[Flightgear-devel] Radio frequency range: min/max/wrap

[Julian Foad]

I noticed that the radios had nav. freq. range 108.00 to 117.95 but com. freq. 0 to 140; this should be 118 to 140. But while playing with that I noticed that the wrapping is a bit unpredictable. With (min=118, max=140, step=1, wrap=true) adjusting it up and down, it sometimes skips 118 and sometimes skips 140. For the nav. frequencies, my 1991 UK Pooley's Flight Guide confirms that the range is 108.00 to 117.95 inclusive. But the current implementation that specifies (min=108.00, max=117.95, step=0.05, wrap=true) tends to cycle (117.85, 117.90, 108.00, 108.05) skipping 117.95.

There is a problem with the way "min" and "max" work when "wrap" is on and discrete steps are being used. "Wrap" is designed for analogue values to go round in a circle where "min" and "max" are regarded as equivalent. On things like our radio frequency controls, it is down to luck (due to floating-point precision) whether (min=118, max=140, step=1) cycles through (139, 140, 119) or (139, 118, 119).

Some of the directional instrument controls are specified as (min=0, max=359, wrap=true). These should, I think, all be specified as (min=0, max=360, wrap=true), so that it doesn't skip 359, because in this case the min/max are the end points of an analogue range (not a set of discrete valid values). It doesn't matter whether it reads "360" or "0" for North.

So:
- Can anyone confirm the min. and max. settable com. frequencies on radios of this general type? I'm fairly convinced now that it must be 118.00 to 139.975 inclusive (or 139.95 on old models with 50 kHz spacing).

- Do they wrap from one end of the range to the other? If not, it is easy to model properly. If they do, we need to look more carefully at the way the wrapping handles discrete steps.

- Julian


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