This page has many virtues. It shows how to do a nice job of research
and how to document it for the reader; it offers useful info about
setting up your RTTY station; and it provides hard data, which is a
lot better than relying on conventional wisdom.
Unfortunately, K0SM's paper has flaws in its premise that render the
conclusions hardly persuasive.
1) the FSK measurements were made with the *original* K3 FSK generator
not the most recent shaped FSK. Even though the original K3 FSK was
better than most, the current DSP implementation is much better and
provides FSK indistinguishable from the filtered AFSK.
2) the AFSK measurements were made using Line In and did not include
effects common to overdriving the microphone preamp as happens in
many rigs.
3) in the K3, AFSK_A disables mic compression and RF clipping. That
is not the case in other transceivers operated in SSB mode. The mic
compression and RF clipping is one source of AFSK distortion and
harmonic generation.
4) the AFSK measurements were made with the K3's AFSK TX filter in
line. That filter removes substantially all the hum and distortion
present in other transceivers when the ground (audio) return is open,
the sound card DAC is driven into the distortion producing range
(typical at output levels greater than 60%), and/or the mic preamp is
being overdriven. Andy's tests with all knobs full bore proved that
the K3 is capable of cleaning up signals that would be a disaster with
many other rigs.
5) The K3 does not use a "closed loop" ALC which generates overshoot
and "pumping" distortion. Again, these effects are common on other
manufacturer's transceivers and are often responsible for a high
preponderance of the "clicks" in both FSK and AFSK signals from those
transceivers.
I have off the air spectrum captures of W1AW with their top of the
line Icom rigs and professional operators that show *all* of the
issues of overdrive, loss of signal return and ALC effects even when
driven by supposedly "clean" AFSK signals from fldigi. Again, AFSK
requires far more operator attention to "signal hygiene" than does FSK
if the result is to be acceptable (that is, unless one is using a K3).
73,
... Joe, W4TV
On 2015-01-30 9:34 PM, Tony Estep wrote:
On Fri, Jan 30, 2015 at 5:29 PM, Fred Jensen <k6...@foothill.net> wrote:
Ummmm ... I've never understood the infatuation with FSK over AFSK....
For those who believe that FSK has some advantage over AFSK for purity
signal or some other reason, K0SM has posted a detailed web page showing
the spectra of signals generated by both, using various transmitter
adjustments. Check it out at:
http://www.frontiernet.net/~aflowers/k3rtty/k3rtty.html
This page has many virtues. It shows how to do a nice job of research and
how to document it for the reader; it offers useful info about setting up
your RTTY station; and it provides hard data, which is a lot better than
relying on conventional wisdom.
73, Tony KT0NY
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