[RTTY] Psuedo FSK

[Joe Subich, W4TV]

> I don't know of programs other than fldigi and cocoaModem that
> generates a single hard edged on-off keyed tone. Joe W4TV probably
> knows of others, if they exist.

As far as I know there are only three programs that generate pFSK -
two for the PC (fldigi and 2-Tone) and one for the Mac (cocoaModem)
- but then I have not looked at several of the packages in a while.

73,

    ... Joe, W4TV


On 10/14/2013 12:32 PM, Kok Chen wrote:
>
> On Oct 14, 2013, at 8:21 AM, Jim W7RY wrote:
>
>> Has anyone built this circuit?
>>
>> http://www.w1hkj.com/FldigiHelp-3.21/PseudoFSK.html
>
> pFSK (what cocoaModem calls OOK -- "on-off keying" in textbooks) has
> been around for quite a while now, but never really caught on.  It
> has been brought up often on this reflector.
>
> There are commercial units (MicroHAM USB Interface III, MicroHAM
> DigiKeyer II) that will take the keyed tones and convert them to FSK
> keying signal for you.  You can be lazy and not build a thing.
>
> Think of it as K4DSP's FSKit, but you only send it one tone instead
> of the two AFSK tones.  In other words, it is equivalent a Mark-only
> (or Space-only) AFSK signal.  Because of that, you just need one tone
> detector, and it need not be frequency selective, so it is much
> cheaper to implement in pure hardware.  Once you have a
> microprocessor such as an AVR chip in FSKit, you might as well
> implement a full AFSK detector.
>
> The FSKit scheme is somewhat superior because of the "DC restoration"
> problem (the two tones ensures equal Mark and Space pulse durations).
> With an OOK signal, a keyed Mark pulse can potentially be slightly
> longer (or shorter) than the Space pulse depending on the detector
> threshold (usually just Vbe of a transistor that is offset by a diode
> rectifier drop).
>
> On the other hand, with pFSK/OOK, you can choose a higher frequency
> audio tone to reduce the uncertainty of the leading and trailing
> edges of the FSK keying pulses.  I think fldigi uses a tone north of
> 3 kHz.  cocoaModem uses 2500 Hz.  If memory serves, when I tested it,
> the DigiKeyer II will take very high frequency tones.
>
> cocoaModem's OOK tone does not use any waveshaping (I am pretty sure
> it is the same with fldigi) since the audio tone is not intended to
> go out on the air. That also helps reduce the uncertainty at the
> rising and trailing edges of pulses.
>
> An active tone detector (for example, a diode in the feedback of an
> op-amp: like what the ST-8000 uses for its detectors) with ensure
> better Mark/Space balance also.
>
> Since it is not frequency selective, the usual pFSK circuit cannot be
> used with regular two-toned AFSK signals that are generated by the
> computer.  I don't know of  programs other than fldigi and cocoaModem
> that generates a single hard edged on-off keyed tone.  Joe W4TV
> probably knows of others, if they exist.
>
> Instead of building actual diode rectifiers, you can of course use an
> AVR chip to do single tone detector too.  You just need to modify
> Doug's FSKit program slightly.  Heck, if you have an FSKit in hand,
> all the wiring it already there and all you need is to burn a
> different program into it.
>
> If you build your own, just look at the FSK keying signal on the
> scope to make sure that it is symmetrical with respect to Mark and
> Space states, and you are good to go.
>
> 73 Chen, W7AY
>
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