VK2DSG on 14 MHz

Technical Description Mode Comparison

MT63 is a DSP based advanced HF mode for Amateur Radio, intended to provide high performance keyboard - to - keyboard conversational operation on HF bands under poor conditions. MT63 utilizes a number of revolutionary ideas, and is technically very complex. However MT63 is easy to tune and easy to operate.

MT63 is reasonably sensitive, and it performs to advantage when conditions are both weak and unstable. It also copes with incredible QRM.

Simple Description

By encoding the data to transmit (what you type on the keyboard) in a complex way, using 64 different modulated tones, the MT63 developer Pawel Jalocha SP9VRC has been able to include a large amount of extra data in the transmission of each character, so that the receiving equipment can work out, without any doubt, which character was sent, even if 25% of the character is obliterated. This technique is called Forward Error Correction. Other modes use FEC (for example AMTOR mode B uses a simple FEC technique), but MT63 has other advantages. Unlike most HF modes where a character can be lost or changed into something else, by a single noise burst, MT63 is inherently very robust, because each character is spread over many tones (to avoid interference such as other radio transmissions) and over several seconds (to avoid bursts of noise, such as lightning).

On each of the 64 tones, the transmission data rate is fairly slow, which suits the nature of ionospheric disturbances. Despite the low data rate, good text speed is maintained, because the text is sent on many tones at once. The system runs at several different speeds, which can be chosen to suit conditions, but 100 WPM, much faster than you can type, is typical.

MT63 sounds unusual, (it sounds like a roaring noise) but the performance is spectacular. There is no connection process, as in AMTOR, Packet or PACTOR. Some users maintain that under poor propagation conditions (excessive fading) MT63 works better than either PACTOR II or Clover. Under good conditions the performance advantages are less obvious. The convenience advantages remain, no matter how good the signal - the mode is suited to nets and random QSOs, as no link need be established, and any number of stations can participate. MT63 is also far more immune to interference and deliberate jamming than any of the more conventional modes. Changeover from transmit to receive and vice-versa is however considerably slower than most modes. It therefore requires some skill and patience to "break in" on a QSO.

There are disadvantages to MT63. First, the mode is broad (see below) and is quite aggressive, i.e. it causes interference to other modes, but itself is little affected by other modes. Also, because of the delay through the error correction and interleaving processes, it is not possible have quick turnaround "slick" QSOs. In otherwords, operation is clumsy.

About Wide Modes

Modes like MT63 are quite different from the traditional Amateur digital modes, in fact even other new digital modes. MT63 is both wideband and relatively slow, gaining performance enhancement by spreading the signal in time and space, although it is not a true "spread spectrum" mode. There is some controversy surrounding indescriminate use of modes such as MT63, because they appear to generate wider signals for a given data rate than other modes. Choose you operating times and frequencies with care! (In other words, 20 metres when the band is open and crowded is not appropriate).

Other examples of wide robust FEC modes are Q15X25 (also by SP9VRC), Stanag 4285 and MIL-STD 188-110A. Experts such as Bob McGwier, Charles Brain and Tomi Manninen are working on other high speed HF modem implementations, including the ALE system, digital voice and digital SSTV modems and the NATO standard Stanag 4285.

MT63 is a little unusual, in that it has extremely strong FEC, and a relatively slow data rate, whereas most other wideband modes are designed for higher throughput and lighter FEC (since they are often used in an ARQ manner).

Operating Comments

These notes are intended to promote and simplify the use of MT63, while at the same time ensuring that, through the use of good operating practices, minimum nuisance is caused to other amateurs.

  • CQ calls should be made using 1 kHz long interleave. (This is the default mode). If possible, identify all calls with Morse or voice ID. It is important to ID, as it is a sure way to educate other amateurs that the weird noises they are hearing are from a genuine Amateur transmission. MT63 attracts more than it's share of attempted jammers!
  • Choose a calling frequency clear of all other signals by at least 500 Hz. Make sure you are well away from adjacent SSB QSOs. Sure, SSB chaff will not affect your operation, but your noises will affect theirs seriously. You CAN operate right next to another MT63 QSO (e.g. one on 14.1095, another on 14.1075 MHz).
  • Use USB for all transmissions above 9 MHz, and LSB below (as for SSB).
  • The international calling frequency is 14.1095 MHz USB, i.e. set the RX display to this value. This places the lower edge of the signal at 14.110 MHz. Leave the receiver running and you'll be amazed what you capture!
  • When starting an over, particularly working DX, send a line of unimportant text first (e.g. callsign exchange) to allow the receiving equipment time to sync. If conditions are poor it may take several attempts before sync is achieved.

Performance Notes

Since late 1998, Les VK2DSG and Fred OH/DK4ZC, with the help of others, have done extensive testing of MT63 under different conditions, long path and short path, with selective fading and flutter, with strong and weak signals and with doppler phase shift. These notes are based on their work as well as more recent observations. FM-Hell and MFSK16 (both arguably better than MT63 under most conditions) were not available at the time of the initial tests.

  1. When conditions are good all modes work well, but when signals are very weak the AM/ASK and 2-FSK modes are less effective. PSK31, MFSK16 and MT63 were then clearly better.
  2. Selective fading does not affect MT63 reception at all, although it is very visible on the waterfall. Fading seriously affects 2-FSK modes, and could be heard as a wooshing noise on the MT63 signal (it can be simulated by moving a notch filter across the passband). This wooshing sound and striped waterfall are characteristic of single-hop MT63 signals, and all part of the fun.
  3. When signals are very poor and unstable, MT63 transfers useful data at normal speed. The long-interleave modes are particularly good, and the 2 kHz bandwidth very impressive. Feld-Hell is similar, or perhaps even a little more robust, and FM245 especially so, since they are decoded by eye. The data rate however is much lower than MT63. MFSK16 is generally more sensitive than MT63, with much shorter delays, but has slower typing speed.
  4. When copy stops as signals fade out, MT63 typically goes from almost perfect copy to almost zero copy, without much garbled text, similar to MFSK16. Other modes suffer a lot of garble at fade-out.
  5. Unless much stronger, and thus depressing the receiver AGC, QRM such as SSB and even some off-frequency MT63 stations have no effect on reception. We now know that two MT63 stations and two Q15X25 or MFSK16 stations can hold separate QSOs on the same frequency without much mutual interference!
  6. MT63 is more affected by polar Doppler flutter than PSK31, because of the lower baud rate, but in fact the performance under such conditions seems little different since the strong FEC corrects the errors well. MFSK16, designed for the purpose, excells under such conditions.
  7. MT63 was equally effective on low bands (well below the MUF) where atmospheric noise and multi-path are the biggest problems. Especially under NVIS conditions, MT63 performs with utmost reliability, even over long ranges and low powers (zero to 3000 km night-time 80m using under 1 Watt). Finding sufficient space on the band is sometimes more of a problem!
  8. Under low band conditions with lots of lightning noise, RTTY and PSK31 are not reliable, although Hellschreiber remains usable (especially Duplo-Hell). MFSK16 also works well, but not perhaps as well as MT63.
  9. Tuning MT63 signals is not difficult, and drift in the transmission is tracked without loss of lock. Newer software is especially good in this regard. Lock is easily lost if the receiver tuning is adjusted. Old rigs which drift can still be used with MT63!
  10. Identifying which mode and which sideband is in use proved very difficult in the early days. Waterfall displays for tuning, and the universal use of 1K long interleave right-side-up have eliminated this problem almost completely.

Copyright © M. Greenman 1997-2005. All rights reserved. Contact the author before using any of this material.
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