MT63 Modes


General Description

MT63 is an Orthogonal Frequency Division Multiplexed (OFDM) mode consisting of 64 parallel carriers each carrying part of the transmitted signal. The tones are differential BPSK modulated. MT63 employs a unique highly redundant Forward Error Correction system which contributes to its legendary robustness in the face of interference and fading.

The tones have synchronous symbols, and are raised cosine modulated. The mode requires a very linear transmitter. Over-driving leads to excessive bandwidth and poorer reception. The mode is unpopular with many operators because of the excessive bandwidth used.

The mode is very tolerant of tuning, as most software will handle 100Hz mistuning. The mode was designed by Pawel SP9VRC.

Protocol

These are unconnected, manually controlled message asynchronous symbol synchronous simplex chat modes, with full-time Forward Error Correction. The 64 PSK tones are spaced at 1/64th of the bandwidth, for example 15.625Hz for the default MT63-1000 (long) mode.

Coding and Character Set

The ASCII-128 user interface is mapped to 64 tones via a Walsh-Hadamard function (rate 7/64), and uses an interleaver to spread each character across 32 or 64 symbols. Character mapping is direct (via the coding function and interleaver, there being no scrambler or gray coder.

There are three modes, which are scaled in all dimensions (baud rate, spacing, typing speed), and two interleaver options (short and long). One shortcoming of the mode is that robustness is compromised with the short interleaver, and latency (delay between overs) is unacceptably long with the long interleaver. For example, with the default mode the latency is 12.8 seconds.

Operating Parameters

Mode Symbol Rate Typing Speed1 Duty Cycle2 Modulation Bandwidth3 ITU Designation4
MT63-500 5.0 baud 5.0 cps (50 wpm) 80% 64 x 2-PSK 500 Hz 500HJ2DEN
MT63-10005 10.0 baud 10.0 cps (100 wpm) 80% 64 x 2-PSK 1000 Hz 1K00J2DEN
MT63-2000 20 baud 20.0 cps (200 wpm) 80% 64 x 2-PSK 2000 Hz 2K00J2DEN

Notes:
1. WPM is based on an average 5 characters per word, plus word space.
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. Default and normal calling mode (long interleaver).