hello Chen,
Thanks for the answer, very interesting, let us know your findings when you
will run the ST-8000 in parallele with other units.
I need to reactivate RITTY from K6STI for serious comparison.
Best 73
Jacques F6BKI
----- Original Message -----
From: "Kok Chen" <chen@mac.com>
To: "Jacques RAMBAUD" <f6bki@wanadoo.fr>
Cc: <rtty@contesting.com>
Sent: Sunday, April 18, 2004 10:11 PM
Subject: **SPAM** Re: [RTTY] fluttered signals
>
> On Apr 18, 2004, at 1:34 PM, Jacques RAMBAUD wrote:
> > -what causes the signal to be so difficult to copy ? multipath ?....?
> > -Why is MMTTY unable to deal with such signal?
> > -Is there any other way , or RTTY decoder that are better in this
> > respect , how old TU with discrete filters (88mh) works ?
> > -Are the KAM or PK232 better ?
> > any litterature on the subject?
>
> If it sounds odd and fluttery and difficult to print even when the
> signal is loud, it is pretty likely to be multipath. It can happen not
> just from transpolar circuits but trans-equatorial as well. I get a
> lot of flutter across the pole from Europe, but I have also noted some
> pretty bad flutter from across the equator with loud S9 signals in
> Portland from FO5, for example. One time when JA1ELY was at PY0FF, his
> PSK31 signals were fluttered beyond copy (but his RTTY signal was still
> barely printable).
>
> My opinion is that simple modems such as the KAM are not going to
> behave better than the software modem you used.
>
> What makes it so hard to decode? The HAL ST-8000 Technical Manual
> says: "Multi-path propagation of an RTTY signal may cause differential
> fading or severe pulse distortion of the signal."
>
> I think the "differential fading" is a specific instance of the
> selective fading that we all have learned to hate.
>
> The "pulse distortion" is understandable from observing that any square
> pulse signal is made up of different Fourier components. If all
> components do not arrive at the same time instance, the result is no
> longer be square. When it is severe enough, this can cause the Mark
> detector output and the Space detector output to actually overlap
> (usually, when the Mark tank detects a signal, the Space tank circuit
> will not see anything since you do not transmit both Mark and Space
> tones simultaneously -- which you can in some other modes, but not in
> Binary FSK).
>
> The ST-8000 has a very simple circuit (just a handful of gates) which
> takes the detected mark signal and the detected space signal and forms
> an output signal from the combination to estimate a signal whose mark
> and space do not overlap; pretty much an exclusive-OR type gate
> configuration. (The ST-8000 has detector settings for normal, mark
> only, space only, and multipath.)
>
> RITTY has selectable matched filters that are designed for different
> propagation conditions, but I don't know any more than that since I
> don't run that software.
>
> I have been planning for some time to investigate DSP counter-flutter
> implementations, but have never gotten around to it. I have even
> bought a used ST-8000 for the time I have more free time to investigate
> it and would need a reference modem to compare against. For now
> however, except for some recorded sound files, this is as much as I
> know.
>
> 73
> Chen, W7AY
>
>
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