[RTTY] DOS computer for improved RTTY reception?

[Kok Chen]

On Jan 18, 2011, at 1/18    10:04 PM, Jim W7RY wrote:

> Sounds like you need some help learning to run MMTTY!  It works  
> wonderfully!

There are many who are not 100% successful when they move from a TU of  
a TNC to a sound card based software modem.

The following are my opinions:

The first thing and probably the most important thing is to make sure  
that (1) the sound card is getting adequate sound and (2) the sound  
card never clips.

There is a happy medium between these two states.  If (1) is not  
satisfied, you will never be able to copy weak signals and if (2) is  
not satisfied, you will probably only be able to copy the strongest of  
signals and at the same time get no protection from any QRM rejecting  
filter in the software.

If the sound card ever clips, even slightly (like pregnancies, there  
is no such thing as "only slightly clipping"), the software filters no  
longer work as filters.

The easiest way to get a "first cut" is to find a program that has a  
spectrum display.  Watch the sound card noise floor with the rig  
turned off.  The spectrum display should be showing the noise floor of  
the sound card.  Now turn on the rig.  The noise floor should rise.   
If it does not rise, you don't have enough audio feeding the sound  
card and need to do something pronto about fixing the situation.   
Monitor strong stations and make sure it does not clip.  There usually  
is some "VU Meter" in the software that warns you when that happen.  I  
don't use Windows and cannot recommend a program, but software modems  
such as cocoaModem has a spectrum display built in, that has enough  
dynamic range to help you to make such adjustments.

Getting a noise floor rise of 10 dB is probably a good stating point.   
A 3 dB rise means that the noise floor of your rig is the same as the  
noise floor of the sound card.  If the rise is more than 10 dB, you  
are just wasting the dynamic range of the sound card.

If you can't get a noise floor to rise by 10 dB and still keep strong  
stations from clipping the sound card, the sound card does not have  
sufficient dynamic range for your rig, and you will need some kind of  
adjustment to "ride the gain." Either a digital attenuator in the  
sound card, or an analog pot.  Some sound cards don't have a digital  
attenuator, or they are only good for 12 dB, in which case you will  
need external pots or the use of the RF attenuator of the rig.

Typically even cheap sound cards today have more than 85 dB of dynamic  
range, so if you take away the 10 dB of noise floor excess, you still  
end up with 75 dB of total dynamic range.  This should be sufficient  
for many rigs, especially if you adjust the AGC of your rig properly.   
Even if the rig advertises 100 dB of dynamic range, it does not mean  
that the line output has 100 dB worth of dynamic range.

Some other things to observe are things like filtering.  If the  
software already has a matched filter, do not turn on the rig's twin- 
passband filters (or whatever they are called).  The software's  
matched filter should already have optimized for the signal and any  
further change of filter shape will ruin the copy.  For that matter,  
many narrow I.F. filters have terrible group delays when operated  
anywhere even near the skirts of their filters.  Our own Jeff AC0C has  
done some experiments on group delays of roofing filters.  This is why  
there is the common observation that when there is a weak signal, it  
is better to widen the filter for better copy -- you are both letting  
in more keying sidebands and also cleaning up the group delay  
characteristics when you do that.

Treat the I.F. filter as the "roofing filter" of your sound card --  
use it only to keep loud QRM from clipping the sound card.  Let the  
software filters handle all the detail filtering.  If the software is  
written by someone who understands RTTY, its filtering will be  
superior to the ones in the rig -- as long as the sound card never  
clips.

Take care of those items, and a good software demodulator will wipe  
the floor with the best hardware TU/TNC/Modem (yes, the ST-8000  
included).

Be sure to follow good audio practices.  Check for ground loops (since  
those can really affect the noise floor).

73
Chen, W7AY