DadsToysBG

My Feedback: (35)

Last Sunday at our field another flyer had my channel on FM. We took his radio to my plane. I turned my plane on which has PCM and then his radio. Nothing happened to my plane. Did this three times and walked around with the radio's. We then took my radio to his plane, turned both on and his plane froze up. PCM workes, now he has to be nice to me. Both were JR. This is why i fly PCM only. Dennis

dsnyder

That sounds like a rare occurrence, not going into hold or failsafe with another TX on on the same channel, but still a good point on the advantages of PCM over FM for interference rejection.

DadsToysBG

My Feedback: (35)

We have done this more then one time. The single conversion will act up part of the time but the duel has never acted up on the ground test. Dennis

dsnyder

What I'm saying is I wouldn't expect this to be the normal situation and I wouldn't expect to never have any types of problems with another TX transmitting on your frequency regardless of FM or PCM use. You do make an excellent point however on the advantages of PCM. Thanks for sharring that information.

BillS

I am confused by the answers and tend to believe the results may have been normal.

Consider the following hypothesis.

When the airplane was turned on the servos went to fail-safe because there was no signal. When the other foreign radio was turned on the servos remained at fail-safe because the signal was not valid.

Isn’t this what would be expected? The actions of the airplane do not indicate that it could have been flown with the foreign radio on.

Please clarify if I am wrong.

Bill

DadsToysBG

My Feedback: (35)

No my plane worked OK. With fail safe they would not haved moved. Dennis

BillS

That is extremely interesting if you were able to operate the servos reliability with a foreign radio on. It would also be interesting to know the results with the foreign radio set to PCM.

An in the air experiment would also be very informative. The experiment could be performed at altitude and not be dangerous to the airplane provided the foreign radio was turned off when requested. We have both been out of control before and landed safely.

All my JR equipment is PPM and my PCM equipment is Futaba but no one else is on my frequency. However I might be able to come up with a way to perform a similar test with Futaba gear.

Probably cannot talk my buddies with JR PCM gear into the experiment.

Bill

DadsToysBG

My Feedback: (35)

The first test we did on the ground was with JR and Futaba, both on the same channel. In each case we only saw very slight interference with each plane. As far as doing it in the air we can use your planes. These simple tests showed me that you have to be very unlucky to get shot down. This still happens, and at our field if you don't check the board you buy him a plane. Dennis

tadawson

All you are seeing here, guys, is the FM capture effect in action. IE, an FM receiver will lock onto the strongest signal present in most cases. When they are about the same strength, then things flip back and forth, and life can get interesting. As such, you werre most likely standing very close (closer that the other guy by a large margin) to your plane, so your signal overrode the other once you turned on. Once you took off, the distances would have been the same, and disaster would have struck . . . . . PPM or PCM, the signal is still FM and all rules of FM still determine what the receiver captires in the front end. How it deals with that signal once received differs radically from PPM to PCM.

- Tim

Jim_McIntyre

Not quite true.

The advantage PCM has over PPM is with signal to noise ratio. The PCM encoded signal can be accurately decifered against a greater background of interference. The modulation may be the same but, the encoding is providing an advantage.

DadsToysBG

My Feedback: (35)

I'm waiting to see the day when we fly 2.4. Dennis

BillS

Not the least bit concerned about using my planes so long as the turn on and off is when requested. Controlled experiments don’t need to be dangerous. Your sense of adventure disappeared quicker than a P38 with an engine out.

Bill

tadawson

\

This will cause PCM to give greater range, overall, than PPM, since the filtering is better. It in no way will affect the capture effect nature of FM, since capture happens at the RF level - coding in not yet in play.

- Tim

Jim_McIntyre

You're right of course.
But... I don't think the capture effect completely explains what is being observed here.
The receiver stage will randomly capture and re-capture both signals, the decoder section will make sense of the PCM frames that do get through on the good captures (provided they are received often enough to avoid failsafe) and continue to provide some modicum of control while the ppm unit glitches wildy trying to respond to everything received.
This is more an observation of practice than any actual measureable result as I've witnessed aircraft on PCM being unaffected by aircraft where the PCM transmitter was further from both aircraft (helicopters in the 2 instances I observed this effect, not that that matters).

tadawson

At equal ranges, then yes, I agree . . . . the PCM unit will not flail about from the PPM signal, and as long as enough frames get in to avoid lockout, then it will have some control. I was speaking to the on-ground test, and considering the distance to cause more than the 3-4 dB needed for a clean capture of one signal. Under similar circumstances, some of the PPM signal would still come in clean, but the violent thrashing that would occur due to the conflicting signal would most likely make any attempt at meaningful control of the model impossible.

- Tim

Jim_McIntyre

Ok, I comprehend, I agree.

I do think the "in air scenario" is more relevent though, as that is where I would want the control....

BillS

“in air testing”

A potential and suggested “in air” non-destructive testing program is outlined below.

A self-righting airplane with PCM should be used for initial testing. I have a 4* 60 that might be appropriate but have never checked the self-righting characteristics. Within the next few days the airframe will be checked to see if it will auto correct. If so the receiver will be changed to PCM. Throttle fail-safe will be set to approximately 20% throttle. The other surfaces will be set to fail-safe at neutral hands off flying position.

Initial tests.
1. On the ground determine the time to reacquire signal.
2. In the air turn the radio off and on to determine the time to reacquire signal.
3. Also determine the time for the human to reacquire control and fly straight and level.
4. Turn the radio off in the air and determine if the airplane will fly.
5. Turn the radio off in the air and determine how long the airplane will fly without control.
6. If the airplane successfully passed the above tests we have a test airplane.

More subjective tests:
1. Controlled turn on and off of an offending same frequency FM/PPM transmitter while the test airplane is in the air at altitude.
2. How soon did the hit occur?
3. How serious was the interference?
4. Is the interference continuous or intermittent?
5. Controlled turn on and off of an offending same frequency PCM transmitter while the test airplane is in the air at altitude
6. How soon did the hit occur?
7. How serious was the interference?
8. Is the interference continuous or intermittent?

The testing outline is incomplete and will change as facts are acquired. What other tests should be made?

Bill

tadawson

No argument whatsoever! The initial poster was referring to an on-ground test, hence that was my focus . . . .

- Tim

DadsToysBG

My Feedback: (35)

It was hot that day and we were sitting in the shade trying to stay cool. This test was not ment to anything except what I said it was. You can test this to death if you want too. I would'nt do any in air testing. This was just a fun thing to do. Dennis