[RTTY] [MMTTY] The Problem with AFSK ...

[Kai]

I think there is something wrong with your analysis, and with your spectrum 
display.
The widths you claim for the signals are not consistent with the spectrum width 
you show
if the width of the center pair are to be 170 Hz.

The pair of signals to the right of the center pair are about 3 times as wide as 
the center
pair, not two. If, as you claim, the center pair are a 170 Hz shift, then the 
ones to the
right must be near 510 Hz shift, not 340 Hz, so it is not a second harmonic.

Perhaps you should investigate your spectrum analysis software and measurement 
technique,
as it does not look robust.

-Kai, KE4PT





On 3/13/2014 4:51 PM, Joe Subich, W4TV wrote:
> 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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