[RTTY] Psuedo FSK

[Kok Chen]

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