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Re: [TenTec] Digital Modes

To: tentec@contesting.com
Subject: Re: [TenTec] Digital Modes
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
Reply-to: geraldj@storm.weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sun, 19 Apr 2009 09:58:34 -0600
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
On Sun, 2009-04-19 at 06:57 -0600, Stephen Cannon wrote:
> Is anyone using a software program that connects direct from PC to radio for
> reception of Weather Fax or other fax operations?
> I have been using a TNC interface between my radio and PC but it sounds like
> I am old school (with everyone using SW programs like FLdigi, etc.) and I
> need to get up to date.
> Stephen 
WEFAX is AFSK on FM, wider FM than your ham receiver will accept. There
is probably software using a sound card for some OS. Receive only needs
little of an interface because ground loops at speaker level are not a
problem. Lots of times vintage hardware like the PK-232 and RTTY
terminal units copy better in QRM and weak signals than sound cards. Old
hardware isn't necessarily bad, though fldigi is handy for lots of

For a radio with line in and out, the only thing you need for interface
is a pair of stereo cables. The high voltage input of line compared to
microphone prevents the ground loops that the boxes like rig blaster
solve being a problem.

As for AFSK vs FSK, that argument has been going on for at least 40
years. The rig receives the same whether AFSK or FSK, then on FSK it can
be that the signal is some cleaner (if driven properly, if abused it has
the capability of being very dirty) but aligning receive and transmit
frequencies is a pain. AFSK with sound card software like fldigi matches
frequency exactly and isn't particular about which RTTY signal in the
receiver pass band is desired to be copied. It copies the one you click
on. And transmits to match the same frequency. As for the arguments,
with proper settings BOTH work. FSK tends to be fixed shift, AFSK via
sound card can be flexible in shift. The rules allow variations on
shift, ever since the rules were changed from 850 Hz shift (no tolerance
in the rules for error) to less than 900 Hz. The optimum shift depends
on propagation, QRM, and sending speed. 170 is often very good. Some
digital modes run 200 Hz a bleed over from 300 baud phone line modems.

73, Jerry, K0CQ

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