(The
following information is published in many places on the internet. It is
therefore impossible to determine and credit the original author)
TOR is an acronym for Teleprinting Over Radio. It is traditionally
used to describe the three popular "error free" operating modes, AMTOR,
PACTOR and G-TOR. The main method for error correction is from a technique
called ARQ (automatic repeat request) which is sent by the receiving station
to verify any missed data. Since they share the same method of transmission
(FSK), they can be economically provided together in one TNC modem and
easily operated with any modern radio transceiver. TOR methods that do not
use the ARQ hand-shake can be easily operated with readily available
software programs for personal computers. For these less complex modes, the
TNC (terminal node controller) is replaced by an on-board sound card or
out-board audio device. These modes may use redundancy or "human processing"
to achieve a level of error correction.
AMTOR [example] is an FSK mode that has been fading into history. While a robust
mode, it only has 5 bits (as did its predecessor RTTY) and can not transfer
extended ASCII or any binary data. With a set operating rate of 100 baud, it
does not effectively compete with the speed and error correction of more
modern ARQ modes. The non-ARQ version of this mode is known as FEC, and
known as SITOR-B by the Marine Information services.
PACTOR [example]
is an FSK mode and is a standard on modern TNCs. It is designed with a
combination of packet and Amtor Techniques. It is the most popular ARQ
digital mode on amateur HF today. This mode is a major advancement over
AMTOR, with its 200 baud operating rate, Huffman compression technique and
true binary data transfer capability.
G-TOR (Golay
-TOR) is an FSK mode that offers a fast transfer rate compared to Pactor. It
incorporates a data inter-leaving system that assists in minimizing the
effects of atmospheric noise and has the ability to fix garbled data. G-tor
tries to perform all transmissions at 300 baud but drops to 200 baud if
difficulties are encountered and finally to 100 baud. (The protocol that
brought back those good photos of Saturn and Jupiter from the Voyager space
shots was devised by M.Golay and now adapted for ham radio use.)
PACTOR II
is a robust and powerful PSK mode which operates well under varying
conditions. It uses strong logic, automatic frequency tracking; it is DSP
based and as much as 8 times faster then Pactor. Both PACTOR and PACTOR-2
use the same protocol handshake, making the modes compatible.
CLOVER [example]
is a PSK mode which provides a full duplex simulation. It is well suited for
HF operation (especially under good conditions), however, there are
differences between CLOVER modems. The original modem was named CLOVER-I,
the latest DSP based modem is named CLOVER-II. Clovers key characteristics
are band-width efficiency with high error-corrected data rates. Clover
adapts to conditions by constantly monitoring the received signal. Based on
this monitoring, Clover determines the best modulation scheme to use.
RTTY [example] or
"Radio Teletype" is a FSK mode that has been in use longer than any other
digital mode (except for morse code). RTTY is a very simple technique which
uses a five-bit code to represent all the letters of the alphabet, the
numbers, some punctuation and some control characters. At 45 baud
(typically) each bit is 1/45.45 seconds long, or 22 ms and corresponds to a
typing speed of 60 WPM. There is no error correction provided in RTTY; noise
and interference can have a seriously detrimental effect. Despite it's
relative disadvantages, RTTY is still popular with die-hard operators.
PSK31 [example]
is the first new digital mode to find popularity on HF bands in many years.
It combines the advantages of a simple variable length text code with a
narrow bandwidth phase-shift keying (PSK) signal using DSP techniques. This
mode is designed for "real time" keyboard operation and at a 31 baud rate is
only fast enough to keep up with the typical amateur typist. PSK31 enjoys
great popularity on the HF bands today and is presently the standard for
live keyboard communications. Most of the ASCII characters are supported. A
second version having four (quad) phase shifts (QPSK) is available that
provides Forward Error Correction (FEC) at the cost of reduced Signal to
Noise ratio.
HF PACKET [example]
radio is a FSK mode that is an adaption of the very popular Packet radio
used on VHF FM ham radio. Although the HF version of Packet Radio has a much
reduced bandwidth due to the noise levels associated with HF operation, it
maintains the same protocols and ability to "node" many stations on one
frequency. Even with the reduced bandwidth (300 baud rate), this mode is
unreliable for general HF ham communications and is mainly used to pass
routine traffic and data between areas where VHF repeaters maybe lacking.
HELLSCHREIBER [example] is a method of sending and receiving text using facsimile
technology. This mode has been around along time; the recent use of PC sound
cards as DSP units has increased the interest in Hellschreiber. The
single-tone version (Feld-Hell) is the method of choice for HF operation. It
is an on-off keyed system with 122.5 dots/second, or about a 35 WPM text
rate, with a narrow bandwidth (about 75 Hz). Text characters are "painted"
on the screen, as apposed to being decoded and printed. A new "designer"
flavor of this mode called PSK HELL has some advantage for weak signal
conditions. As with other "fuzzy modes" it has the advantage of using the
"human processor" for error correction; making it the best overall mode for
live HF keyboard communications.
MT63 [example] is
a new DSP based mode for sending keyboard text over paths that experience
fading and interference from other signals. It is accomplished by a complex
scheme to encode text in a matrix of 64 tones over time and frequency. This
overkill method provides a "cushion" of error correction at the receiving
end while still providing a 100 WPM rate. The wide bandwidth (1Khz for the
standard method) makes this mode less desirable on crowded ham bands such as
20 meters. A fast PC (166 Mhz or faster) is needed to use all functions of
this mode.
THROB
is yet another new DSP sound card mode that attempts to use Fast Fourier
Transform technology (as used by waterfall displays) to decode a 5 tone
signal. The THROB program is an attempt to push DSP into the area where
other methods fail because of sensitivity or propagation difficulties and at
the same time work at a reasonable speed. The text speed is slower than
other modes but the author (G3PPT) has been improving his MFSK (Multiple
Frequency Shift Keying) program. Check his web site for the latest
developments.
MFSK16 [example]
is an advancement to the THROB mode and encodes 16 tones. The PC sound card
for DSP uses Fast Fourier Transform technology to decode the ASCII
characters, and Constant Phase Frequency Shift Keying to send the coded
signal. Continuous Forward Error Correction (FEC) sends all data twice with
an interleaving technique to reduce errors from impulse noise and static
crashes. A new improved Varicode is used to increase the efficiency of
sending extended ASCII characters, making it possible to transfer short data
files between stations under fair to good conditions. The relatively wide
bandwidth (316 Hz) for this mode allows faster baud rates (typing is about
42 WPM) and greater immunity to multi path phase shift. A second version
called MFSK8 is available with a lower baud rate (8) but greater reliability
for DXing when polar phase shift is a major problem. Both versions are
available in a nice freeware Windows program created by IZ8BLY.
NOTES:
Frequency-shift keying ( FSK) shifts between two known states. Phase-shift
keying (PSK) changes PHASE of a signal against some reference. FSK is sent
by either shifting a carrier frequency (F1B) or modulating SSB with two
shifting audio tones (AFSK). When sending PSK, a complex audio waveform is
transmitted by SSB. Tracking is much more critical for PSK, thus requiring
more frequency stability.
DSP (Digital Signal Processing) techniques use high speed processing to
convert audio into digital coding, so that a program can manipulate the
coded audio in ways not possible with traditional hardware filters. The 16
and 32 bit sound cards found in modern PCs provide this capability.
FUZZY MODES are those modes that allow the human eye/ear/brain to be used to
its maximum potential. In order to do this, a number of rules are required,
to ensure that any electronics or logic circuitry is not allowed to make
decisions which may be less inspired than human decisions. Examples of
potentially Fuzzy modes are Morse Code, HFFAX, SSTV and Hellschreiber. The
rules are:
The
transmissions must be uncoded. (The signal is sent as a real-time language.)
The receiver must not decide when data is present. (Untouched by any prior
decisions.)
The receiver must not decide what data is present. (It must be presented as
received.)
