[RTTY] AFSK to FSK converter

[Kok Chen]

On Sep 17, 2012, at 6:41 AM, Frank wrote:

> Now anyone want to admit that this entire thread was an April fools joke?
> 
> AFSK to FSK converter indeed!

Scoff as much as you like, but K4DSP originally had the same problem that Bill describes.  He simply wanted some way to use a software modem with older rigs.   And indeed, he had used an old discarded TU for just such a converter.

You can see him describe it right on this very reflector:

http://lists.contesting.com/_rtty/2005-03/msg00035.html

The FSKit makes use of a $2 programmable Atmel AVR chip instead of an old clunky TU that few people have in their junk boxes anymore.   

Engineers design schemes to solve other people's problems.  Good engineers design schemes before others even know they need the solution (think Edwin Armstrong).  Over time, a regenerator like the FSKit might become the best solution out there for keying an FSK transmitter.

Also for a couple of bucks, an Exar XR-2211 outperforms an ST-6, but that is neither here nor there since for this purpose, the demodulator only needs to decode signals that has SNR in excess of 60 dB (the transmit AFSK tones from a sound card).  Time moves on, and a tiny SOIC chip outperforms a rack full of tubes.

With an SDR tarnsmitter that starts with in-phase and quadrature baseband signals, you don't really need direct keyed FSK anymore since the distinction between FSK and AFSK pretty much melts away (the same can be said of the old keyed AFSK rigs, such as the FT-1000D; there is no difference between using AFSK and FSK in those rigs, either).

There is another style of FSK-AFSK conversion too, but sort of in reverse.  TUs such as the ST-8000 and Timewave DSP-599-zx support a FSK Regenerator mode that turns the demodulated RTTY pulses from the receiver back into a 2215/2295 AFSK tone pair.

Some of us used to depend on that flavor of regeneration because HAL and Timewave did not support Mac OS.  We simply fed the regenerated AFSK tone pair into some TNC (e.g., KAM Plus) that can run under Mac OS.  The regenerated tone pair again has very high SNR and the second (inferior) TU does not throw any extra error.  The regenerated tones includes the errors from the front end (superior) TU, and the entire TU chain functions as if it is just the superior TU running by itself.

That second flavor of FSK regeneration is also exactly the way you can receive with an ST-8000 in the modern word.  You turn on the ST-8000 regenerator and feed the regenerated tones into the sound card of a modern modem.  Not that you would want to, since a whole bunch software modems today wipe the floor with the ST-8000 if you have set a decent sound card up properly.  For transmitting? -- well, we are back at Bill's dilemma -- how to key the FSK input of a transmitter from a sound card based software modem.

73
Chen, W7AY