You change the shaping by moving from FSK where you have zero control over
to AFSK where you have full control. Another advantage of AFSK is that you
get to set the tone pair where it sounds good to your ears instead of being
stuck with the 2100 hz rasp of rigs without a low-tone pair option force.
As Joe likes to point out, full control with all knobs turned to 11 is a
problem. But I think that a guy who has learned to use his mic gain control
can learn how to set his AF drive level on AFSK.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Kok Chen
Sent: Tuesday, October 25, 2016 4:51 PM
To: RTTY Reflector
Subject: Re: [RTTY] FSK is bad?
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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