[RTTY] MMTTY difficulties

[Kok Chen]

On Feb 8, 2013, at 10:01 AM, ham at n0sq.us wrote:

> Already tried reversing the polarity.
> 
> Joe Subich, W4TV <lists at subich.com> wrote ..
>> 
>> Lee.
>> 
>> Check the RTTY Polarity menu in your TS-2000.  It probably needs to be
>> set to REVERSE.  IIRC, the KAM takes care of that but your serial port
>> interface would not.

Lets see if we can drill down a little bit.

If I am reading this correctly, you are 

1) transmitting FSK through MMTTY, using EXTFSK,
2) receiving through a second receiver with a KAM.
3) and the KAM tuning meter shows that you have centered the two tones correctly.

And, when you transmit, your transmitter keys properly, and you can hear diddles (instead of a single tone) coming from the receiver.  I also assume that the receiver and KAM can copy signals off the air?

Correct so far?

If so, your transistor circuits are working.  And EXTFSK may be working (read on).

You said that you have tried reversing polarity, is that at the receiver (KAM end) or at the MMTTY end?

If you are trying to reverse at the KAM end, do not do it by using the FSKinv command on the KAM.  That one controls the KAM transmit direction.  Instead, turn the KAM's INVerse state on and off.  (FSKinv and INVerse are different commands, one to control the transmit and one to control the decoding)

If that makes it work, and you can copy off the air signals on the KAM without inverting, you will need to invert MMTTY's output (see if MMTTY has an EXTFSK invert setting, or if the transmitter has an FSK invert setting). 

If you hear modulation through MMTTY's EXTFSK, and you still cannot copy by reversing the KAM, there is one more thing that is possible: you are throwing too many errors when using EXTFSK.  EXTFSK is really a hack, where the computer attempts to send the bits out every 22 millisecond.  If there is any timing latency in the OS, the output will not be close enough to the 22 millisecond bit periods that the KAM expects at the receiving end. Unfortunately, you need an oscilloscope to tell you exactly what is happening.  

With most computers, the latency is around one tenth of a bit time (people will have more problems copying you when you are weak, but they will copy you fine when you are loud, so most people don't even know they have a problem, they just think the receiving end has a crummy demodulator).  But if you have an old, slow machine, it could be bad enough that few characters get through.  

If that is your case, some of your choices are: change software, use the K4DSP FSKit to convert AFSK to FSK, or just change to using AFSK transmit.

BTW, you did not tell us what the KAM is printing while all of this is going on... is it printing nothing, is it printing complete gibberish, or is it printing a bunch of weird but repeating characters during diddles, or is it printing correct characters now and then?  Telling us that can help zeroing in on the problem.

73
Chen, W7AY