[UK-CONTEST] RTTY operation (was ARRL Contests)

[Ian White GM3SEK]

David, G3YYD wrote:

>Chris Tran ZL1CT wrote:

>> The advantage of true FSK is that it is difficult to have a poor transmitted
>> RTTY signal, whereas with AFSK it is easy to overdrive the Mic input stages.
>> However, our initial impression is that true FSK has the disadvantage that
>> when S&P, you have to carefully tune in every station you wish to call (with
>> our AFC off) to be accurately netted (you could argue that their AFC would
>> find us but if we're in a pile-up I guess not)   Standing by to be corrected
>> by experienced RTTY ops. !
>>

>I have always wondered why FSK is used when AFSK is so much more
>flexible and simpler to implement. I have used AFSK since the early
>1970s. I have never had anyone complain about my transmitted signal.
>Recently I listened on my FT1000MP to my K3 on AFSK and could not detect
>any problems at all even at very low levels. I suspect that many of the
>wide RTTY signals are FSK generated as they will not have the filtering
>that is available on AFSK.
>
>When using AFSK:  RUN net off and AFC on is the configuration - ensures
>auto track off QRG callers without TX moving. When on S&P net on and AFC
>off is how I operate - net on insures where you RX you TX.
>

Both Chris and David make valid points.  FSK has no user settings so the 
signal quality is always the same; but that quality depends on the rig. 
Sudden frequency shifts produce similar key-clicks to unfiltered CW 
keying (which is why I always felt a bit nervous about FSK in the 
FT-1000).  AFSK can manage these problems in software, so in some rigs 
it can produce a cleaner signal than the hardware FSK... but the user 
has to adjust the drive level into the rig, so AFSK can also be truly 
awful.

Coming back to the business about AFC and NET: when it's so easy to tune 
in an FSK signal, who really needs them? For RTTY I keep both of them 
permanently switched off.

The FFT spectrum display in MMTTY makes it very easy to tune an RTTY 
signal. Just drop the two tones onto the two yellow bars  (or anywhere 
reasonably close) and MMTTY will start decoding.

Another advantage of the FFT display is that it's like having a little 
panadaptor permanently on-screen. It helps you find clear frequency 
slots and keep aware of nearby QRM. Keying sidebands spilling into your 
slot are clearly visible, and so are the extra 170Hz sidebands from 
overdriven AFSK. The two yellow bars in the FFT are separated by exactly 
170Hz, and in heavy QRM that's a big help in sorting out which pairs of 
tones belong together.

The traditional "crossed bananas" display does nothing that isn't done 
better by the FFT, so I keep that switched off.


Having said all that, apologies for the config trouble in the very first 
QSO last night (that was the logger, not MMTTY). Also apologies for the 
hang-up later on, when I discovered I'd kicked the keyboard lead out of 
its socket. What with one thing and another, it never felt comfortable, 
but 76 turns out not too bad.


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek