[RTTY] BARTG 75 RTTY Sprint?

[Kok Chen]

On Jun 10, 2010, at 6/10    4:24 AM, Charlesjr at afgimail.com wrote:

> As far as loosing power at 850hz shift RTTY we never worred about  
> that as we could run 40kw from
> some  FRT-40's (transmitter).

The power problem when going from narrow shift to a wide shift is bad  
but not terrible.  The reason is that you are keeping the baud rate,  
but just widening the separation of the two tones.  The matched filter  
for each tone remains narrow -- visualize two band pass filters whose  
centers are separated by 850 Hz, each one having the equivalent  
bandwidth that is equal to about 1/2 the baud rate (but drops off  
slowly as 1/f).

Thus, the extra noise bandwidth is not much greater than using 170 Hz  
shift, with the baud rates that we're talking about.  I.e., and loss  
in SNR has already been accomplished by raising the baud rate :-).

The problem of using excessive bandwidth is however a problem.  If  
everybody uses proper matched filter, you can start overlapping  
signals and get along just fine.  Notice that you can place quite a  
few amateur RTTY of PSK31 signals in between the two tones of that  
loud wide shift commercial RTTY that we hear on 30m and we don't  
really suffer ill effects -- and they probably have the appropriate  
filters to remove the weak gnats in between their two tones, so they  
don't suffer either.

To overlap, you need a higher dynamic range receiving system.  If you  
are trying to copy a weak signal, the stronger signal that is  
overlapping must not exceed the dynamic range of the receiver/sound  
card/numerical range, etc.   Therefore, in general, I don't think it  
will work too well.

You can copy using mark-only or space-only of course.  But when you do  
that, you do lose that 3 dB of SNR which can affect copy of a very  
weak station.

A good reason to use a wide shift is to counter the kind of selective  
fading that you find on the lower frequency bands.

To see the effect, you can watch very wide signals, for example the  
Coast Guard HF-FAX stations like NMC (in Europe, there are DDH and  
DDK) or even a wide DominoEX or Olivia signal on a waterfall display.   
What you will notice is that when selective fading occurs on the  
higher bands, the QSB "notch" is often very narrow in frequency.   
Narrower than 170 Hz.  When selective fading occurs there, quite  
often, only one of your two RTTY tones will be taken out.  Thus, the  
demodulator's slicer will automatically compensate for losing the more  
attenuated tone (albeit at a loss of up to 3 dB in SNR).

The fading "notch" is much broader at the lower HF like 4 MHz (and I  
suspect it is even worse at MF frequencies, but I have not found  
appropriate signals to test with).  When the fade take away both tones  
of your signal, then all bets are off.  The receiving system now looks  
like it is copying a flat fading signal and you have lost the  
advantage of using FSK.

73
Chen, W7AY