Timewave DSP-232, Руководство по эксплуатации

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Chapter 11 SIAM and NAVTEX Operation
When preparing to decode a digital signal, the only decision you need to make before using SIAM is whether the
signal is narrow or wide shift. Inside the Amateur bands, most digital signals are at 170 or 200 Hz shift so set
WIDESHFT OFF
. Outside the Amateur bands, most stations use a frequency shift greater than 200 Hz, so set
WIDESHFT ON
. (Commercial SITOR is an exception—set
WIDESHIFT OFF
.)
Tuning in an FSK signal properly is critical to successful SIAM operation. Follow the tuning procedure below:
1. Select RADIO 1 or 2 using the
RADIO
command to switch the DSP to the port that’s associated with your
radio.
2. Put your HF receiver is either the LSB or FSK/RTTY mode.
3. Turn any IF-shift and passband-tuning controls to their center or “off” position.
4. Adjust the volume to a comfortable listening level.
5. Set
WIDESHIFT ON for commercial “bands” or leave it OFF for the Amateur bands, including MARS
and SITOR.
6. Listen for the distinctive two-tone warble of an FSK signal. When you find a station, slowly vary your
receiver’s tuning control so the DSPs tuning indicator’s light pattern looks like this:
Frequency too low Tuned in! Frequency too high
7. Using either the
THRESHLD
or
AUTOTHR
command, adjust the Threshold so the DCD light goes on
when a tuned RTTY station is being received. After about 10 seconds, the DSP should respond with a baud
rate indication and confidence factor similar to the one shown below.
0.47: 50 Baud
About 15 seconds later, the DSP will display one of the following signal classes and tell you whether or not the
signal is reversed by giving the status of
RXREV
:
ASCII ALIST Unknown 6-bit
AMTOR Baudot noise TDM
The complete signal analysis will look something like the following:
0.47 50 Baud, Baudot, RXREV OFF
This means that the DSP has determined the signal to be a 50-Baud Baudot signal that isn’t inverted (since
RXREV
is
OFF
). The
0.47
means that the DSP is only 47% sure that this analysis is correct. (You’ll receive a higher
percentage when either the signal is louder or clearer, or tuned in better.)
If the DSP determined the signal to be
Unknown
,
6-bit
,
or noise
(which it can’t decode), typing OK will
get you this response:
?bad
The signal analysis routine will run repeatedly until the operating mode is changed by either typing
OK
or by
changing to another mode. If you decide to tune to a different signal during an analysis, simply type SIGNAL again
to immediately restart the analysis routine, or wait for the next analysis cycle.
Note : Adjust the THRESHOLD so the DCD light goes out when no station is being received. That way,
you’ll prevent “garbage” characters from printing on your screen.
Copying Encoded RTTY Transmissions
Many RTTY stations can be found on the HF bands that aren’t transmitting in plain text. Most of these stations use
sophisticated encryption techniques that make decoding them almost impossible. There are a few stations, however,
that use a relatively simple bit-inversion technique; for these stations, there’s the
BITINV
command.
If the text you’re receiving isn’t plain but appears to be encoded, you can try different settings of
BITINV
; it will
Exclusive-OR a number from $00 to $1F with the received character of a Baudot signal, thus inverting specific bits.
11-2 September, 05