AFAIK, all non-DSP rigs with synthesizers work this way.
Assuming you want the output frequency to be derived
from the master clock frequency, there is no easy
way to shift an RF carrier. You can't use a free
running oscillator, because it won't be derived
from the master clock frequency. You can't switch between
a mark synthesizer and a space synthesizer because of
transients. If you try to key the programmed frequency
of a BFO synthesizer, the PLL will probably go out
of lock momentarily, producing garbage. Also, it
may not be fast enough to keep up with RTTY. After
considering all this (as a very experienced synthesizer
designer) it is hard for me to blame the designers
for using AFSK.
It is easy to do it much better, and it would only cost pennies extra at the
most. The signal could be generated by the normal SSB system and then run
through a narrow IF filter. Problem solved. They could have done CW the same
way, or in a similar fashion, with an unshaped off-on carrier through a
narrow filter. They just didn't think to use the parts they already had in
the radio.
But that isn't this issue. This issue is they run baseband audio from a
computer into a SSB transmitter to generate TX signals. This means it is
really a SSB transmitter processing the tones, and they don't even restrict
bandwidth with a narrow filter. That is really the entire issue. Instead of
a narrow filter cleaning up stuff, it all goes through a SSB filter. Anyone
with a computer and a little skill can invent a "new mode". It's just bad
engineering to stick that stuff near weak signals, because the problem can
only be fixed at the transmitter.
73 Tom
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