An excerpt from Pawel Jalocha's source code distribution:
The MT63 modem is intended for amateur radio as a conversation (RTTY like) mode
where one station transmits and one or more other stations can listen. In
short, the modem transmits 64 tones in its 1 kHz bandwidth: the audio range for
the tones is 500-1500 Hz. The differential bipolar phase modulation is used to
encode 10 bits of information per second on each tone. The user data in the
form of 7-bit ASCII characters is encoded as a set of 64-point Walsh functions.
The bits are interleaved over 32 symbols (3.2 seconds) to provide resistance
against both pulse and
frequency selective noise or fading. The character rate equals to the symbols
rate thus the modem can transmit 10 7-bit characters per second.
This modem can as well run in two other modes obtained by simple time scaling,
the possible modes are summarized here:
BandwidthAudio Range Symbol Rate
Character Rate
Interleave / Char.
500 Hz
500 - 1000 Hz
5 baud
5 char / sec
6.4 or 12.8 sec
1000 Hz
500 - 1500 Hz
10 baud
10 char / sec
3.2 or 6.4 sec
2000 Hz
500 - 2500 Hz
20 baud
20 char / sec
1.6 or 3.2 sec
For each mode the interleave factor can be doubled thus each character becomes
spread over twice as long period of time.
The MT63 modem is made for single side band operation. The audio generated by
the modem (sound card output) is applied to the SSB modulator. On the receiver
side, the output of the SSB demodulator is put into the sound card input. The
envelope of the MT63 signal is not constant as in other multi-tone systems - it
is rather noise-like. One must be carefull not to overdrive the transmitter.
The receiver of the MT63 is self-tuning and self-synchronizing thus the radio
operator is only required to tune into the signal with +/- 100 Hz accuracy for
the basic 1000 Hz mode. The modem will tell the actuall frequency offset after
it is synchronized. The operator should not try to correct this offset unless
he is able to tune very slowly his radio receiver, because the MT63 as a low
rate phase modulated system does not like sudden frequency changes.
The MT63 is a synchronous system and it relies on the sampling rate to be the
same at the receiver and the transmitter. At least the sampling rates should
not be different by more that 10^-4. MT63 samples at 8000 Hz thus if your card
runs at 8000.5 it's probably OK but if it runs at 8005 Hz it's not good ! An
extreme example can be my Soundman-16 (PAS-16 clone) which when asked to run
8000 Hz tell me, that it can only do 8008 Hz and it reality it runs at 7910.3
Hz which makes an error of more than 1% - far too much for the MT63 even at
infinitely good S/N.
My other two cards (DSP-16 and Ensoniq 1371) are more reasonable: they show an
error of 0.3 to 0.5 Hz at 8000 Hz sampling.
