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<h1 style="text-align: center;">MT63</h1><div style="text-align: center;"><div style="text-align: left;"><div style="text-align: center;"><div style="text-align: left;">MT63
is an Orthogonal Frequency Division Multiplexed 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 it robustness in the face of interference and facing.
The tones have synchronous symbols, and are raised cosine
moduled. This mode requires a very linear transmitter.
Over-driving leads to excessive bandwidth and poorer reception.<br><br>The
mode is very tolerant of tuning and fldigi will handle as much as 100
Hz of mistuning. This is very important since MT63 is often used
in very low Signal to Noise ratios. There are three standard
modes:<br><br><table bgcolor="#eeeeff" border="1"><tbody><tr bgcolor="#eeffff"><td><small><b>Mode</b></small></td>
<td><small><b>Symbol Rate</b></small></td>
<td><small><b>Typing Speed</b></small></td>
<td><small><b>Bandwidth</b></small></td>
</tr>
<tr>
<td><small>MT63-500</small></td>
<td><small>5.0 baud</small></td>
<td><small>5.0 cps (50 wpm)</small></td>
<td><small>500 Hz</small></td>
</tr><tr>
<td><small>MT63-1000</small></td>
<td><small>10.0 baud</small></td>
<td><small>10.0 cps (100 wpm)</small></td>
<td><small>1000 Hz</small></td>
</tr><tr>
<td><small>MT63-2000</small></td>
<td><small>20 baud</small></td>
<td><small>20.0 cps (200 wpm)</small></td>
<td><small>2000 Hz</small></td>
</tr></tbody></table><br>In addition there are two interleaver options (short and long) which can be set on the <a href="configMT63.html">MT63 configuration tab</a>.
The default calling mode is MT63-1000. If the short
interleaver is used then one can expect some compromise in robustness.
The long interleaver results in somewhat excessive latency (delay
between overs) for keyboard chatting. MT63-1000 with the long
interleaver has a latency of 12.8 seconds.<br><br>You
can change from receive to transmit immediately upon seeing the other
stations signal disappear from the waterfall. You do not need to
wait until the receive text completes. Any remaining data in the
interleaver will be flushed and the associated receive text printed
quickly to the Rx pane. Tx will commence right after the buffer
is flushed.<br><br>MT63
is the only fldigi mode that does not allow random placement of the
signal on the waterfall. Your transmit signal, and also the
received signal should be centered at 750 Hz for MT63-500, 1000 Hz for
MT63-1000, and 1500 Hz for MT63-2000. If you click on the
waterfall to move the tracking point it will be restored to the
required position.<br><br>The default mode, MT63-1000, looks like this on fldigi's waterfall.<br><br><img style="width: 308px; height: 121px;" alt="" src="Modes/MT63-1000.png"><br>
<br>
<h4>Edited excerpts from Pawel Jalocha's official mt63 code release<br>
</h4>
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 baudrate specific
bandwidth. 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.<br>
<br>
This modem can as well run in two other modes obtained by simple time scaling, the possible modes are summarized here:<br>
<br>
<table style="text-align: left; width: 60%;" border="0" cellpadding="2" cellspacing="2">
<tbody>
<tr>
<td style="vertical-align: top;">Bandwidth<br>
</td>
<td style="vertical-align: top;">Audio Range<br>
</td>
<td style="vertical-align: top;">Symbol Rate<br>
</td>
<td style="vertical-align: top;">Character Rate<br>
</td>
<td style="vertical-align: top;">Interleave / Char.<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">500 Hz<br>
</td>
<td style="vertical-align: top;">500 - 1000 Hz<br>
</td>
<td style="vertical-align: top;">5 baud<br>
</td>
<td style="vertical-align: top;">5 char / sec<br>
</td>
<td style="vertical-align: top;">6.4 or 12.8 sec<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">1000 Hz<br>
</td>
<td style="vertical-align: top;">500 - 1500 Hz<br>
</td>
<td style="vertical-align: top;">10 baud<br>
</td>
<td style="vertical-align: top;">10 char / sec<br>
</td>
<td style="vertical-align: top;">3.2 or 6.4 sec<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">2000 Hz<br>
</td>
<td style="vertical-align: top;">500 - 2500 Hz<br>
</td>
<td style="vertical-align: top;">20 baud<br>
</td>
<td style="vertical-align: top;">20 char / sec<br>
</td>
<td style="vertical-align: top;">1.6 or 3.2 sec<br>
</td>
</tr>
</tbody>
</table>
<br>
For each mode the interleave factor can be doubled thus each character becomes spread over twice as long period of time.<br>
<br>
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.<br>
<br>
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 actual
frequency offset after it is synchronized. The operator<span style="font-weight: bold;"> should not</span> try
to correct this offset unless he is able to tune the radio
receiver very slowly, because MT63 as a low rate phase modulated system cannot tolerate sudden frequency changes.<br>
<br>
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 a Soundman-16
(PAS-16 clone) which when asked to run 8000 Hz reports 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.<br><br>
<br>
<br>
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