Don et al,
I also echo support for the development and experimenting with new modes.
And I, for one, will continue to try them and learn about their specific
performance characteristics... both pro and con.
Since this also relates somewhat to the recent thread on RTTY bandwidth and
receiving filters I wanted to pass on my filtering experiments during the
contest last night.
>From Don's note:
> I saw that George, W1ZT, commented that he likes transmitting AFSK and
> has worked on filtering solutions. I was using an external JPS NIR-12
that
> knocked the audio down to just a sliver of bandwidth and it helped, but as
soon
> as someone came on within .5 khz, weak signals would disappear! Maybe
it's
> my radio, I don't know. I do know that I can sit next to a station
running a KW
> at .5 khz and barely notice he's there if I'm running my 250 hz with my
radio
> in the FSK mode.
I had the Icom 756PRO set to LSB with 300 Hz filtering and a center
frequency of 1500 Hz. This is my "normal" RTTY setting. I like to listen
to lower tones that those experienced guys (with better hearing). MMTTY
was set to a mark freq of 1415 Hz as my "normal" setting (although I had
trouble making the HAM profile stay there). The receiving filter was way
too wide for a single RTTY23 signal but narrow enough to stop Don's signal
when he was loud and only 300 Hz up from UX0FF. I was able to copy and work
UX0FF only because the narrow IF filter could keep Don's signal from driving
my AGC. Otherwise I would have experienced the identical problem Don did.
This is exactly what used to happen to me with my Icom 765 and it's regular
SSB filters last year. Don, I wonder if your 300 Hz PSK filter is an IF or
audio filter?? I am not that familiar with the TS 870 signal chain.
I tried setting the PRO filter to about 150 Hz and got the single signal
effect so I experimentally agree with Chen's spectrum but would like to try
it again now that I see his plot of the shapes. I bet I could step down
even narrower.
But these narrow settings seemed to aggravate the AFC and cause excessive
delay in locking onto a signal for copy. Even when I forced the tuning by
clicking on the waterfall spectra, it took several seconds to get clean
copy. As some have said, this caused a total miss of a call sign. There
may be a mismatch in my radio filter settings and the AFC capture range
settings in MMTTY. Adjustment of MMTTY parameters could improve this
situation.
As with 170 Hz RTTY, I was successful in calling CQ with NET off and AFC on.
I observed that the AFC seemed to have a long way to "swim" to find a signal
that was apparently right on frequency. Again, this may be an AFC parameter
setting refinement in MMTTY I can make.
For S&P, both NET and AFC were on and I "hope" I was calling people on
their frequency. I would have liked to know how to "freeze" the frequency
once I acquired copy because I seemed to have to reacquire again to get the
report after a successful call.
All of these techniques are the same for AFSK work whether 23 Hz or 170 Hz
shift. I learned all of them from people on this reflector as well as
several years' use of K6STI's RITTY program.
I could envision a future contest with signals packed 100 Hz apart similar
to the present conditions where 300-400 Hz spacing frequently occurs. But
it needs a new set of transceivers with adjustable IF filtering comparable
to but even more flexible than the Icom 756PRO. ... maybe I am talking
myself into a completely computer receiver... and I would certainly miss
the big knob !! and the musical tones.
George .. W1ZT
----- Original Message -----
From: "Don Hill AA5AU" <aa5au@bellsouth.net>
To: <rtty@contesting.com>
Sent: Saturday, September 01, 2001 5:24 PM
Subject: Re: [RTTY] RTTY 23 - Someone please kill it!
> Andy et al,
>
> Perhaps I was a bit harsh in my first post. I hope I didn't offend
anyone.
> That wasn't the intention.
>
> I know several people put a lot of time and effort in developing these new
> modes, which is fine. I thank them for that and hope they continue to
create
> new modes for us to try.
>
> I think the tuning problem could be solved with a better display or tuning
> indicator. At first it appears to be difficult to tune, but it's just
like a RTTY
> signal except it's very tiny on our displays. AFC didn't help much
either.
>
> You need both a fast and slow AFC. You need a fast AFC to tune in
> a signal. You need a slow AFC or none when calling CQ. I noticed that
> when I go to receive after CQ'ing, the AFC wonders off frequency so fast
> that if someone does call you, by the time the AFC locks onto the signal,
> you've missed the callsign. Many stations, including myself, resorted to
sending
> a short string of RY's at the start of each message to allow others time
to
> tune. This is, of course, a step in the wrong direction.
>
> The problem I see is on receive. A weak signal is barely printing, then
someone
> fires up .5 khz away and you lose the signal. I was using a Kenwood
TS-870
> trying both regular sideband filter in its narrowest position 400hz
(1000hz on the
> low side, 1400 hz on the high side) and the PSK filter which is 300 hz,
but
> neither seemed to help knock down nearby signals. I know many radios can
> use the CW filter in SSB mode and this may help.
snip...
> 73, Don AA5AU
|