[RTTY] Rascal FSK

[Kok Chen]

On Nov 26, 2008, at 11/26    6:33 AM, Doug Hall wrote:
> FSK won't work unless you have a USB-to-Serial adapter that can be
> programmed for 5 data bits and 45 baud. Many (most?) of these devices
> do not support this configuration, whereas nearly all COM ports do.

What Doug say is true --  off the shelf USB-serial adapters are today  
almost universally nonfunctional for 5 bit/45.45 baud.  Some hardware  
can take it (e.g., a couple of the Keyspan adapters) but Keyspan's  
drivers no longer appear to support 45.45 baud.

However, there is one way these serial ports can still be used for  
FSK -- and that is to use DTR signaling, rather than TxD signaling.   
I.e., let the software determine the bit timing.  "Bit banging" as  
some people call it.  A number of software packages do that.

That being said, I personally don't recommend using DTR signaling  
with modern multiprocessing operating systems (Mac OS X, Linux,  
Windows) unless you can embed the Baudot to DTR function inside the  
kernel.  Without that, the DTR timing would have to be done in  
software that is in user space, which can be swapped out occasionally.

RTTY standards have bit periods of 22 milliseconds.  If the software  
is swapped out for a short period, a particular bit in the Baudot  
will be stretched.   That will reduce the probability of the  
following bit of being copied correctly, and might affect subsequent  
bits too until the start-stop synchronization is re-achieved.  If the  
stretching is greater than 11 ms, subsequent bits will definitely be  
lost and it might take a few characters before the receiving end  
recovers character synch.

What K6STI's RITTY calls the "digital flywheel" also loses it  
effectiveness when bit timing changes sporadically.

The insidious part is that you won't notice it -- it is the receiving  
end that will feel the effect.  It also happens rarely, so most  
people will just think it is "line noise."  You just have to hope  
that it doesn't happen just at the time you send your call sign to  
that P5 station.

73
Chen, W7AY