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Digital HF Modes

 

An Overview of Modern Digital HF Radio Operating Modes

THE ARQ MODES (Connected)

TOR is an acronym for Tele-printing Over Radio. It was originally  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). This method includes a return signal from the receiving stations either requesting a repeat or an acknowledgement for each frame sent. Since they share the same method of transmission (FSK), they can be economically provided together in one TNC (Terminal Mode Controller) modem and easily operated with any modern radio transceiver. TOR . However, another ARQ mode , WINMOR has been developed by the WINLINK 2000 team. This mode  uses the computer soundcard and elimininates the need for a hardware TNC. WINMOR resembles PACTOR in operation but does require some special software to use it.  .

WINMOR Winmor stands for Winlink Message Over Radio and is a new transmission protocol by Rich Muething KN6KB of the Winlink development team. Unlike Pactor, only a simple computer soundcard to radio interface is required.and it runs as a virtual TNC together with host software such as RMS Express and /V4Chat .  More WINMOR details below.

AMTOR is an FSK mode that has been fading into history. While a robust mode, it uses only 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.

HF PACKET radio is a FSK mode that is an adaptation 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 communications and is rarely used in MARS now if at all.

PACTOR (Or PACTOR 1) is an FSK mode and is a standard on modern TNCs. It is designed with a combination of Packet and Amtor Techniques. 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.

* PACTOR-III is a proprietary mode used for message and traffic handling over an HF radio circuit. Use of Pactor-III protocol is limited for US hams and some other countries due to the very wide bandwidth of the Pactor-III signal. Presently digital signals that occupy the bandwidth of PCT-III are restricted to a few bands. Only the embedded hardware (modem) from the German company that owns the rights to this mode, is capable of operating Pactor-III.

* CLOVER 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.

* The asterisk indicated proprietary modes

WINMOR Ever since the "handshaking" modes of Packet and Pactor were introduced in the late 1980s there have been efforts made to develop soundcard methods of producing ARQ communications without the need of expensive hardware TNCs. WINMOR may finally be the answer to these efforts. This new development is distributed free with the software RMS EXPRESS found at http://www.winlink.org/WINMOR?page=1 Tests are proving to be very successful. See the WINMOR Primer for a full discussion of the mode and instructions on setup and configuring.

The set up will ask for a Federal Access Code if you use the program for MARS. That code number may be found on the RMS Express Agency Code page in the Members Only section on the USAF MARS North Central Area web site or at www.groups.yahoo.com/group/mars_winlink/ . If you use the site, you will need to join the Yahoo group as a MARS member and wait for an authorization but that usually comes within an hour or sooner. Once all this is accomplished the WINMOR Primer will give you excellent step by step instructions for operation and setup.

DIRECT MODES FEC (Non-Connected)

The modes listed below are all included in the Software listed in the previous link. MixW, Fldigi, DM780 and Multipsk.

RADIO TELETYPE (RTTY or RATT) is an 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 its relative disadvantages, RTTY is still popular with die-hard operators.

PSK31 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.

HELLSCHREIBER 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 FM HELL has some advantage for providing better quality print, at the expense of a greater duty cycle. As with other "fuzzy modes" it has the advantage of using the "human processor" for error correction.

MT63 is a 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 and 2 Khz for faster transmission) 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.  ARMY and  US NAVY/MC MARS use this mode for a number of reasons so AF MARS members are encouraged to stay familiar with MT-63 in order to  achieve interoperability with our siister systems.  

THROB is yet another  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 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. This mode is becoming a standard for reliable keyboard to keyboard operation and is available in several popular programs.

OLIVIA Is another MFSK mode that is one of the most robust methods of text keyboarding. It can perform superbly for long distance communications in ionospheric noise conditions where other modes fail. It is possible to communicate worldwide using Olivia with as little as a few watts of transmitter power. Olivia is different from some other types of amateur radio digital keyboarding methods, because it can often be decoded perfectly in the most poor signal-to-noise conditions, even when the human ear cannot discern the presence of the signal, and even when it cannot be easily seen on the conventional waterfall spectrum display.   There are different combinations of formats. The variations are in the number of tones and the bandwidth. However The 2 most common formats are: Olivia 500/16 and Olivia 1000/32

CONTESTIA is very similar to Olivia in both sound and structure but “* CONTESTIA is a bit less sensitive than Olivia (+1.5 dB on minimum S/N). It is also a bit less robust (due to a smaller block size) but it is twice more rapid (with a reduce set of characters).” Some brief tests done in the North Central Area have shown Contestia slightly less robust than Olivia.

Last updated on 7/27/2012 send comments to AFA7VP

 

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