Is MT63 much enough to need this much rework instead of stability?
At one point it was slated for removal (from fldigi or gmfsk, I can't
remember) but I put in a good word for it with either you or Tomi for
keeping it, mostly because of the MARS usage, based on some comments
from current MARS members. I've never seen it in use on HF ham bands
though. Is there a place to look? If so I'd like to try it out.
Yeah, I lobbied Dave too about MT63.
It's in use above 14.100 on 20 meters, not sure about bands below. Try 14.109.
It's definitely not dead.
I've seen regular use of MT63 on 80 meters, usually above 3583.0
Attached is an excerpt of the README that Pawel included with his last
posting for the MT63 code base.
Dave
This is the release 0.4 of the MT63 modem for LINUX.
Date: 21-NOV-1999, Author: Pawel Jalocha, SP9VRC,
This is another "quick" release, the new feature is the CW identification
which can be transmitted below the 64-tone digital signal.
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. 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:
Bandwidth Audio range Symb. rate Char. rate Interleave/char
500 Hz 500-1000 Hz 5 baud 5 char/s 6.4 or 12.8 sec
1000 Hz 500-1500 Hz 10 baud 10 char/s 3.2 or 6.4 sec
2000 Hz 500-2500 Hz 20 baud 20 char/s 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 first experiments with this mode were done on the EVM56K DSP
evaluation board from Motorola and the package was named MT63ASC.ZIP.
This LINUX implementation is written to be compatible with that package.
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 the other multi-tone system - 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.
Driving the radio transmitter:
The signal envelope is not constant (!) - it is rather noise like.
It's again the operator who should care not to overdrive the SSB
transmitter. If you have the ALC meter it is best to start with
the setting which make NO movement of the ALC. If you only have
the output power meter I suggest you start with the average power
equal to 1/10th of the peak power.
Soundcard sampling rate issue:
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 ! **
About the C++ code:
All the code used to compile the modem executables is provided here.
I don't mind if someone makes a use of it for his/her freeware program
for amateur radio or shortwave listenning. In contrary I do not give
permission for this code to be used in a commercial or military software.
IF you want to implement this code in your software, you will most likely
need the files mt63.cc mt63.h mt63.dat dsp.cc and dsp.h - look into mt63.h
for the description and comments of the classes MT63tx and MT63rx.
They are not dependend on the platform as this is pure arythmetic
(you will need -lm on linking). When you add the sound I/O and user I/O
you have the modem ready.
The code can be compiled under pure MSDOS and I do this with
the Borland C++ 3.1. However, as there is no sound support, you can
run the mt63tx/rx/trx on raw audio files, which can still be usefull
for decoding pre-recorded audio files or off-line experiments.
Pawel, SP9VRC.
** fldigi uses precision resampling algorithm's to convert from the sound
card native sampling rate to/from the 8000 Hz required by the mt63
encoder/decoder. Calibrating the sound card for sample rate error is
absolutely essential to the performance of mt63. Pawel included a command
line program which sampled time-tick signals to measure the sound card
errors. It produces results identical to the fldigi WWV calibration modem.
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