833 baud 22 wpm 100% 244 Hz 244HF1B
THOR8^5 7.8125 baud 28 wpm 100% 346 Hz 346HF1B
THOR11^6 10.766 baud 40 wpm 100% 262 Hz 262HF1B
THOR16 15.625 baud 58 wpm 100% 355 Hz 355HF1B
THOR22 21.533 baud 78 wpm 100% 524 Hz 524HF1B
*Notes:*
1. WPM is based on an average 5 characters per word, plus word
space. Values based on sending 100 "paris " words.
2. Transmitter average power output relative to a constant carrier of
the same PEP value.
3. This is the "Necessary Bandwidth" as defined by the ITU.
4. A summary of the ITU Designation system can be found at
http://en.wikipedia.org/wiki/Types_of_radio_emissions
5. Double spaced mode.
6. Default and normal calling mode.
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Implementation details are contained in the GPL software source code for
fldigi which can be downloaded from the following site:
http://www.w1hkj.com/fldigi-distro/fldigi-3.03.tar.gz
This is a tar zipped format that will be familiar to all Unix, Linux,
Free BSD and OS X developers. Windows developers can unzip this type of
archive using one of several archive managers including PKZIP.
Fldigi is open source source software that is licensed under the General
Public License, http://www.gnu.org/copyleft/gpl.html. You are free to
use the source intact, to modify, to improve and even to incorporate
into a commercial product. You must however abide by the the license
under which it has been developed and published. To date one other
amateur product has used fldigi source with great success, DM-780, by
Simon Brown.
73, Dave, W1HKJ
