Mode MT63

MT63 keyboard mode (500, 1000 and 2000 Hz, short and long interleave), found automatically.

Group	Amateur: digital
Default frequency	14.1095 MHz
Default sample rate	2 Msps
In the mode list	Keyboard mode, 500 / 1000 / 2000 Hz

New in 0.2.7. Tested on generated signals only so far; recordings of real signals are welcome.

What it does and how it works

MT63 is the keyboard-to-keyboard mode of Pawel Jalocha (SP9VRC) that is built to survive fading, interference and a badly tuned dial: 64 carriers, every character spread over all of them and over several seconds.

What the mode does

• Three widths, found automatically: MT63-500 (5 characters/s), MT63-1000 (10) and MT63-2000 (20), each with the short (32 symbols) or long (64 symbols) interleave. The receiver also accepts forcing either setting.
• Upper sideband, the signal anywhere between dial + 100 Hz and dial + 3.9 kHz. Usual dial frequencies: 3.583, 7.0775, 14.1095, 18.1025, 21.1095 and 28.1095 MHz.
• Tuning tolerance: the receiver finds the block within the passband and its AFC follows the carriers; tests run with a dial error of 90 Hz and a symbol clock 150 ppm off.
• Text only (7-bit characters). Callsigns are highlighted; the log shows one row per line.

How it works

Each carrier carries differential BPSK once per symbol (bit 1 keeps the phase, bit 0 reverses it); the symbol rate is 10 per second in the 1000 Hz mode. One 7-bit character per symbol is turned into a Walsh-Hadamard codeword over the 64 carriers (code 0..63 selects a row, 64..127 the inverted row). The 64 bits of one character are sent on the 64 carriers at different times (a fixed delay per carrier, up to 31 or 63 symbols later). The receiver gathers them again, correlates with all 64 rows and takes the best: a character survives when only some of its carriers are hit by a fade or a burst.

Receiver chain:

1. The channel is mixed to 0 Hz and brought to 8 kHz.
2. A 2048-point spectrum looks for a flat block as wide as one of the three modes (power in the block against the bands beside it).
3. The synchroniser takes the last 16 symbols through the modem's front end (block mixed to 0 Hz, rate 2 x the bandwidth), cuts 512-sample frames with the modem's pulse as window, and looks for the symbol timing and the carrier comb (carriers 4 FFT bins apart) for which the phase steps between symbols are real. The residual rotation gives the frequency error.
4. Three comb positions run side by side until one gives clean Walsh codewords (this settles the carrier numbering). Both interleaves are decoded all the time; the cleaner one is shown.
5. An early/late timing loop and the frequency loop follow the signal.

Screens

Band presets, bandwidth and interleave selectors, Listen (the sideband on the sound card), the waterfall of the passband with the block found marked on it, the level of each of the 64 carriers, the big text and the chat log. The receiver tab shows the codeword quality (about 3 for noise, 8 and more for a clean signal), the AFC and the character counts.

Test signal

The built-in simulator sends an MT63-1000 short-interleave test message. The generator (Mt63Generator, core/include/dect2/mt63/mt63_gen.h) can make any width/interleave with noise, a tuning error, drift, a symbol clock error, fading and an echo.

What is verified and what is not

Verified against the generator written for this mode: all six width/interleave combinations, the tuning error, off-centre blocks, noise, fading, echo, clock error and drift (tests/mt63).

Not verified against a real MT63 transmitter: there is no recording of one at hand. The parameters (64 carriers, 512-sample pulse table with symbols 200 samples apart and carriers 4 bins apart at 2 x the bandwidth, Walsh-Hadamard character code, differential BPSK, interleaver step patterns) come from the published description and from reading fldigi's MT63 sources, but details the sources did not make clear (the Walsh row order, the exact interleave delay table, the phase convention of the idle character) were chosen to be self-consistent. If a real recording does not decode, those are the first things to compare.

Credits

MT63 by Pawel Jalocha SP9VRC; the pulse shape table and interleaver pattern are from fldigi (GPL-3.0-or-later, W1HKJ, Dave Freese and contributors). See THIRD_PARTY.md.

Tips and caveats

• Receive only; nothing is sent. Encrypted traffic is never decoded.
• Try it without a radio: the mode has a built-in synthetic test signal.
• Receiving and the law: see Legal.

Related pages

• All modes, Amateur: digital
• Supported radios, Troubleshooting, FAQ