Comments inline, below...
----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: <rtty@contesting.com>
Sent: Friday, June 11, 2010 11:42 AM
Subject: Re: [RTTY] BARTG 75 RTTY Sprint?
> ORIGINAL MESSAGE:
>
> On Wed, 9 Jun 2010 18:37:33 -0500, "Robert Chudek - K0RC"
> <k0rc@citlink.net> wrote:
>
>>
>>No one will be using a wider shift. It wastes bandwidth and is prone to
>>more
>>errors.
>
> REPLY:
>
> It's true that wider shift does use more bandwidth, but in fact,
> errors are actually reduced, provided you have a receiver and software
> which can take advantage of it. Wider shift reduces the deleterious
> effects of selective fading.
Agreed about the selective fading.
> Narrow shift causes the mark and space to selectively fade together,
> where a wider shift causes them to selectively fade differently. If
> the RX bandwidth is wide enough to accommodate both and the software
> is capable of correctly decoding when only one tone is present,
> reception is actually improved.
I believe this is where the technical aspect and real-world experience begin
to separate. For example, the Icom Pro III has the special twin passband
filter setting for RTTY, but (as far as I know) it is limited to the 170 Hz
tone
pairs of 2125 and 2295. For larger shifts you have to open up the receiver
bandwidth to capture both tones, allowing more "noise" to enter the system.
I do not know of any recivers that incorporate the twin tone filters like
Icom,
so I am not certain the majority of operators could reduce errors by using
850 Hz shift.
> The 170 Hz shift used by hams is a compromise between selective fading
> and crowded bands. Commercial and military stations who care little
> about crowded bands commonly use a much wider shift, for good reason.
Understood. In addition, it was mentioned the military often have higher
powered transmitters they could call into action when propoagation was
poor. When I started my RTTY "career" in 1964, 850 Hz shift was the
standard, but 170 Hz started gaining in popularity. For a period of time I
had both shifts installed in my TX4B.
It's interesting (to me) that the idea of using 850 Hz shift has become a
topic of discussion. I am curious how this thought was incubated. :-) But
maybe it's just that I've "been there, done that" that makes it a non-issue
in my mind?
I do have a question about the MMTTY software and the AEA PK232,
Kantronics KAM, and other hardware decoders. Do they effectively "fill in"
the missing mark or space tone when selective fading completely takes one
or the other tones out of the audio stream?
Kok Chen has talked about some demodulators that will receive FSK using
a single tone. I always thought this was a specific feature of a higher
quality
demodulator that needed to be enabled.
73 de Bob - KØRC in MN
> 73, Bill W6WRT
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