Which is better: FSK or AFSK?
In the field of RTTY transmissions with audio boards, you can
transmit either in FSK or AFSK mode.
In FSK (frequency Shift Keying), the mark and space frequencies
in transmission are operated directly by RTX (which thereby must have
the FSK mode) through a built-in modulator. Reception is in any case
operated by the sound card.
In AFSK (Audio FSK) instead, mark and space are operated by an audio
oscillator outside the RTX, which therefore needn't have the FSK mode.
This function is implemented by the PCs’ standard sound cards (e.g. SoundBlaster)
and by software like MMTTY, TrueTTY, RITTY, and so on.
Advantages and Disadvantages
Now let's see which are the pros and cons of either mode.
Surely the highlight in AFSK is that due to AFC (Automatic Frequency Control),
which modifies the mark's and space's frequencies, the software can automatically
"track down" and target perfectly the other station, without having to do it
manually with the VFO: this will do both for reception and transmission.
This function is quite useful both when calling and when replying, as you just
have to tune in on the station even if approximately and wait (pretty little)
for the program to work it out.
On the other hand, as far as I know, the AFSK has a couple of negative
aspects, which both have to do with use in contests: when the number of adjacent
stations with strong signals is high, you risk "losing" the tuning because the
AFC "feels" a stronger signal nearby and moves, targeting the other station:
in these cases you should disconnect it, retarget by hand the OM and ask him
to repeat the report...
The second disadvantage regards the filters: to be able to use the AFSK,
the RTX must be switched to LSB or USB with reverse, if the program allows it;
the problem is that not all radios (like mine, a TS940) allow to use the CW's
narrow filters even in SSB (whereas in FSK this is possible), which are
indispensable in the chaos of the contests in order to "isolate" a single signal.
As to the FSK, the disadvantage is that the AFC can be used only in reception,
which makes the manual targeting of your partner indispensable if you want to
answer a call.
I can thus say that both the FSK and the AFSK give very good results; the
choice of the mode is up you, and depends mainly on the radio you are using and
the clogging of the frequency.
Bye and Good RTTY to all!
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