[RTTY] Help with MMTTY

[Kok Chen]

On Sep 7, 2013, at 4:46 AM, Tom Haavisto wrote:

> I tried the TX offset in MMTTY - set it to 170 hz/RIT off, but again back
> to no response when calling another station.
> 
> Any ideas what I am doing wrong?

There are two things (at least :-) than can cause the receive and transmit tones to be at different places on the RF spectrum.

One, as you noted, is the RIT/XIT position of the transceiver.

The other is the so called "tone pair" of the RTTY signal.

For historical reasons (surplus landline modems back in the 60s), a popular tone pair is 2125/2295 Hz.  The separation of the tone pair determines the "shift" of 170 Hz.

However, other tone pairs can be used and with the proper RIT/XIT, no one on the air even knows what tone pair you are using as long as the shift is 170 Hz.  But their locations on the RF spectrum are different.  This holds for both FSK and AFSK.  With FSK, the transmit tone pair offset is set by the transceiver.

Assuming that the RIT/XIT are known to be zeroed out, I would check MMTTY's settings and look for the tone pair settings (look for numbers like 2125, 2295, etc) and make sure that the tone pair that is used in the receive direction is precisely the same as the tone pair that is used in the transmit direction.  I don't use MMTTY, so I can't tell you where the menus are, but they should be there.

If you have an older rig that does not use DSP to generate AFSK, choose a high tone pair.  The 2125/2295 Hz pair is a good choice, since it will pass through the SSB transmit filter, but any harmonics will not.

You can check the tone pair out aurally by sending the AFSK output from MMTTY to the computer's speakers instead of the modem's sound card. (That is the RTTY analog for a dummy load, HI HI.)

73
Chen, W7AY