[RTTY] Spot frequency

[Kok Chen]

On Jun 13, 2010, at 5:18 PM, Gary AL9A wrote:

> I seem to remember this was due to the 2125 Hz shift.  Is this a difference 
> between running FSK or AFSK?


By convention, RTTY signals are identified by the Mark frequency.

Most radios when operated in FSK mode will display the mark carrier as the VFO dial frequency.  So, the problem is most likely *not* be originating from you.

But take the case of an AFSK op who just worked you (and presumably zero beat with you)...

If his VFO dial is showing the suppressed carrier frequency of an LSB transmitter, and he using the 2125/2295 tone pair, then his (and your) mark carrier will be 2.125 kHz below his dial frequency.   If he is spotting from his VFO dial, the spot will read 2.1 kHz higher than your actual mark frequency.

Now, what if he is using USB?

In this case, assuming he is using the same 2125/2295 tone pair, his mark carrier will be 2.295 kHz above his suppressed carrier.  (Notice that the mark, by convention is always the higher frequency of the two RF carriers.)  If he is reading off a VFO dial that displays the SSB suppressed frequency, his packetcluster spot will then be 2.3 kHz too low.

That said, good rigs won't have this problem if you set them up correctly.  The menu of the FT-1000MP for example, allows you to apply a dial offset to the AFSK ("PKT") mode.

Even if the radio does not have that feature, software modems often have a place where you tell it to how to label the waterfall frequency. cocoaModem for example has a VFO offset box, plus a menu to tell it if you are using USB/LSB and it will apply the arithmetic gymnastics to display the number that has to be added to or subtracted from the VFO dial to get the actual frequency of your RTTY mark.  cocoaModem will in fact reverse the waterfall (lower tones appearing on the right side of the waterfall instead of higher tones always on the right) when you transceive in LSB so that the waterfall will show the higher RF carrier always to be on the right of the waterfall. 

With all the tools at our fingertips, (including a calculator :-), there should be no excuse for not spotting the mark frequency of an RTTY signal properly.

73
Chen, W7AY