pilott34

Ive personally seen a case at FL jets where an aircraft on the downwind and went into failsafe. The turbine shut down and pilot had no control. The receiver did what it was supposed to do and the aircraft continued to fly straight. The plane was headed for Tampa, but all of a sudden control was regained and the Kingcat glided all the way back and made the runway. ECU showed over a 2 second failsafe and everything was as advertised. I dont know that in this case if it were a PPM that the aircraft would have continued to fly straight because of the servos glitching. Cats have 9 lives...that one has 8 left.

Mike

erbroens

As a real field experiment, My P-80 powered Kangaroo with a PPM futaba normally has 0 hits each flight. Sometimes (every 6 flights or so) I got 1 hit of .2 or .1 seconds... and it never shows off in the plane´s reactions, (believe me, I fly this plane fast and close to the ground, so I would see it).
And all this with a club field full of fiery glow powered planes and helicopters trying to tune it´s engines on ground while I am flying, nor to talk about the r/c cars, the cell phones, the broadband over power lines, and sun flares!

mr_matt

My Feedback: (10)

That's amazing, evry PPM hit I have seen causes massive servo movement.

erbroens

Well I think that a 0.1 or 0.2 second glitch the plane wouldn´t have time to wobble at all, or if you are flying a PCM receiver, you wouldn´t notice that you didn´t have command on the plane for such a short period.... anyway I flew this same receiver in a F3A airplane airplane for a year and based in this confidence I installed it on the roo.

But although i am currently safely flying a PPM receiver on the roo, I would instead prefer to use a
PCM receiver on it, like I said before,because if it is properly setup, you can do something if you got a massive hit, and not because PCM reception is better than PPM ....

Gary Arthur

My Feedback: (29)

If this was the case, we would all be driving around with wheels made from stone. I am glad we now have rubber tires! Anyway, I think PCM is better. I had a ppm rec. in my spot, it worked well for a while. Then I got one of those small glitch's when I was landing at about 6 ft. Needless to say , I cracked the nose on my spot. I do not think a PCM would have done that.

rhklenke

My Feedback: (24)

Maybe I wasn't clear. When I said ECU, I ment commercially available turbine ECU, e.g., JetCat, Artes, AMT, etc. As far as I know, none of these ECU's monitor the incoming signals to make sure that they "make sense" and aren't experienceing glitching. Therefore, as long as the glitching PPM signal stays within the operating range, the turbine will continue to run whereas the PCM failsafe function can (and SHOULD) be used to signal the ECU to shut the turbine down.

Bob

dwayne_doberman

Gents,

For me, the issue is'nt to do with PPM versus PCM. I have once experienced complete lockout: i.e. a situation where a perfectly working system suddenly appears to die and all further stick inputs are completely ignored. Only by switching the Tx off then back on brought things back to normal. Luckily this happened on the ground but I (stupidly!) continued with the day's flying - but happily with no further occurences. Having read about similar instances in this forum, I now know "lockout" can happen with some regularity.

My setup was a PCM Futaba 149DP Rx and T9CP Tx. I've no hard evidence to suggest that any other system is better or worse, so I'm not going to switch on the basis of one occurence.

PCM is definitely far less susceptible to interference - I, like many others on this forum no doubt, have first-hand experience of the improvements in control stability that PCM can offer. However, PCM is more complex and with increased complexity there is greater scope for errors/shortfalls in the design. There may be a case where the Tx or RX does not cater for an unforseen or unusual combination of circumstances or events. Thus when this situation occurs, the result could be a "lockout" condition where the electronics/software enters into an unkown and unrecoverable mode.

For now, I'll continue with Futaba PCM - until I too have lost an expensive jet due to unknown causes!!

Bob

erbroens

I humbly request to anyone in this forum evidence or a scientific explanation for this statement.

Best regards, Enrique

Gordon Mc

My Feedback: (11)

Sounds like you just have a defective TX ... and that can happen with any TX brand and any modulation / encoding system.

Best example I've seen of that is a club member who was using an old Futaba PPM TX. A few times he got himself in a situation where he was a bit "over his head", and then he crashed and blamed the radio. Of course everyone refused to believe him because he only crashed after getting himself confused / worried, and the radio always worked after the crash. Well, turns out he was actually right. We found out that when the radio was "squeezed" slightly it stopped transmitting and the LCD (showing the TX voltage etc) went blank; when pressure on the case was removed, it started working again. (The reason he crashed when he got "worked up" was that he was subconciously gripping the TX a bit harder.)

Gordon

rhklenke

My Feedback: (24)

That is at the same time quite easy, and quite difficult to do. Easy if you are an electrical engineer with a background in communications systems, and difficult if you are trying to explain it to someone who is not. It simply comes down to the fact that in PCM reception, you are trying to determine the presence or absence of a pulse (a binary digit), whereas in PPM, you are trying to find the exact edges of a pulse. Someone who is well versed in digital communications (its not my field, I'm a computer engineer, not a communications type) can show that the signal-to-noise ratio in PCM transmission is much higher than in PPM transmission which translates to much lower susceptability to interference.

Bob

erbroens

Well I am a computer programmer, and I studied electrical engineering.. althought I never worked in telecommunications or never got much into it, I never found a satisfactory explanation about how a digital converter could be smarter to skip "errant edges" (PCM) than a well designed analogic filter receiver (PPM) ....

This is the kind of question that IMO has a "grey area" or not only just 1 and 0´s... and so unless someone has a solid explanation of it and real field experimentation data , statements like "PCM is better" are simply invalid.

All I could said is the empirical testing that I have done myself in the last 25 years. flying jets, giant scale aircraft with gasoline engines and helicopters with PCM and PPM with the same grade of success on the quality of the radio link... I only prefer the PCM receivers because of the factory build feature of the failsafe. If the manufacturer would provide this feature in a PPM system, based in my experiences, I would buy it without bothering if it is PCM or PPM...

Best regards, Enrique

Yann

Enrique,

To make it simple, PCM means Pulse Code Modulation.
That means that a proprietary code is encapsulated in the frame sent by your TX.
The receiver reads only frames with this code. Whatever is received without this 'password'
is trashed. This way you filter better the signal and you increase the reception range as well.

Yann

SJN

Damn radios [:@]

http://www.rebelflyingclub.com/photo...7/kangaroo.avi

mr_matt

My Feedback: (10)

Wow tough crash, but what was that cool song?

rhklenke

My Feedback: (24)

Actually, its not invalid. If you take a class in digital communications and can wade through the math, a communications engineer can prove the higher signa-to-noise ratio to you in a closed form solution. It has to do with the effect of the noise on the quality of the pulses and the statistical probability that the noise will raise (or lower) the pulse past the detection threshold vs. the probability that it will corrupt the pulse enough to change the location of the edge...

Well my empirical testing shows different results, but that's the way of the world... If you want PPM with failsafe, why don't you go to the Multiplex IPD stuff?

Bob



[/quote]

erbroens

Yann: thanks for the explanation of what PCM means...

Rhlenke: the grey area is how the current hardware processes this statistical probability.. pretty much like a pixelized edge of a blurred digital pic... as you know not all digital cameras have the same image quality, althought they share the basic digitalizing algorithm. They way how this data
is processed can often lead to errors equal or greater than a non standarized data filter.(sorry if
I didn´t used the correct english technical terms).

About why I didn´t use Multiplex stuff is because it is not available as the Futaba... and anyway,
why change for a brand that worked OK for me so many years? I had in my bench a PPM never crashed receiver that I flew a lot in a plane and I know that is 100% reliable. Instead of spending U$ 300 (yes, that´s the price of it here!) in another PCM receiver, I saved this money helping myself buying more and better acessories for my jets.

Best regards, Enrique

rhklenke

My Feedback: (24)

Your analogy doesn't really fit at all - either you didn't translate it correctly or your concept of how PCM coding works is not correct. The problem doesn't come in where the signal is digitized and where it is returned to analog, its in the communications channel and digital communications has a greater signal-to-noise ratio - period.

Anyway, to each his own...

Bob

erbroens

Yes you are right . it really not fits at all, I just tried to explain it a way that most people who are not
information technology professionals or engineers could grasp the basic concept of the problem, nor
to say that I am typing in a language that I learned mostly reading old model airplane magazines...

and yes, to each is own. in this little planet there is no absolute truths or periods,, we are blessed to see and do things in infinite different ways. (of course some ways are more elegant and efficient than others)

Enrique

SJN

Enrique, which futaba PPM rx do you use ? the 138 DF ?

mikehannah

Hi
Okay talking as a profesional in the electronics industry. The reason that PCM is better than PPM can be explained with this simple analogy.
You are in a crowded room with the a very loud band playing in the background. You are trying to hold a conversation with someone.
Is it easier to follow a conversation that uses all different kinds of words ( eg analog)
or simple yes or no ( digital)

Obviously the simple yes or no win out, as even if you misinterpret one yes/no answer it will have a very small effect on the overall message. Where as get an entire word wrong in using full word example it will change the meaning of the entire sentence.

If you want bored by the maths I can dig them out.

As to the debate if PPM is better than PCM is in my opinion a no brainer. This belief that PPM will give you warning of a poor signal is frankly nuts. Unless it is very light and very short lived interference then you can kiss goodbye to your jet ten times over before a good well maintained PCM reciever decides to go into failsafe ( signal to noise ration several orders of magnitude higher than a PPM receiver could ever hope to cope with).
I have flown PCM in helicopters and planes ( off all varieties) for several years with out any problems. To me use PCM there is no substitute.

Mike

currently building a sand bag wall for cover.

Woketman

My Feedback: (6)

Mike, you are flat wrong. The stuff you said may apply to reular, un-post-processed PPM systems, but DEFINITELY NOT a system like the MPX IPD. IPD has fail safe protect just like PCM and it works very well. Harry C can tell us all the smallest of tech details, but he has not yet chimed in here. In fact much of what has been said here by the PCM proponents about PPM only applies to non-post-processed PPM.

I have flown Futaba and JR PCM. In my experience, the MPX IPD has been light years ahead, for me. I don't know if that is due to the IPD or the legendary high output of the MPX Profi TX. But frankly, it don't matter cause it has REALLY worked well!

Ebroens, MPX makes the IPD RXs in + & - shift, so they should work with any PPM transmission.

mikehannah

Hi
I will respect your opinion and I know nothing about the Multiplex system however simple truth. if you can't get the signal through in the first place then you can post process to your hearts content it wont make a wit of difference.

Mike

Woketman

My Feedback: (6)

Harry C, we need you!

erbroens

Wocketman: this feature is cool, + and - shift options allow the use of a Futaba or JR tx without restrictions. and like you said many experienced modelers around are happy using Inteligent Pulse
Decoding instead of PCM, the only problem (at least, for me) that MPX gear is not as easy to find as the most popular brands of radios around...

Mike & rhklenke: perhaps we could be more productive discussing how we could create a way to
test a PPM and a PCM receiver side by side, comparing the data and find some conclusions, instead
of debating theory... perhaps a low power radio signal emissor or something like this.

Best regards, Enrique

specialFX

That´s where the diversity system gets hands on. It works with 2 independent RX´s with perpendicular antennas. These two signals are set into relation to each other and faults (if any) are tryed to correct by the unfaulted signal. For best diversity received on a different band, with two different frequencies. All done with PPM. And if signal integrity gets lost, programmable failsafe comes into action.

Thomas