Independent tests prove lack of frequency hopping with XPS

[XJet]

	Quote: Originally Posted by zoomer260
	Xjet, I'm NOT a basher on this thread I am following with accute interest. But I am wondering why you said a particular system needs failsafe for one airborne craft but not another ? They both can be lethal. Is it because meet rules require it on one but not the other or is it a personal requirement ?

Well it's just a personal preference but I would not feel at all comfortable using any RC system that didn't have failsafe on a model larger than .40-size.

Imagine the mayhem that could ensue if you're taxiing even a 50cc model and got hit by interference -- even a 22x8 spinning at 7K RPMs will make mincemeat of anyone who gets in its way.

All my larger planes have PCM receivers, I keep FM/PPM sets for the smaller "somwhat less" dangerous ones ;-)

Does anyone fly 50cc+ without failsafe these days? Why would you?

[XJet]

	Quote: Originally Posted by ChrisHays
	xjet.. please include a pic of the tx!..thanks

Here's a picture of the transmitter, 2.4GHz receiver (with satellite) and 2.4GHz module.

Looks suspiciously like a JR module but it won't work without a small mod in my 9X II (9303).

[zoomer260]

	Quote: Originally Posted by XJet
	Well it's just a personal preference but I would not feel at all comfortable using any RC system that didn't have failsafe on a model larger than .40-size. Imagine the mayhem that could ensue if you're taxiing even a 50cc model and got hit by interference -- even a 22x8 spinning at 7K RPMs will make mincemeat of anyone who gets in its way. All my larger planes have PCM receivers, I keep FM/PPM sets for the smaller "somwhat less" dangerous ones ;-) Does anyone fly 50cc+ without failsafe these days? Why would you?

No particular reason to not use it I've just seen severe rush to the hospital damage from a .40 size airplane. My current radio is both but all I have is PPM receivers. The one PCM receiver I've used (I got with a used radio system) cost me a plane to lockout even though I had it checked out before and after the incident. As a result I'm not sure about it's benifit. I've had "hits" and with PPM you know it, with PCM you can get hit and maybe not ?
But to answer your question is I have no PCM receivers and have plenty PPM.

[zoomer260]

And to show that I'm not trying to start anything, I've never used two receivers either. But just for the heck of it I'm gonna do it in this plane I'm building now just because I have them and thought why not ? No reason to do it other than it's something I haven't tried and I'm thinking maybe two antennas are better than one ?

[DesertHucker]

	Quote: Originally Posted by zoomer260
	No particular reason to not use it I've just seen severe rush to the hospital damage from a .40 size airplane. My current radio is both but all I have is PPM receivers. The one PCM receiver I've used (I got with a used radio system) cost me a plane to lockout even though I had it checked out before and after the incident. As a result I'm not sure about it's benifit. I've had "hits" and with PPM you know it, with PCM you can get hit and maybe not ? But to answer your question is I have no PCM receivers and have plenty PPM.

In Australia you can not fly IMAC without fail-safe at least on the throttle.

So a system without fail-safe could not be used for IMAC. I'm not sure about other disciplines.

[sweetpea]

There are cheap after market failsafe device that can be put inline with the throttle servo.

So this system could still be used, not that I would use it.

[DesertHucker]

	Quote: Originally Posted by sweetpea
	There are cheap after market failsafe device that can be put inline with the throttle servo. So this system could still be used, not that I would use it.

Did not think of that

[XJet]

These devices are ony useful if the receiver stops outputting pulses when the signal is lost or interference is received -- a lot of the DSP-based PPM receivers just go into a default "last position hold" for quite a while before they stop outputting pulses.

I'll check this cheap Chinese 2.4GHz set and see what it does.

[KrisW]

	Quote: Originally Posted by XJet
	These devices are ony useful if the receiver stops outputting pulses when the signal is lost or interference is received -- a lot of the DSP-based PPM receivers just go into a default "last position hold" for quite a while before they stop outputting pulses. I'll check this cheap Chinese 2.4GHz set and see what it does.

Any idea how many frequencies the Chinese items can use? It would be nice to know if they can switch at all.

[RH1N0]

	Quote: Originally Posted by XJet
	These devices are ony useful if the receiver stops outputting pulses when the signal is lost or interference is received -- a lot of the DSP-based PPM receivers just go into a default "last position hold" for quite a while before they stop outputting pulses. I'll check this cheap Chinese 2.4GHz set and see what it does.

I've noticed that JR PCM receivers change frame rate when they are in fail safe. It's only a slight change and not enough to upset the servos but it is a useful way to detect that the receiver is in failsafe when the servo pulse duration is set to hold.

I'm interested to know if any of the 2.4 systems do this as well. Worth a look when you test the system...

[3DDevil]

XJET... when you tested the IMAX 9x system, did you notice that the plane was way more sensitive than when flown on PPM or PCM? I found this when I first flew my plane on XPS. I really liked the extra responsiveness...They recon its because with 2.4Ghz you get a lot more signals/sec to the servos?

That was until my XPS locked out and the plane crashed!!....

[XJet]

	Quote: Originally Posted by 3DDevil
	XJET... when you tested the IMAX 9x system, did you notice that the plane was way more sensitive than when flown on PPM or PCM? I found this when I first flew my plane on XPS. I really liked the extra responsiveness...They recon its because with 2.4Ghz you get a lot more signals/sec to the servos?

I suspect that, much as folks claim "but it's true", this talk of "locked in" feeling and extra responsiveness is more placebo effect than anything.

Why?

Well no matter how fast your transmitter and receiver communicate, the fastest your servos can accept data is once every 20mS (50 times a second). That means there's an average latency of 10mS between when you move a stick and the servo becomes aware of that move.

Since a module-based 2.4GHz system can't send the data any faster than it's being created by the transmitter, it can't produce a faster response-time than the old "dumb" FM/PPM system.

Any module-based 2.4GHz system is actually going to introduce *extra* latency because it has to process each frame of data (1.5mS x num of channels) and convert it into a set of digital values, then send it to the receiver. Once the receiver gets that data, it then has to convert the digital values into a pulse for each servo.

The *fastest* and most responsive of all radio systems is actually the good old FM/PPM setup with a "dumb" (ie: no DSP or other computer stuff) receiver. With these old systems, the average delay between when a stick is moved and the servo receives the new position data is still still 10mS but the signal is passed on to the servo immediately, without the additional latency of processing or frame-reconstruction. In fact, the delay between when a pulse enters the transmitter module and when the same change appears at the servo lead on an old FM/PPM system is measured in *microseconds* rather than the milliseconds we associate with PCM, DSP or module-based 2.4GHz systems.

In fact I noticed a definite reduction in latency when switching from a Berg 5/II in my Katana profile to a plain old "dumb" FM/PPM receiver -- because the Bergs (like most DSP receivers) actually tend to buffer a whole frame before they output it. The difference *was* noticeable when flying. I've noticed no such difference when comparing my JR 9XII (9303) PCM to either the XPS or this cheap Chinese 2.4GHz gear.

The latency of an old FM/PPM system is also a whole lot better (a frame less delay) than a PCM system too -- but I don't recall folks saying that their FM gear felt more responsive or "locked in" than their PCM systems.

And let's not forget the weakest link of all -- the mechanical parts of the servos. Most servos are rated somewhere between 100mS and 250mS transit-time. This makes a delay (or improvement) of a few mS in latency elsewhere in the system seem rather insignificant.

If people are finding their 2.4GHz module-based systems feel more responsive then just what were they using before?