[RTTY] AFSK to FSK converter

[Kok Chen]

On Sep 17, 2012, at 2:20 PM, Rex Maner wrote:

> I don't understand any of this  ""OOK""  or such stuff.

OOK is described in communications textbooks as On-Off Keying.  You can see it mentioned here, for example,

http://en.wikipedia.org/wiki/On-off_keying

With OOK, the presence of a tone represents a Mark and the absence of a tone represents a Space (or vice versa, for inverted signal).

Think of it as what we know as "CW" and indeed, the original amateur RTTY ops had used OOK to send signals between their teleprinters.  Feld Hellschreiber is another mode that uses OOK (but it is really more of a facsimile mode than a digital mode)

A second complementary tone is added to the single OOK tone to form 2-tone FSK.  Some digital modes use even more tones; DominoEX uses 18 tone FSK, for example.  

FSK works much better than OOK for HF because of fading (the two tones automatically establish the threshold level for the ones and zeros even as the signal fades up and down), and especially for selective fading, when you can still copy a single tone (as an OOK signal) after the other tone has faded under the noise.

If you have used Mark-only or Space-only reception with an ST-8000 (or ST-6000), you would have experienced OOK copy.  Some software modems implement it too.  I know of one for sure, but I only use one digital mode program and don't know for sure how many others have Mark-only or Space-only receive capability.

By the way, Mark-only and Space-only is almost a lost art nowadays.  If they are not already doing it, more software modems should implement it.  There are numerous times when I can copy through QRM that is clobbering one FSK tone by using Mark-only or Space-only to decode the surviving tone.

The reason OOK transmission is bandied about here is that one can convert the on and off audio tones from a sound card into an FSK keying signal even when the single tone is moving around.

There are two software modems that I know for sure can transmit OOK.  But the implementation of OOK is rather trivial -- if the software author knows how to generate an AFSK signal, it is child's play for him to generate an OOK signal.  Programs that don't implement OOK transmission probably are that way because there has been no justification for it.

73
Chen, W7AY