FSK Support?

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  • Question
  • Updated 3 years ago
  • Answered
What are the plans for supporting FSK on the Flex-6000 series? It seems to me that this should be easy to implement.
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Jerry Gardner

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Posted 3 years ago

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Jim Gilliam

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Have you tried fldigi? The CAT interfaces very nicely and you will have just about all the digital modes.

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Tim - W4TME, Customer Experience Manager

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Official Response
We have the FSK included as a feature in our enhancement database for a future release. However,it has not been scheduled for inclusion in a specific future release of SmartSDR.  I should mention that running ASFK digital modes with DAX can generate an incredibly clean signal that is indistinguishable from FSK generated RTTY. 
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Doug Hall

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I was always a huge FSK enthusiast, and was initially disappointed when I found that it wasn't supported in the Flex with SmartSDR. But Tim is correct. Ever since Flex added the RTTY enhancements to SmartSDR (back in version 1.5, I believe) running AFSK with DAX provides performance that is as good as any rig I've ever owned that had hardware FSK.

For some legacy rigs there is a good technical reason to use FSK RTTY - on some rigs you can only select the narrow IF filters for RTTY if you are using FSK. But that's not an issue with the Flex.

One issue with FSK is how the FSK signal is actually generated by the digital mode software. For sound card modems it pretty much requires a serial port. Honest-to-goodness COM ports are going away, and as time goes on there will likely be fewer and fewer USB-to-Serial adapters that support 45.45 baud operation. And none of the old standalone boxes (AEA, Kantronics, etc.) provide the performance of PC sound card modems. My HAL, IRL, AEA, Kantronics, and homebrew TUs have been gathering dust for years.

Bottom line (for me) is that I won't bother switching to FSK if/when Flex adds support for it. Can't see any reason to. And this is from a guy who wrote an article for QST in 2011 promoting the use of FSK.

Doug K4DSP
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Alex - DH2ID, Elmer

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I fully concur with you about how FSK works, Doug.

Programming it into firmware should be fairly easy, as the SDR only has to
generate mark and space tones which are triggered by either an internal setup
analogue to CWX or an external software program.

The main advantage of FSK over AFSK has always been that the tones for
AFSK are always triggered by an external program and then generated by
either the internal PC/notebook soundcard, which could be susceptible to
tone frequency drift or an external soundcard. I have always relied on
external soundcards as I worked a lot of SSTV and FAX and anybody who
does this knows about slant correction... There is also a very small time delay
before the tones arrive in the TRX and are transmitted or decoded and there
always has been the Windows latency problem. All this is eliminated by
FSK, but do we still need FSK in a modern SDR TRX?

My FSK setup: IC-756PRO set to RTTY or TS-2000X set to FSK
- microHAM DigiKeyerII interface - HamRadio Deluxe DM780 in FSK mode.
This works very well and the IC-756PRO even has a dedicated RTTY filter
for FSK. But testing and working under all condx did not show a significant
difference between FSK and AFSK, which I can also work with this setup.

My AFSK setup: Flex6k5 set to RTTY and HamRadio Deluxe DM780 in AFSK
mode. This setup works equally well and I see no difference in character
recognition and behaviour in QSB situations.

73, Alex - DH2ID
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Jerry Gardner

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What is the trick to getting RTTY to decode in DM780 with the Flex set to RTTY mode? It doesn't work for me unless I set the rig to DIGU or USB.
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George Molnar, KF2T, Elmer

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RTTY mode in SSDR is a narrow LSB. If your software allows, try using "reverse" tones.