Which modulation is better, why?, why not?

Jets are generally quite heavy. In the UK any models over a certain weight require a failsafe to be legal. PCM has this built in. PCM is generally more resistant to intereference as is dual conversion.

Only slight downside is that interference which causes PPM to be 'twitchy' will possibly cause PCM to failsafe.

IPD. The reason I feel it is best is that hopefully you can see some PPM-like glitching when you are getting into trouble, before the sh_t hits the fan. That would let you know you have an issue and let you land ASAP. But when the signal really does go to h_ll, then you still have PCM-like failsafe. With the unbelievable range of the MPX Profi 4000 and the MPX 12 channel IPD receiver, I have never had a range issue (that I did not cause myself) so I don't have first hand experience with failsafe. The one time that I did something WAY stupid and brought my on-board 5 cell voltage too far down during flight, I had some glitching. But I never went into failsafe, just a glitch or two. This let me look for a problem, I found it and all was fine. No failsafe happened.

PCM every time!
Most interference in momentary, PCM will ignor it, PPM will glitch, which may rip your wings of you 200mph model, or even worse flip it into spectators.
You can still fly PCM even with a lot of interferance, providing some frames get through, you have control. The control may be slugish, this is the warning. PPM in that situation would have the plane leaping about all over the place in god knows what direction, and as i have said, may rip it apart.
When total signal is lost, i program mine to throttle back and hold, if you are lucky the interference will stop and you have control again, if not, the plane stays on the same tragectory but slows. PPM in this situation, well it is anyones guese as to where it hits.

I have flown Futaba PCM for 10 years and can honestly say i have only had 4 or 5 lockouts. In all but 1 of these cases i have had signal return and safley landed the plane.

i would not trust a Jet to anything else!

Stig,

I agree with Gray, dont trust my jets to anything but PCM for all the reasons stated. Another plus to PCM is the additional DB gain over ppm. Of course, you do have to take the time to properly setup PCM to benifit from it .....
See you soon my friend...
Todd

What you have all quoted as PCM's advantage is not PCM but the advantage of failsafe. The same error detection, glitch avoidance and user programmed failsafe settings can be had on PPM as well. FMA is claiming higher noise suppression for its PPM DSR than PCM. So, now that the playing field has been levelled as far as glitching and failsafe is concerned, what's to choose between the more expensive, larger, heavier, slower, brand specific PCM or the cheaper, smaller, lighter, faster, works with any brand PPM?

H

Looking forward to it. The Roo is ready to go with my SimJet 1200.

I hear you guys. I know the PCM modulation is supposed to be better. I'm not trying to say it isn't. Still, when I look at the statistics in my home country about crashes due to radio lockout, I see that ALL of the models that crashed, were using PCM modulation. I have been using a standard Futaba F118 PPM receiver allways, and I never had any trouble with. When I talk to people about, what receiver they should use, I tell them about my experiences, and they go on buying a standard receiver, and never have any trouble with it. How can this be? Is it because PCM users hide some of their potential system errors, by using PCM, or what?. Theese errors would show up at once, when you use PPM, I think.

Paul: I know we have descussed this a few years ago. You told me the same theori, that you are telling me now. But, it is only theory. I'd rather talk about the statistcs. Things we know. What has happened in the past, and why. Would it have been any diferent, if another type of modulation had been used?

The previous comments about PCM are due to the modulation and not the failsafe. As guessed from its title, PCM is a code. If interference occurs the rx knows because it does not fit in with the code. The model therefore tends not to glitch. This is nothing to do with failsafe. Failsafe will kick in after a specified period of not receiving the correct code and is another, entirely separate, benefit.

I would be interested to know how you can get the same glitch avoidance on PPM. How can the Rx tell what is interference and what is a command from the TX?

Failsafes can be fitted to PPM receivers but you still don't get the added 'glitch' protection.

The comments about PCM are due to the failsafe and not to PCM. PCM code can be sent without checksums and the Rx simply convert the binary code back into PPM for the servos, including all the errors in the binary code. Even by adding a checksum and detecting an error does nothing, since it could be programmed to detect the existence of an error but still pass it on to the servos anyway. PCM can be made without error detection and without failsafe and it will glitch and crash the same as PPM. A few years ago I was at a demo by Philips about CD technology and the difference between digital and analogue. They demonstrated a CD player in which they could turn off the error detection and error correction circuits and let you hear raw digital music. It was worse than a scratched vinyl record, the digital data on a CD is very poor and if passed on to the speakers without the "failsafe" it shows all the "glitching" of analogue. It is only useable when the CD's version of failsafe is switched on.

It is the failsafe mechanism, not PCM, that does the brief hold that avoids glitches, and then goes into programmed positions. For this both PCM and PPM failsafe are exactly the same, the only difference is their methods of detecting errors. Failsafe uses two sets of memory. In one set it stores the pre-programmed settings for full failsafe, and in the other it stores the last good data packet which is constantly being erased and replaced with the next good data packet. When the error is detected the faulty data is discarded and the last good packet is taken from memory and sent to the servos instead. This avoids momentary glitches. After some time if the incoming data is still corrupted the data from the pre-programmed memory is sent to the servos instead. This sytem is absolutely common to both PCM and PPM failsafes. It is totally independent of the data modulation type, only the error detection mechanism is different between PPM and PCM.

I use exclusively PPM failsafe system. It has all the same features as PCM with failsafe. It does not glitch and if the corruption lasts 1/2 second it goes into the programmed failsafe positions. The PPM failsafe Rx is lighter, smaller and cheaper than PCM, it gives full status to 12 channels at a frame rate that PCM systems do not yet match, and if I should ever change my brand of Tx it will work just as well with another brand, I am not locked into one system. It also gives me the higher resolution of PPM, I have demonstrated my PPM system resolving to greater than 1400 places, though in practice given the fuzziness of servo pots and gear slop this is somewhat academic.

FMA has gone even further. The system I use looks for errors in the servo data. FMA has developed a system that also looks for errors in all the data that is not servo position data, such as numbers of channels in the frame, length of synchro pulse and so on, all the little bits that create a fingerprint of your individual Tx that distinguishes it from other Tx even on your frequency. They claim a very secure link between Tx and Rx because of this.

H

You'll have to excuse me as I'm not an electronics guru.

With PCM will any intereference cause errors? I thought that you would have needed a very similar code input to 'confuse' it, or a loss of transmitter signal?

Given the same RF section and demodulation scheme of raw data there is actually a very small window of RF interference where it makes any difference at all whether you have PCM or PPM. i.e a poor signal will have to be very consistent at some level for PCM to give you better control. In practice this does not happen. PCM has a benefit over PPM but it is NOT a significant improvement in dataling security. However PCM under transient glitching is safer because the aircraft can be made to behave more predictably during those instances and is therefore the preferred scheme for jets. Another important factor is that the demodulation of PCM is more resistant to power bus noise from multiple digital servos. The analog nature of PPM demultiplexing is often very prone to power glithes. These tend making the servo to glith even more and you have an unstable system. Under these conditions the RF section of the RX also suffers from this noise. So it kills itself. THIS IS THE MOST IMPORTANT BENEFIT OF PCM...
I am busy with an R/C system that is not subject to general interference at all. The technology is not new but has never been taken up by R/C mnfs due to cost. However the cost has reduced significantly over the last few years making it viable to have a duplex link with 100 times the bandwith of PCM. It is FCC approved and requires no frequency control or peg system... Its not science fiction, we are using it on UAV,s as the default choice.
Imagine as many channels as you can put servos in a plane, feedback of inflight data, airspeed, altitude, battery and engine status.
All for less than $2000
Andre Baird

But I have that same benefit with PPM failsafe, so again this is not a benefit of PCM. It is a benefit of failsafe, regardless of whether you use PPM or PCM.

The system I use detects errors, discards them, sends last good information to the servo (glitch avoidance) and if the errors continue it sends my pre-programmed instructions to the servos. That is not a description of PCM. That is a failsafe system and it is as valid on the PPM system I use as it is on a PCM system.

Too many of you are confusing PCM and failsafe as being the same thing, and thinking that failsafe is exclusive to PCM.

PCM with the error detection and failsafe memory switched off will suffer the same glitches and unpredictable flight pattern from external interference that a PPM without failsafe will do. It is failsafe that makes the difference and it can be built in to both PCM and PPM.

H

Wow.. that didn't sound like an FMA ad much. Just because a manufacturer claims some new thing doesn't automatically make it true. People are funny.. they tend to trust things that are proven to work on their investments ($6000+ jets) over someone standing on a soapbox claiming his 'snake oil' is the best.

FMA as a brand is of no interest to me since it does not make radios on the frequency band I use. I merely point out that there are makers of PPM making the same claims as the makers of PCM claim for their equipment.

H

I would certainly not suggest that you switch from your current PCM to a PPM failsafe. However you should be aware that the benefits you believe that you get from PCM are also equally available on PPM though it is only from a limited number of brands at the moment, as this could affect a future purchasing decision. This ties in with another thread that is current at the moment, jet fliers want lots of channels, there are two brands that give you four different 12 channel radios. Both brands give 12 channels only in PPM, but only one of them offers that PPM with the full glitch protection and failsafe that PCM offers. If you did not know any better then you would assume that both brands do not have failsafe if you believed that failsafe is exclusively a PCM behaviour.

H

Harry, the only confusion here is your belief that a failsafe scheme based on PPM and PCM will behave the same way. PCM is binary in nature and error detection can be made within a SINGLE frame (bitsteam of data) to do the same with PPM you will need several frames of data before you can make any assumption of errors. Also a valid PCM frame can be recovered IMMEDIATELY wheras a PPM Signal would have to be consistent for many frames before you can make that assumption. On top of that errors in PPM are timing dependent which can be introduced due to the data even within the RX. This is NOT the case with PCM which is why it is used in almost all other forms of communications that requires low data rate errors. It the Mars landers used PPM they would never had made the news...
Sure you can have fancy software filtering which appears to behave the same in tests but under dynamic conditions it WILL be worse. The reason mnfs are employing these scemes is taht under ideal conditions you can have more resolution and more datachannels that with PCM for the same bandwith required for narrow band systems.
Andre Baird

BMT
I could not have put it better myself, PCM every time.
There may be special PPM sets that 'apear' to offer what PCM offers but we are talking about normal field flying with the most prevalent brands (Futaba and JR).
What is available is PPM and PCM, what is best is PCM.

No mater what the theory, try a simple test. Get 2 sets of radio on the same frequency (on the ground!) One PCM one PPM.

Switch them both on with both ariels down. The PPM set will be all over the place, you will still have control over the PCM. It will 'frame' due to lost data frames but it will follow your actions.

To most modelers, that is what they want, they don't care about the electroncs or software.

I have even proved this with a slow sports model in the air, both tx's next to each other. I was still able to fly the model.

For me, untill somone demonstrates a system with GREAT avantages over this, i will stick with my good old PCM.

Some interesting views, mine is a compromise.

I start out with ppm on a new jet model. With all the metal against metal, pumps solenoids, long leads and ecu's the potential sources of interference are far greater than in any other type of model. I would rather know I have a problem to solve, rather than use technology i.e pcm to be more tolerent to any problems that may exist.

Once the model has been proven on the ground at various distances/revs and is shown to failthfully follow my commands without glitching, will I put in a pcm receiver and fly it.

If you go straight to pcm and interference(noise) does exist, you might be at an attitude/distance where failsafe does not protect you or your model once the noise exceeds the signal.

I agree with this in principle, this is one of the things we had considered doing.
I think you have hit the nail on the head with regard to turbine models. if you consider what you actually have, it is more like an electric model!
ECU (spead controller but more complex) pump (motor) high power nicads, valves, heavy cables etc.. all add up to a lot more electrical noise. More care must be taken on the instalation to isolate these items, keep power cables and data cables apart.
The other thing i have just recently had experience with is carbon fiber causing problems.
My L39 had many uneventfull flights under its belt, but after a fuel spilage the foam in the composite on the bottom of the fuselage nose became soft and week. I removed it and replaced it with carbom fibre strands running front to back on the base of the fuselage.

Flew the model and had 2 'holds' lasting about half a second or so, not long enough to engage failsafe.
Landed and range checked, my range was reduced considerably. Fully checked model, changed receiver, xtal, checked tx with another plane.
All seemed well but still had bad range. All i had done was put carbon strips in.
I ripped the strips out and replaced them with balsa section and thin glass cloth.
All of my range returned! Have flown uneventfully ever since.

The carbon i had put in laid under the output of the rx and ran forward about 1 inch away from and parallel to the ariel.
I beleive it was picking up output from the rx and acting as a tx ariel, feeding it back into the rx ariel.
I have repeated the condition by layng strands in the same place, and guese what? no range.
I believe that most turbine crashes that are atributed to lock out are as a result of instalation problems.
So it is a good use of a PPM rx to check out your instalation before you fit your PCM to fly.

Also we have much more carbon in models these days, from my recent experience i would warn against carbon being anywhere near power cables or data cables where there is a clear electrical path back to the ariel.

Totally wrong. The PPM error detection see an error immediately, it does not require subsequent frames. Similarly, it comes out of failsafe on the very first good frame. You clearly think the error detection is based on comparing data from one frame to the next, which is not correct.

H

I'm an aero engineer, not an electrical engineer. All I know is that I have owned PPM & PCM systems, in years past, from both JR and Futaba. The MPX Profi has simply blown away all of those systems for range, period. Also, it has been more reliable (at least more so than the Futaba stuff). Not to mention far far more capable. Perhaps I am just lucky now or unlucky back then!

Doug was right, I tend to trust what I know works and the MPX IPD technology works VERY well! (far better than I could say for the two Futaba PCM receiver failures or lock-outs that cost me two planes)

Could you please explain how the PPM failsafe works?

Yeah I would like to hear it again with Andre listening in.

The following is a quote from the Multiplex literature. Note that the error detection is done on each individual servo pulse and has nothing to do with needing subsequent frames to compare.

"A micro processor in the IPD receiver detects the control signals from the transmitter, checks them for validity, and then passes on the approved signals to the servos. To the IPD receiver "valid' means signals whose pulse width lies within the range 890 msec to 2350 msec. These limits are outside the normal range of most radio control transmitters, even those not made by MULTIPLEX.
If invalid signals are picked up, the receiver blocks them, and passes on to the servos the last received valid signals. It continues to do this until it picks up "good' signals again, or until the time limit of 0.5 seconds has elapsed. This behaviour corresponds to the HOLD function familiar to any PCM user.
If the IPD receiver picks up no valid signals for a period longer than 0.5 seconds, it can pass a programmable safety position (fail-safe position) to the servos."

Sounds remarkably similar to PCM!