| Doug McLaren Wrote:
| >
| > You'd be surprised how much damage an out-of-control R/C car can do.
| > Consider an 1/8th scale car running into your shins at 60 mph -- or
| > worse, hitting something and going airborne, flying at your face ...
| >
| I'll take your word for it. :) I have little personal experience with
| RC cars.
As do I. But I have seen some crashes that could have really hurt
somebody had somebody been in the wrong place ...
...
-- let me re-insert some text that was trimmed --
| > > Also when the Tx frequency hops, the Rx has to do likewise, which
| > > means the two have to sync up again after every hop. And what
| > > happens when something goes wrong and the sync up doesn't happen
| > > immediately as intended?
--- re-introduction complete ---
| > This sync-up thing is pretty well worked out. People have been doing
| > spread spectrum like this for what, 50 years? They know how to make
| > it work, and it works. Basically, this is just FHSS FUD.
| >
| I'm not so sure...most 2.4 GHz FHSS RF links we use have the odd short
| dropout that simply isn't of much consequence. Anyone who's used a
| cordless phone or a Wi Fi ethernet card has probably experienced these
| little glitches. They simply don't matter in the applications mentioned
| above. But even a very short glitch causes loss of pilot confidence when
| flying an RC model, and a slightly longer glitch can cause a crash.
I was talking specifically to the idea of FHSS `losing sync' when it
hops. The only time that this will generally happen is if the TX or
RX loses power, or gains/loses some time somehow (going through a
wormhole or something similar might do it, let's hope your flying
field doesn't have many.)
As for WiFi, it doesn't use FHSS at all, so it's a poor example of
FHSS glitching. And cordless phones could use either form of spread
spectrum -- or perhaps no spread spectrum at all.
As for glitches not causing problems, even a 1% packet loss in a
TCP/IP link, over whatever medium, will really mess things up. The
system can recover, yes, but throughput will drop by a very large
margin. TCP does handle lost packets, yes, but not very well.
Losing 1% of your R/C frames probably wouldn't even be noticeable by
most flyers, because once the next valid frame arrived, all would be
well again. With PPM, frames usually aren't lost but are instead
corrupted, so you might have a visible `glitch' in the controls where
the servos do something you didn't tell them to, but with PCM and SS
usually invalid frames are quietly dropped and you probably won't even
notice.
| From memory, John Adams said that the FHSS systems had more short
| dropouts in the RF link, and the problem was never overcome. There was
| also reduced range and S/N ratio with FHSS due to the increased
| processing gain of a DSSS system over an FHSS system.
In any event, it sounds like Futaba worked out whatever the problem
was. They have been doing spread spectrum in their industrial R/C
stuff for well over a decade now, so they probably do know what
they're doing -- they just moved slowly in getting it packaged for us.
This problem with the zero GUID has nothing to do with FHSS vs DSSS --
it's either a manufacturing process glitch, or overlooking a
`weakness' in the components they use (and forgetting to add some sort
of failsafe to detect that. They could certainly make their firmware
throw an error if a 0 GUID is detected or an invalid CRC in the flash
memory, had they considered it.)
(And which is more dangerous? An out of control R/C plane, or crane?
Could probably go either way ...)
In any event, Futaba's system does sound like it has one big advantage
over Spektrum's system -- how it handles
_bad_ interference. Suppose
somebody brings their 2.4 GHz camera to the field, and it's using the
usual six MHz of bandwidth. Suppose that both 1 MHz channels that
your Spektrum system are using are within this 6 MHz (and if Spektrum
keeps them far apart, suppose that two people bring their 2.4 GHz
cameras ...)
Spread spectrum can cut through interference to some degree, but the
range will be greatly reduced. Since Spektrum can't hop channels at
all, you'll crash once you get out of the greatly reduced range.
In the Futaba case, OK, less than 10% of the band is `ruined' -- but
the rest is fine. So maybe you'll lose 10% of your frames -- the
plane might seem slightly `jerky' (or maybe you'd need to lose a lot
more than 10% for it to be noticeable) but it's still completely
flyable. In fact, you could probably lose 95% of your frames and
still fly the plane.
Of course, this is all guesses, because I don't have any Futaba
spread-spectrum gear and have not tested any of it.
| That was one of the reasons I attended John Adam's talk back in Jan
| 2006. This guy is an engineer and RC pilot
To be fair, if you get a job designing R/C equipment, people will take
you a lot more seriously if you actually do R/C. Even if he knew
nothing about R/C before getting hired, I'll bet his job/boss probably
*required* that he pick up the hobby if he wasn't already into it.
(It would be nice to be able to deduct R/C stuff on your taxes!)
| Exact title notwithstanding, after his talk and the flood of
| questions he answered from the very interested audience
Being first is neat like that!
| I came away with a quite positive impression of the Spektrum
| engineering teams work.
Futaba's team probably would have put on a similar performance, had
they been first. They could do it now, but people wouldn't be quite
so interested anymore -- now there's several R/C spread spectrum
vendors, lots of people are using it, ho-hum.
--
Doug McLaren, snipped-for-privacy@frenzied.us
`If one studies too zealously, one easily loses his pants.' -- Albert Einstein
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