rajul

My Feedback: (58)

Hi all, I would like to find out if PCM receivers are less susceptible to metal noise ? I just tried simulating some metal noise with two metal rods near my PCM receiver and antenna and seems like no effect. Thanks..........

Forgues Research

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PCM will mask problems but not filter it as you might want to beleive.
And when the problem gets a bit worse, that's when it will go into failsafe.
People are falsely led to believe that PCM is immune to everything. Not true. The best way to do it is to use a regular PPM FM receiver and make sure that everything is OK and then if you want to go to PCM, then go ahead.

mulligan

Roger,

I've seen this kind of statement made a couple of times, and I'm confused. My understanding is that the square wave pattern used in PCM makes the receiver less susceptible to interference, which is normall sinusoidal in nature. These interference patterns are dropped and only the correct pattern is accepted. IF some sort of interference does get through, then failsafe is activated.

The helicopter pilots I know swear by PCMs ability to reduce/eliminate hits- their copters, which also seem to be more interference prone with all the vibrating metal on board, are not going into failsafe, so it is presumed that the potential interference sources are being ignored.

Please explain more how you believe PCM does not help reduce interference.

- George

Gordon Mc

My Feedback: (11)

The point is simply that you should try to get rid of the source of the interference rather than hiding it. Your receiver can't possibly reduce interference, because the signals from your TX and from the interference source will mix regardless - all that your RX can do is accept or reject the resulting frame.

PCM won't "glitch" when a small number of corrupted frames occur, so you may think that you have solved your radio interference problems by using PCM, but you haven't. It's still there, and its just waiting to bite you as soon as the interference source manages to hit a sufficient number of consecutive frames.

Here's a simple, though not quite perfect, analogy: painkillers are great at getting rid of pain such as headaches, right ? But if you get headaches constantly, and consequently take painkillers constantly, you are managing to keep the pain at bay for a while but are ignoring all the signs that your body is trying to give you that there is something wrong with you. Now instead of taking the fact that you have constant headaches as a promt to go to the doctor and find out what the cause of the problem is and fixing it, you are electing to just hide the symptoms. You might be lucky and have this work out okay, but on the other hand you might be allowing some brain tumor to grow from the point where it had been treatable to the point where it kills you.

Same with interference - you can try hiding it, or you can fix it. The latter is highly recommended.

If you don't understand how PCM works, see also: http://www.rcuniverse.com/showthread...627#post229475

Gordonn

mulligan

Gordon, I understand and appreciate what you say... and what you say in your linked post. In addition to the good information you provide, can you answer my question regarding wave forms?

I thought that in addition to the encoding, frame counting, and failsafe process, there was a difference in wave form (i.e., sinusoidal for PPM, square for PCM). While everything you say I agree with, if what I thought I've read in the past is true, then what is interference on PPM is not necessarily interference on PCM strictly speaking (not because of the encoding technology, but rather because of the signal itself). In other words, if you took away the encoding/decoding process and frame analyization in a PCM system, you still would not see the hits you get in a PPM.

Others, take note: I AM NOT AN EXPERT ON PCM/PPM. I'm just trying to rationalize/correct my understanding here- don't take what I wrote above as fact... yet

- George

Forgues Research

My Feedback: (7)

Couldn't have said it any better, thanks..

HarryC

My Feedback: (1)

George, both PPM and PCM are effectively the same shape, just interpreted differently.

Both PPM and PCM are just square on/off signals. In PPM you code the position by the length of time of the on signal, in PCM you code by the series of on/offs representing a number in binary. A series of data for 6 servos is just a stream of square wave on/offs in either case, the the PPM will have 6 ons and the PCM will have lots of ons and offs. Both of these are then modulated onto the same standard FM sine wave in order to be transmitted by radio. So both PPM and PCM are square wave data modulated onto sine wave radio and both are equally hit by the same interference.

The method by which we handle the PCM data gives it its advantage, because it carries much more extra data with it than just servo position. PPM relies on the data stream starting with a synchronisation pulse and then it sends each servo's data in turn. Should the synchro pulse alone be corrupted the whole of the rest of the data is useless even if it is in perfect condition because the Rx does not know it is the start of a new sequence. If any one servo's data is corrupted beyond the time limits the Rx can recognise for a servo, then all following servos' data is also lost. But in PCM we send a marker at the start of each servo's data to identify it, so if any one servo's data is corrupted, the next servo's data still tells the Rx where it should be sent. So in PPM a tiny blip of interference can in the worst case ruin the entire set of data for all channels if it hits at the synchro pulse or at the first servo's data, in PCM the same tiny blip might just ruin as little as one servo's data.

Failsafe is no longer the preserve of PCM now that Multiplex make the programmable failsafe PPM Rx. But it still suffers the problem that the tiniest spot of corruption can ruin a whole data set, and so it can never be quite as good as PCM in that respect.

Harry

Phil Cole

The main difference between the PPM and PCM signals is that the PPM pulses vary in width, and PCM pulses are all the same width.

It's a lot easier to decide whether or not you have a pulse when you know when it should occur. PPM receiver have to decide exactly when a pulse occurs, not just whether it's there or not at fixed intervals.

This means that if a PCM receiver appears to be "working" when a PPM receiver would not, it probably is. By "working" I mean no detectable holds, not the apparent lack of servo jitter.

I mostly fly helicopters, and am always making some minor correction to the flight path, and I do feel the occasional hold, usually for a half-second or less. It happens less often than the jumps I see from PPM-equipped models. This carries over to my airplane flying as well, since I can't help fiddling with the sticks even if the plane is going where I want.

The Mulitplex IPD receivers do a lot more processing than PCM receivers to get their perofrmance. That's not a cost or weight disadvantage any more due to the march of progress.

I would use them if they made them for 50 MHz.

Volfy

My Feedback: (23)

I don't need to reiterate what has bee explain about the differences between PPM and PCM. However, the claim that square waves are more noise-immune than sinusoidal waves is a misconception.

A square wave, when expanded out in the form of a Fourier Series, consists of a sinusoidal wave at the fundamental frequency superimposed with a myriad of other sinusoidal waves at odd and even harmonic frequencies at various amplitudes. So in essense, a square wave is nothing more than a bunch of sinusoidal frequencies added together. As such, it affords no more noise immunity than a pure tone sinusoidal wave.

mulligan

Thanks for setting me straight, Harry.

Volfy, you lost me there, although I understand your thesis. We're getting off the thread topic, but somewhere else, I'd like to know more about "a square wave is nothing more than a bunch of sinusoidal frequencies added together." I am not a EE, so I'm almost afraid to ask I'll probably start another thread later, but thanks for the info.

- George


- George

Gordon Mc

My Feedback: (11)

Here's some simple graphical explanations:

http://axion.physics.ubc.ca/341-02/fourier/fourier.html

http://www.mit.tut.fi/research/ptplo...ierSeries.html

http://www.gmi.edu/~drussell/Demos/Fourier/Fourier.html

Gordon

Volfy

My Feedback: (23)

Thanks Gordon, for the URLs. Perfect example of a "movie" worth a thousand words.

To correct myself, I should say that the Fourier Series of a square wave consists of only odd harmonics in addition to the fundamental (as the 3rd URL clearly illustrates).

rajul

My Feedback: (58)

I would like to thank all of you for the great exchange of useful information on PCM..........