FLDIGI Users Manual 4.0
MT63 is an orthogonal frequency division multiplexed mode consisting of 64 parallel carriers each carrying a part of the transmitted signal. There are 3 bandwidths and baudrates that fldigi implements in MT-63:
The lowest frequency transmitted is always 500 Hz. If you have a scheduled MT63 qso or are trying to copy what you think is MT63 you should tune the signal so that the lowest observable signal is at 500 Hz. Fldigi is capable of decoding signals that are mistuned by as much as +/- 100 Hz.
Selection of interleave (short/long) is made from the modem menu; MT63-500S, MT63-500L, MT63-1000S, MT63-1000L, MT63-2000S, MT63-2000L.
Interleave and 8-bit extended characters are usually agreed upon before a QSO exchange begins. The default is to use the long interleave. 8-bit extended characters allow the transmission of Latin-1 accented characters.
To assist the Rx operator you can transmit a short tone at the lowest (and highest) tone frequencies. You can set the tone duration.
You may also elect to use manual tuning (position on the waterfall) of the Tx / Rx signal. Be careful when doing this so that the signal does not extend beyond the limits of your transceiver SSB filters.
MT63 is used extensively in the Military Affiliate Radio System (MARS).