On Sep 5, 2008, at 1:30 AM, Logan Zintsmaster wrote:
I hope the group will pardon me while I engage in a bit of RTTY
heresy.....
When you use a PC sound card for decoding RTTY, the center frequency
is no
longer important, just the tone spacings. With MTTY, I can pick any
center
frequency I want to use (typically 1000 Hz so I stay away from the
roll-off)
and tune until I see it in the MTTY spectrum window. I make any fine
adjustments with the cursor to pick the signal I want if there are
multiple
signals shown. If I want a bandpass filter, I use the bandpass
function in
MTTY. MTTY uses the same audio frequency for encode as it used for
decode
so the signal is inherently locked to the received signal. It even
has an
AFC function to track any drift.
All this is true for receiving. You can pick just about any center
frequency you want.
One problem with using the waterfall display and selecting from a wide
bandpass is you don't get the level of unwanted signal rejection that
is possible when using narrow crystal or DSP filtering. If there is a
strong signal adjacent to your desired signal, you may not be able to
decode properly.
The K2 is equipped with a variable-bandpass filter that works very
well on RTTY as it does on CW. In order to use this effectively,
you've got to select a particular center frequency and adjust the
filters around that frequency.
This does require one to "tune in" a RTTY signal, but that is a pretty
easy process using the crossed ellipse display.
Using just the SSB filter for receive and transmit avoids the
problem of
center frequency tracking between the SSB filter that is always used
on
transmit and whatever receive filter has been selected.
Except that you often don't want to use the SSB filter for receive. I
generally use the variable-bandwidth filter set for 1.0 kHz. I also
have settings for 0.5 and 0.3 khz. I only use the SSB filter to
receive RTTY on a very quiet band.
One other consideration -- when you are transmitting, you want only
the AFSK signal to fit in the passband. This is the reason traditional
RTTY was so high in the audio bandwidth. If you send tones of about
1000 Hz or so, then their second and possibly third harmonics may be
transmitted as well. To avoid this, pick frequencies higher in the
passband. With the limited BFO ranges on some K2s, it may be hard to
center the variable-bandpass filter. I chose 1500 Hz as the center
frequency for my RTTY tones for these reasons -- it places the second
harmonic outside the passband of the SSB filter.
I also chose a whole number as the center frequency so I can bring the
DSP passband filters into play.
It also has the advantage of allowing you to hear what is happening
across
the audio bandwidth so you know whether to go higher or lower for
the next
signal. Very useful in a contest.
In a contest, I don't want the radio's AGC pumping against nearby
signals -- I want those signals rejected -- another reason to use a
narrow filter.
Bill Coleman, AA4LR, PP-ASEL Mail: [EMAIL PROTECTED]
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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