[RTTY] CWSkimmer use in Rtty

Kok Chen chen at mac.com
Tue Jul 26 09:14:22 PDT 2011

On Jul 26, 2011, at 5:04 AM, Bill wrote:

> For each rtty signal, there 'appears' to be two traces. Separated slightly.
> I'm assuming Mark and space.

Yep.  That is the Mark-Space shift.  

For most people, the separation is 170 Hz.  It could be 200 Hz for many who still insist on using an AFSK TNC, and the shift can pretty much be anything for an off-calibrated older generation Direct FSK rig like the Omni V or Omni VI (I have seen an Omni V put out 80 Hz shift on FSK :-).

> The upper (higher freq) is always longer.

It depends by what you mean by "upper."  It all depends on what the software author did, and also whether you are using an USB or LSB receiver.  It could as well be left and right, or some 3D plot where frequency is in the "depth" dimension of the plot.

By convention, Mark is the one that is the higher of the two FSK frequencies on the RF spectrum.

The Mark is the "resting" state of FSK.  For FSK transmitters, Mark is the "unkeyed" state.

Since the Stop bit, being the bit that places the FSK signal into "rest" state is a Mark, stations that transmit with 1.5 or 2 stop bits will transmit a tad more often in Mark than in Space, if all Baudot characters are random .  You can often find a "reversed" station by just watching signals in the waterfall.

See Figure 16.1 in Chapter 16 of the 2010 and 2011 ARRL Handbook.  I have found that figure to be so absolutely invaluable, I had taken it lock stock and barrel from the older Handbooks when I rewrote the RTTY section of chapter 16 of the handbook.  The same figure appears as a Fig 9.10 in chapter 9 of the 2009 Handbook.

> The upper trace starts first and ends last.

Many modems often begin with an undiddled Mark tone (could last 100ms or so) to allow a receiver to tune the signal (Mark tone is by convention, also where you spot an RTTY signal).  Another reason is to let the transmitter settle after issuing a PTT, before keying it.

Since an RTTY transmission ends with a stop bit, you will also naturally see the transmission end with a Mark.  The Mark signal can is also slightly extended at the end -- i.e., the diddle is turned off a little before the PTT is unkeyed.

In your case, "upper" on CW Skimmer is most likely the higher frequency in the RF spectrum :-).

Chen, W7AY

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