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

[Jay WS7I]

The use of low tones in RTTY has always yielded even with FSK major 
problems which is why years ago, tones were changed from Low-Tones to 
the high tones.  You can have the same problem with FSK using low tones 
as AFSK it just occurs somewhat less.

I recall a phone call back in 1986 while operating a contest with Hal 
WA7EGA.  He was operating and I was logging.  The phone rang, and I 
answered.  The voice asked for Mr. Howard Blegen, I asked who was 
calling and they said the FCC!  We of course were running FSK.  However, 
after I passed Hal the phone, the guy asked him some questions.  Seems 
the fine old Kenwood Twins had a significant spur right dead on the Air 
Force MARS transcon frequency, ouch.

The "Twins" were scrapped and shortly after the old Johnson Thunderbolt 
as well.

So its not only the new guys but every digital operator needs to check 
their signal.

Jay

On 3/13/2014 6:25 PM, Joe Subich, W4TV wrote:
>
> 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.
>