[RTTY] Locating the Correct or Best Tx Sweet Spot for fldigi

[Dick]

Since posting the subject topic, it has been suggested that I should define 
the term Sweet Spot as used in fldigi because it was being confused with 
other uses of term sweet spot. Some thought I was talking about the low 
and high tones for AFSK RTTY and one even thought it was the most resonate 
point on the antenna system for a given band.

As used in fldigi the Sweet Spot is the audio frequency at which your 
transceiver provides the best filtering for a particular signal type. We would 
like to be able to decode (copy) and work stations at this location on the waterfall. Some operators use 1500 Hz for the RTTY Sweet Spot so the
rig's receive filters, when narrowed, will center in the middle of waterfall 
display. While this looks nice and balanced on the screen, it may or may not 
be the best place for your rig.

With the receivers used in modern transceivers you can mouse-select, 
decode and copy stations up and down the waterfall over a range of
frequencies.  In fldigi, if you want to work a station removed from your 
defined Sweet Spot, the QSY button moves the selected station to the 
Sweet Spot on the waterfall and tunes the VFO accordingly. This is the best 
place to work someone.  Also, in fldigi your TX is usually slaved to the Rx 
mouse position. If you mouse-select someone removed from the sweet 
spot and transmit (without using the QSY button), your transmission could 
be slightly off frequency and may generate the weird signals mentioned 
in the posting. This is why the fldigi QSY capability is your friend. In addition 
to moving the station to your Rx Sweet Spot, it tunes your rig's VFO to the 
same location. 

In the posting you will note I started out using 1500 Hz and recently changed 
to 1000 Hz that seems to be better for my Kenwood TS-590S.  I changed 
because my transmitted RTTY signal sounds (to me) more like a true FSK 
signal at this location. This has nothing to do with the modulation level. I 
monitor the TS-590's ALC meter so no ALC is showing. 

The price I paid is my Sweet Spot location is offset to the left a little on the waterfall and that effects nothing. 

Dave W1HKJ, who is the fldigi developer, has excellent documentation on this and other topics on his web page at 

<http://www.w1hkj.com/FldigiHelp-3.21/index.html>

Put away the Angry Birds and spend a few minutes on Dave's FB web page.

73 Dick AA5VU


On Apr 17, 2011, at 12:47 PM, Dick AA5VU wrote:

> This posting is a revision to my posting on the linux-fldigi reflector that may 
> be of interest to some rtty-contesting operators. I am aware many contesters 
> prefer and use FSK, but some of us use AFSK.  
> 
> The topic is about an experiment in determining the correct or best transmit
> Sweet Spot location on the waterfall display in fldigi. The setting is under 
> menu item Configure/Misc/Sweet Spot. For those not familiar with W1HKJ's 
> fldigi Dave included a really cool QSY button that moves or centers the station 
> the operator may want to work to the defined Sweet Spot location on waterfall 
> and adjusts the rig's VFO accordingly. This Sweet Spot applies to both Rx and Tx. 
> 
> It is my understanding that many operators use 1500 Hz for the Sweet Spot 
> to center the rig's Rx filters to the middle of the waterfall display.  When I
> received the Kenwood TS-590S, I used 1500 Hz for the Sweet Spot and that 
> setting worked fine for receiving and decoding. For AFSK RTTY transmit we 
> try to replicate the sound of a FSK signal and I noticed by using the TX Monitor 
> that my transmitted signal sounded good but not what I expected.
> 
> Having the rig's Sweet Spot centered in middle of the fldigi waterfall is nice
> but it is not necessary. As an experiment I have changed the sweet spot from 
> 1500 Hz to 1000 Hz and adjusted the TS-590's bandwidth accordingly so the 
> narrow IF filter bandwidth is correctly located (offset to the left of center) on 
> spectral display. With this adjustment, my transmitted RTTY signal sounds 
> (to my original issue ears) more like a true FSK signal.
> 
> I think many of us tend to worry more about receiving and decoding than 
> we do about what we are transmitting.  As you listen to digital signals on the 
> bands some really sound strange. I am not talking about being over 
> modulation or too much ALC. Some are high pitched, do not sound right 
> and are probably off frequency. Some sound card generated PSK31 and CW 
> signals sound really sad.
> 
> I have changed my sweet spot settings from 1500 to 1000Hz and it seems 
> to be working better. The optimum setting for the TS-590S may be closer to 
> 1100 Hz but I like using 1000 for easier correlation of the rig's dial frequency
> to the QSO frequency. 
> 
> I do not want to startup another FSK vs. AFSK discussion. This is an AFSK topic 
> that may be for interest to other AFSK users. I am still learning the best way to
> use TS-590's IF filter. Being able to adjust the bandwidth is just one of the new 
> (to me) additions from Kenwood. 
> 
> Thanks for reading. I hope it was useful and not too boring.
> 
> 73 Dick AA5VU