[Elecraft] K3 - 250 Hz and 400 Hz Filter Settings

[Kok Chen]

On Jul 18, 2010, at 6:11 PM, Jim Miller KG0KP wrote:

> Made me wonder though if one could decode two signals intertwined but 
> individual mark and spaces traces in the clear form each other?. 

Yes, you can, and the method is regularly used by some RTTY folks.

It is called Mark-only or Space-only copy.  

You basically treat the corrupted FSK signal as an OOK (on-off keyed) signal by only listening to one of the two FSK tones.  If you go back to your communications theory textbooks, you will notice that under quiet band conditions -- the so called "Additive White Gaussian Noise" or AWGN case, you just need to raise the peak transmit power by about 3 dB to attain the same error rate as binary FSK 

OOK radioteletype was actually the original mode that is used by hams.  Hams eventually migrated to binary (two tone) FSK to do better under selective fading.

With Mark-only or Space-only, you basically compare the single demodulated output against the noise floor (setting the decision threshold at 1/2 of the long term peak voltage) instead of comparing the matched filtered output of the Mark tone against the Space tone in the case of binary FSK.

Binary FSK got another boost after the discovery of the automatic threshold correction (ATC) circuit (see US Patent 2,999,925 that was issued in 1961; the same patent also has claims on a method of copying RTTY using diversity reception).

With good demodulators today, when you switch from FSK to OOK, you also lose the additional robustness that the ATC circuit provides under selective fading, in addition to the aforementioned 3 dB.

Many modems support Mark-only and Space-only copy.  The legendary HAL ST-8000 has "MO" and "SO" as two of the settings in the "Detector Mode" switch.  Many big gun RTTY contesters and DX'ers still have an ST-8000 or two sitting next to their radios (when new, they sold for about $3,000 :-).  (The CEO of HAL is a ham, by the way.)

Some software modems also directly support Mark-only and Space-only copy.  For modems that don't support it directly, you can still get a good approximation of Mark-only or Space-only copy by using a very narrow filter that passes only the uncorrupted tone as long as the modem also has a decent ATC circuit/algorithm.

By using Mark-only copy on one signal and Space-only copy on a second signal, I have successfully copied both RTTY signals that are overlapped to the point were a tone of one signal is located at the center of the second signal.

73
Chen, W7AY