Lest anyone misunderstand the problem with AFSK and why I am still
so adamant that the only way to generate RTTY is FSK, take a look at
the spectrum capture I placed on the web site this afternoon:
http://www.microham-usa.com/appnotes/DX_AFSK.jpg
It shows a well known DXpeditioner who is obviously over driving the
the microphone preamplifier in his rig. The center signal is the
"normal" FSK, to the left by 1565 Hz is an *inverted* copy of the
FSK (perfectly readable in RTTY-REV) and to the right by 1565 Hz
is a *second harmonic* (340 Hz shift) copy of the signal.
This demonstrates *three* of the things to avoid with AFSK:
1) *never* overdrive the mic input of the rig - always use a
"Line" level (typically, the PACKET, DATA or ACCESSORY)
input.
2) *always* use the lowest possible audio drive level to avoid
clipping/distortion
3) Use *high tones* - keep the audio frequency *above 1500 Hz*
so the second harmonic stands a chance of being rejected by
the IF filter (if the rig actually has one these days).
Again, *USE FSK* it is far less likely to exhibit the garbage shown
here. This isn't a case of hum from a defective interface - it is
quite simply the fingerprint of an operator who does not know how
to properly set-up AFSK and who probably has no spectrum analyzer
to be ensure proper configuration.
73,
... Joe, W4TV
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