> On Oct 25, 2016, at 2:22 PM, Thom <ki8w@ki8w.com> wrote:
>
> and exactly how do I change that shaping?
With FSK, you are at the mercy of the rig manufacturer. Some rigs use the
"FSK" facility in the synthesizer chips, so even the rig manufacturer has no
control of it, the chip manufacturer does. And the "FSK" requirements from the
chip consumers are different from ours (we are a tiny market for the chip
manufacturers).
With AFSK, it depends on the software modem designer. Among popular modems,
2Tone and fldigi are currently the only two I know that individually waveshapes
the Mark and Space signals before combining them for transmission.
The AFSK modulators that use a bandpass filter to limit the keyclicks from a
composite Mark/Space signal will inevitably be wider than the signals from
2Tone and fldigi. See here to get an idea of how wide the transmit filters
need to be so that the receiver's demodulator at the other end is not impacted:
http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html
By the way, since all filters are in a cascade (RTTY generator -> transmit
filter -> receive filter -> demodulator), the above also applies to how narrow
you can make a superhet's receive crystal filter before you start to impact
error rates.
FSK keyclicks are no different from CW keyclicks. They are (a) not useful for
decoding, (b) they interfere unnecessarily with nearby stations, and (c) they
eat up transmit power needlessly. If your PA is limited by RMS power instead
of PEP power, and given the same amount of average transmit power, a well
filtered AFSK signal will deliver more useful signal power into the
demodulator's filters at the receiving end.
73
Chen, W7AY
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