RTTY Tuning Indicator

A 'scope is still the best way to tune a RTTY signal. Your brain's visual processing system has a very powerful pattern analyzing capacity and spreading the tuning information out across a small piece of your screen gives your brain its best shot at decoding the tuning information.

Here's what the RTTY tuning indicator looks like when tuned in:

The SPACE tone draws a vertical bar on the screen, while the MARK is horizontal. Because the asynchronous codes used for RTTY always have more time at SPACE than at MARK, the vertical bar is brighter. Colors indicate the quality of the signal: the brightest color (white) indicates the strongest signals with green/blue/yellow indicating progressively less. Black is no signal at all, and the dimmer colors indicate weak signals.

The scope has an AGC function that automatically keeps the indicator roughly full window size. You can also resize the window, but remember its easier to read if you keep the window square. If you expand it, you'll notice its quantitized to 64 by 64 "pixels" in the picture. They are all updated 4 times per second to limit the latency as you tune across a signal.

WinRtty maintains two separate sets of MARK/SPACE filters. The ones used for the actual signal decoding are quite narrow and show little or no response when detuned by 30Hz or so. The second set is much wider-about 170Hz at 2 dB down-and is only used for the tuning indicator. They give the visible response when you're tuned as far as 80 or 90 Hz off center, and they also converge much more quickly. This makes the tuning indicator faster.

The output of both sets of filters are drawn into the tuning indicator display. More than 30Hz off center and what you see is coming only from the wide filters except for a very bright spot in the center-both MARK and SPACE on the narrow filters are very strongly indicating "no signal". When you tune closer, the narrow filters contribute the tones they're hearing and make the tuning "plus" indicator much narrower and brighter than it would be with the wide filters alone. Combining the filters this way gives a very reliable "plus" drawing when tuned correctly (from the narrow filters) while at the same time giving a fast visual indication of which way to tune when you're off (from the wide ones).

Here's a quick tour of what you'll see. These examples were made with strong signals.

At center tuning, the two bars should be the same size (make sure you've sized your window to be square!). If they are not, then tweak your IF shift on the receiver until they are the same.