Danny Douglas is correct with his information. With RTTY, most of the frequencies have been mark tone, not carrier frequency since most serious RTTY operators used on FSK and not AFSK. Since there is over 2000 Hz difference between the carrier and the tone, if using AFSK high tones of 2125/2295 with 170 Hz shift, and over 1000 Hz with AFSK low tones, you would have widely different spotting frequencies depending upon whether you were using FSK or AFSK.
Some rigs will read the FSK mark frequency as a direct readout on the display. There were some rigs that read the space frequency instead of the mark, but the normal convention was to read the mark frequency and the 170 Hz difference would not be as difficult to find compared with a discrepancy of over 2000 Hz. If you have a rig that is designed for RTTY FSK operation, you might try tuning in a steady carrier in RTTY mode and set it at 2210 Hz which is a convenient number averaging the standard high tones of 2125/2295 for 170 Hz shift on a waterfall and read the dial frequency. Then switch to USB and record the new dial frequency when the waterfall shows 2210 Hz. Finally, switch to LSB mode and record the dial frequency. They will be VERY different dial frequencies, right? As an experiment with my ICOM 756 Pro 2, here are my rough results to the nearest 10 Hz: RTTY 28233770 Hz USB 28231460 Hz (difference of 2310 Hz from RTTY mode) LSB 28235900 Hz (difference of 2130 Hz from RTTY mode) note also that the difference of 2310 and 2130 is 180 Hz which if I did this more exactly, would likely be 170 Hz, which is the shift between mark and space. If someone gave you the FSK mark frequency, and you were using AFSK and turned your dial to the mark frequency, you would be placing your carrier frequency more than 2000 Hz off from where you are expecting to tune in the signal. It just will not be there. Remember, when you are operating in SSB mode, your display is reading out the carrier frequency. Of course there is almost no carrier since it is suppressed many dB down. That is why when you toggle between USB and LSB, your dial frequency remains the same. You are just showing the carrier center frequency which does not change. What has complicated things is the proliferation of new modes, most of which are being operated with sound cards as AFSK. Some of these modes are extremely wide, as much as 2 kHz wide at this time. So we don't really think of a mark and space with PSK and OFDM, etc. We just have some wide band signals with various tones within a range of frequencies. It is easier to just specify the dial frequency if they have a fixed relationship to the carrier as an AFSK mode. This could be true of 141A or MT-63. With narrow modes, such as PSK31, the signal could be well up the band assuming USB is being used which is SOP these days. 73, Rick, KV9U John Becker wrote: > In my 37 years I have always seen it given as dial frequency. > Reason: RTTY has fixed tones - never changes. Therefore > if you give the dial frequency there is no math to be done > just tune and go. You will be right on. > > > >
