[RTTY] AFSK to FSK converter

[Joe Subich, W4TV]

> 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.

And here's where we are at the mercy of the software developer -
he/she can either provide support for a traditional UART, provide
on/off keyed tone that can be converted to "dry contact" for the
transceiver, or provide a fixed tone pair mode that can be used to
drive a regenerator (microcontroller a la K4DSP, NE565 or XR2211
or an old TU).  If the software author insists on "all AFC, all
the time" then Bill's dilemma is essentially unsolvable.

Some developers understand that there is more than one operating
paradigm and that all approaches have their merit.  Others become
religious zealots and show no respect to historical techniques and
those who have transceivers that are designed around those traditional
techniques.

Bill has discovered another apparent reason to use FSK (RTTY mode)
instead of AFSK ("Data" mode) with Icom transceivers.  It appears
that there is no way to adjust the center frequency of the "DATA"
filters.  One can narrow (and save) the bandwidth but the filters
are fixed at 1500 Hz unless the passband shift is used each time.
Although I am no an Icom user, I can understand how frustrating
(and inconvenient) that must be.

73,

    ... Joe, W4TV


On 9/17/2012 1:33 PM, Kok Chen wrote:
>
> 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
>
>