kopia lustrzana https://github.com/jamescoxon/dl-fldigi
155 wiersze
6.3 KiB
Plaintext
155 wiersze
6.3 KiB
Plaintext
/**
|
|
\page mt63_page MT63
|
|
|
|
\tableofcontents
|
|
|
|
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>
|
|
|
|
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>
|
|
|
|
<center>
|
|
|
|
Mode | Symbol Rate | Typing Speed | Bandwidth
|
|
:--------:|:-----------:|:------------------:|:---------:
|
|
MT63-500 | 5.0 baud | 5.0 cps (50 wpm) | 500 Hz
|
|
MT63-1000 | 10.0 baud | 10.0 cps (100 wpm) | 1000 Hz
|
|
MT63-2000 | 20 baud | 20.0 cps (200 wpm) | 2000 Hz
|
|
|
|
</center>
|
|
<br>
|
|
|
|
In addition there are two interleaver options (short and long) which can
|
|
be set on the \ref mt63_configuration_page "MT63 configuration tab". 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>
|
|
|
|
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>
|
|
|
|
MT63 may be operated in the default fixed audio frequency mode. In this
|
|
mode you are not allowed to randomly place 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.
|
|
|
|
The default mode, MT63-1000, looks like this on fldigi's waterfall.
|
|
|
|
\image html mt63-1000.png "MT63-1000"
|
|
\image latex mt63-1000.png "MT63-1000" width=1.75in
|
|
<br>
|
|
|
|
You can also elect to operate the MT63 modem in a "manual tune" mode
|
|
(\ref mt63_configuration_page "MT63 configuration tab").
|
|
The manual tune allows you to place both the Rx and the Tx signal to be
|
|
anywhere within the confines of your SSB bandwidth. This screen
|
|
shot shows this capability:
|
|
<br>
|
|
|
|
\image html mt63-500.png "MT63-500 with QRM"
|
|
\image latex mt63-500.png "MT63-500 with QRM" width=6.0in
|
|
<br>
|
|
|
|
This view also demonstrates how immune MT63 is to interference.
|
|
The multiple PSK31 signals that appear on top of the MT63 signal did
|
|
not degrade the decoder. MT63 is usually used above 14073 MHz to avoid the
|
|
possibility of this type of mode conflict.<br>
|
|
|
|
<h4>Edited excerpts from Pawel Jalocha's official mt63 code release</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>
|
|
|
|
This modem can as well run in two other modes obtained by simple time
|
|
scaling, the possible modes are summarized here:
|
|
<br>
|
|
|
|
<center>
|
|
|
|
Bandwidth | Symbol Rate | Character Rate | Interleave / Char.
|
|
:--------:|:-----------:|:--------------:|:------------------:
|
|
500 Hz | 5 baud | 5 char / sec | 6.4 or 12.8 sec
|
|
1000 Hz | 10 baud | 10 char / sec | 3.2 or 6.4 sec
|
|
2000 Hz | 20 baud | 20 char / sec | 1.6 or 3.2 sec
|
|
|
|
</center>
|
|
<br>
|
|
|
|
For each mode the interleave factor can be doubled thus each character
|
|
becomes spread over twice as long period of time.
|
|
<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 careful not to overdrive the transmitter.
|
|
<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. The modem will tell the actual frequency offset
|
|
after it is synchronized. The operator <b>should not</b> 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. Signal synchronization is improved by filtering the timing
|
|
measurement of the received signal. For very low signal to noise ratios you
|
|
can improve the synchronization by selecting "Long Receive Integration" factor.
|
|
Think of the process as a digital AFC. PSK31 does something very similar. One
|
|
would expect there to be some correlation since mt63 is the equivalent of 64
|
|
simultaneous binary phase shift signals. The synchronizer filtering can be
|
|
reduced to 1/2 it's normal value by selecting "Long Receive Integration". It
|
|
takes twice as long for decoding to begin and the synchronizer is more immune
|
|
to noise.
|
|
<br>
|
|
|
|
You probably never need to enable "Long Receive Integration" unless you are
|
|
trying to receive an mt63 signal buried in the noise.
|
|
<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<sup>-4</sup>.
|
|
<br>
|
|
|
|
If you have calibrated your sound card to
|
|
\ref digiscope_display_wwv_mode "WWV", then you will meet this requirement.
|
|
<br>
|
|
|
|
<br>
|
|
\ref mt63_page "Return to Top of Page"
|
|
<br>
|
|
\ref main_page "Return to Main Page"
|
|
|
|
*/
|
|
|