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

[Joe Subich, W4TV]

The spectrum shown is a direct capture from the Elecraft P3.

The leftmost two "signals" are most certainly as described - they
were copied directly with MMTTY using the default profile in RTTY-REV
(USB) for the lowest frequency pair (LSB "Image") and RTTY (LSB) for
the middle tone pair ("normal" signal).  The rightmost "tone" has the
same "offset" (approximately 1580 Hz) relative to the mark tone as the 
"inverted" mark (leftmost tone).  That would make  the right hand tone
pair the *third* harmonic of the modulating frequency and the shift 510
Hz (3 x 170 Hz).  Interestingly, the LSB suppression is *at best* 25 dB
(-110 dBm for the image vs -85 dBm for Mark) and the third harmonic is
down only 11 dB (-96 dBm vs. -85 dBm).  One can also see the *fifth*
harmonic products as the two pips to the right of the third harmonic
- only about 31 dB below the Mark.

In any case, the analysis is still the same - there are at least
three problems:

1) Insufficient lower sideband suppression - one can even see the
    suppressed carrier position as a "notch" in the phase noise
    "hump" at approximately 24.9249 MHz
2) audio clipping and (third, fifth) harmonic generation
3) the use of a very low tone pair - approximately 790/620 Hz Mark/Space
    - which easily allows the third harmonic at approximately 2370/1860
    Hz and fifth harmonic at 3970/3060 (24.9289/24.9280 MHz) to pass
    what filtering exists (it can't be much given considering the poor
    LSB suppression).

Of course, this is *exactly* what one would expect if an inexperienced
RTTY operator were to use AFSK and follow the instructions of certain
AFSK interface vendors to operate with all knobs/sliders to the max.

73,

    ... Joe, W4TV


On 3/13/2014 8:27 PM, Kai wrote:
> 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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>