I have not done RTTY for as long as some people here, but here are my
personal observations.
Long, long ago, I had used an MFJ 1278 for SWL'ing. I shouldn't even
mention having owned one... there goes my reputation. Chalk it up as
stupidity of youth.
I have used a few stock KAM Plus, and one which I modified so that the
switched capacitor filters are closer to that needed for RTTY. I
still own a beige KAM Plus and a black KAM plus.
The modified one is a significant improvement, but still not that
great. The problem with many stock TNCs is that their filters are
designed for Pactor and are way too wide for weak signal RTTY use.
After modification, the filters in my modified KAM Plus were too
narrow for Pactor; that is why I have always kept more than one KAM
around.
A PK-900 (long ago given away) came in somewhere between the modified
KAM Plus and an unmodified KAM Plus.
Garry NI6T had also modified the PK-232 to match RTTY (it is actually
much harder to modify than the KAM Plus, since the PK-232 didn't use
switched capacitor filters, which only involved resistor changes) and
the modified PK-232s were not just used at 3D2CU but other DXpeditions
had borrowed them.
If you choose the hardware route, get a HAL ST-8000. There is nothing
better in the hardware world, period, punkt.
If you can't get hold of a used HAL ST-8000 (not the ST-8000A, they
are very different animals), I have found the Timewave 599zx to be
quite solid. Both the 599zx and the ST-8000 have AFSK regenerators so
you can use them as front ends to other TNC/TU that you already own.
They both also have analog AGC, and have decent dynamic range as far
as hardware modems go.
I have also used the HAL DSP-4100 and the HAL DXP-38 side by side with
the KAMs and they are both better than the KAM as long as you adjust
the audio level properly, but neither can match the ST-8000. The
problem with the DSP-4100 and the DXP-38 is that they only have 14-bit
ADCs without an analog AGC stage -- so their dynamic range is
limited. You have to carefully adjust the audio level so you don't
clip the codec on strong signals, and when you do that, it won't work
well on very weak signals. You pretty much have to wire a pot ahead
of them and ride the pot manually.
I have never used an ST-6000, but many people on this reflector have.
Lets say I have never heard a bad word about them. Plus, like the
ST-8000, there is a nice crossed ellipse display built into the 6000
(not as nice as the one on the ST-8000, but better than the tuning
mechanisms that you find on the rest of the field). For weak signal
DX'ing it is imperative that you are able to tune a signal within 10
or 20 Hz under poor SNR conditions. It does not matter how good the
demodulator is if you cannot *accurately* and *quickly* tune an RTTY
signal in. Except for clicking on a waterfall, the crossed ellipse is
one of the few indicators that can do *both* well if you are tuning
with a VFO knob.
However, any modern software modem that is worth its salt should beat
even the ST-8000.
As finely crafted as they are, the ST-8000 is still limited by what
you can do in hardware. Its countermeasure to multipath causing mark/
space overlap is quite rudimentary, for example. But at least the
ST-8000 has some, albeit simple, circuitry to handle overlapped mark/
space. The most modems that are built as TNCs don't even have a way
to switch in a different demodulator under multipathed conditions. If
you want to dig out a fluttery signal across the pole, you need good
multipath performance.
After saying all that, the best way to convince yourself is to do your
own tests. If you run Windows, get a copy of AE4JY's PathSim and if
you run Mac OS X, get cocoaPath. Both are free. Run a clean RTTY
signal through different propagation and SNR conditions through these
HF Channel Simulators and make recordings of them to tape or sound
files. cocoaPath has built in RTTY, PSK and CW generators, so you
don't even have to first create a clean AFSK signal yourself.
Then play the recorded audio back into each of the modem that you are
testing. All modems will get precisely the identical inputs. The use
of HF Channel Simulators is how the industry test demodulators (the
prevailing standard is the ITU-R F.1487). Then, make your own plots
for the modems you are testing, like what Alex VE3NEA has done here
http://www.dxatlas.com/RttyCompare/
73
Chen, W7AY
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