RE: Jet radio modulation
 

Not sure if I can properly convey my question, but here it goes:

What if an interference causes the signal to be "valid" but still not what the pilot intents. Like say, "simulating" applying full down elevator when plane is going straight and level. I mean the check for validity in PCM is a function of confirming that what was transmitted was received. Does IPD validity check means any signal that is within the transmitter possibilities, irrespective if that signal is what was transmitted?

Posting this question is close to my limits of understanding, be gentle:)

Thanks,
Edgar

RE: Jet radio modulation
 

Signetics (subsequently Philips) made a complete PPM RC system chipset for a number of years back in the 80's. The NE544 servo chip the NE5045 TX encoder and NE5044 decoder chip. The chip did exactly what was described above. It would only pass on signals between a preset "window" and discard the rest. It did not make any contribution except that the glitching was between 1 and 2 ms which helps the airframe nothing at all. Its been tried before on orbit radios of the sixties. Its old news. These conditions where you get a few good frames and a few bad frames simply does not happen in practice. Its close to being a on/off scenario.
I designed and build RX receivers and TX systems about 10 years ago using all of these scemes for use in high RF environments at long range . ie target drones for radar ACK ACK. I know what I am talking about. But dont believe me ask any communications engineer that works on actual data links.
Edgar, you have identified the problem exactly. Unless you have a history of good data you cannot eliminate a bad pulse within the "window" defined by simple timers, hardware or software. This scheme relies on the fact that the start of a pulse is valid. it may or may not be, completely negating the "error" detection.
And that is all there is to it. I am sure to dont want to see the mathematical model that proves this.
Andre Baird

RE: Jet radio modulation
 

after reading alot, i still don't get it. What is the differance between FM, PPM, PCM duel covertion etc.

Help

RE: Jet radio modulation
 

Interference will either lengthen or shorten the pulse. If it lengthens it, the pulse merges with the immediately following pulse making one double length pulse which is outside the limit reagrdless of whether the first pulse would have been inside the limit. To shorten it will require a wave that is 180 degrees out of phase or thereabouts, switched on at just the right time in the pulse and switched off at just the right time later in the pulse. It's possible but of very low probability. Remember that PCM is not 100% foolproof, it has some very low probability of reading errors as a correct signal. In both cases the probabilities are low enough not to worry about them. The reality is, at my airfield where we have two known areas of trouble where every model without PCM or PPM failsafe glitches as regular as clockwork, my model never glitches, occasionally it has gone into failsafe for less than a second and then come back in control.

H

RE: Jet radio modulation
 

Volkan is right. Allthough I'm very glad that you guys tune in with your knowledge, I think that most of us don't understand what you engineers are telling us. Try and keep it simple, guys. Thanks. ;)

There are also some things I don't understand. First you make a range test with a normal PPM/FM receiver. If you are sure, that there is no interference, you switch to a PCM receiver.:eek::eek: In my opinion, you could save yourself the extra money, and stay with the PPM receiver, What makes the PCM system better than PPM. If PCM and IPD are better, why do they still produce PPM receivers?

The fact is, that in my country, and in some of my neighbouring countries, jetplanes keep falling out of the sky with their PCM receivers locking in failsafe. I know of one guy in Germany who lost three jets last year, due to his radio going into failsafe. A few of us have always used PPM receivers, and never had any trouble with interference. How can that be?

You guys keep saying that you would never trust you xxxxx dollar jet to anything but PCM or IPD. Why? Because the more expensive receivers are supposed to be better, or what?

I want to use a good quality receiver, but I don't want to spend more money than I have to.

Also, let us know what your experiences are. Tell us some actual stories about incidents. In most cases, theory is one thing, and practice is another.

RE: Jet radio modulation
 

The position of the controls has to be turned into some sort of signal. The pots make a varying voltage, which is a signal but not very good for our purposes. We could convert it into morse code, but that is not a good code for our purposes. There are many types of data code we could use. We have settled on PPM which is analogue and PCM which is binary. Both are simply data codes, nothing to do with radio. We could send them from one place to another by drawing, by light flashing, by smoke signal, by sound, by electricity down a wire, etc. PCM can be transmitted by clapping your hands.

We want to send the PPM or PCM data by radio. There are two ways of putting data onto a radio wave, AM or FM. So there are 4 model radio systems to choose from - PPM AM, PPM FM, PCM AM and PCM FM. No-one makes a PCM AM as far as I know. A lot of people talk about FM versus PCM but as we have seen above, that is impossible, many people say FM when they should say PPM.

Dual conversion is a way of dealing with a problem created by the way receivers work. Receivers generate their own frequency a little off the frequency thay are receiving. The two frequencies mix and make many results, one of which is a much lower frequency of 455khz which still contains the data but the lower frequency is much easier to filter and amplify. This mixing is done once and is single conversion. The snag is that there are two incoming frequencies, yours and another, that will create a 455khz signal in the Rx, and there are various other multiples and fractions of frequencies that can also create these false images. So if the receiver does the mixing twice, first to 10.7Mhz and then down to
455khz it removes the false images just leaving your correct signal. Doing the frequency mixing twice is dual conversion. Note that it does nothing about interference on your proper frequency, what it does is block other specific frequencies from creating false 455khz signals in the Rx.

H

RE: Jet radio modulation
 

Checking with PPM then changing to PCM is an excellent strategy, if you don't have PPM failsafe. A plain PPM system will show any interference and give you a chance to change the radio installation, find the problem at your site and so on. PCM or PPM failsafe will mask many problems during the hold phase, you would only get warning when the interference lasts long enough to send the radio into full failsafe. What is a small problem on the ground will probably become more serious when you are in the air and at greater range so it is good to find problems and deal with them first. Once that is done you want a failsafe system so that when external interference occurs you don't get a massive glitch at 200mph, you don't get a model going straight into the pits at full power, and so on. Until recently that meant using PCM as it was the only system with failsafe.

Plain PPM Rx are still made because they are cheap and most modellers still make purchasing decisions based on low price and nothing else. Also many modellers just can't cope with technology, I have seen plenty try a a basic computer radio and give up and go back to their tatty old plain 4 channel radio.

If planes fall out of the sky in PCM failsafe the obvious answer is that the failsafe was doing its job. No modern PCM system should "lock up" in failsafe, as soon as the interference stops they return control to the pilot. This of course may be of no use if the jet is at 100 feet in a 200mph steep dive. Control is returned but the jet still crashes a fraction of a second later and the pilot probably never even realised he had got back control. Had the model been using a plain non-failsafe PPM it would likely have jerked around out of control rather than go in a straight line out of control. Jets tend to be fast so if it all goes wrong they are into the ground in a moment, so the interference can be brief yet fatal. This can give the illusion of a "lock-out".

H

RE: Jet radio modulation
 

Your summarry is quite correct.
Also the "lockup" of PCM failsafe is safer than no failsafe. Something that is a worry is the abovementioned masking effect of failsafe. Would it be to much trouble for R/C manufactures to just have a simple LED for us to look at when checking an installation or range indicating the data errors that come along BEFORE failsafe engages?
Andre Baird

RE: Jet radio modulation
 

There is another, slighlty overlooked, benefit of failsafe (whether it be PPM or PCM). Everybody talks about its protection against interference and 'returning control to the pilot'. These are obviously the best scenarios.

The standard setting of throttle to idle also has the benefit that it stops your model flying away (specially if you combine it with gear down and airbrake out). I am not suggesting it will allow you to get your pride and joy back, but in crowded areas like the UK it will stop the model from flying miles away ind injuring somebody. Although you don't want to lose your 10K jet I would much rather do that than have it kill an innocent person.

This can have its downside as mentioned earlier in the thread. A boy was killed in the UK when a model when into failsafe. I cannot remember the exact story but I think it involved copying a model memory which reset all functions to 'HOLD' and the throttle was not closed when interference was encountered.

Anybody who uses these things, please make sure you know exactly how your particular system works to prevent further tragedies.

RE: RE: Jet radio modulation
 

OK Stig, my experiences. Brand new Futaba 8UAP radio with PCM RX. One of the first flights the plane's controls go hard over in flight and crash, nothing left. Did not go into fail safe, just everything to extremes, came back for an instant, then back to extremes. Futaba could find nothing wrong with the receiver. A mystery. A few years later, Futaba 9Z with 9 channel Futaba receiver: original Isobar goes into fail safe and flies into the ground. No cause ever determined. By that point I had had it with brand F. I had some radio issues with JR, but that was years ago and was not exclusive to PCM. I always am meticulous with radio care, but nothing I can do about sloppy quality control at the manufacturers.

RE: Jet radio modulation
 

It is a sad irony that the only death in the UK caused by interference was a PCM model in its failsafe mode. There are a lot of related elements about the case, the main item was that the failsafe had not been programmed because the operator found it to be too difficult for him to program it, so by default it held everything at the last position and flew on at full throttle until it hit and killed an 11 year old boy who was standing in the pits. Had the failsafe been programmed, or had PPM been used, the model would still have crashed but in a different place, maybe safely, maybe not. Make a big note, failsafe can be faillethal if not used properly.

H

RE: RE: Jet radio modulation
 

Very interesting topic!

While reading this I realised something. I have a MPX failsafe PPM RX in a PicoJet, a small speed400 powered plane (don't remenber the exact type RX, I'm at work now). Until now I didn't know that it should not be possible to to have any glitching, it should "hold" when something interferes. When I switch off the TX is goes into failsafe like it should.

Thing is, it almost CONSTANTLY glitches when I turn on the motor! How is this possible??

Eelco

RE: RE: Jet radio modulation
 

I am NOT an FMA dealer, but a great fan of the product after owning FS5 and FS8 receivers. They work very well and as advertised. I am actually split in my preferences between PCM and PPM because of my positive experiences. I have taken the liberty of cutting and pasting a description of their Digital Signal Recognition technology from the website.

DSR stands for Digital Signature Recognition and is the latest technology for secure receiver decoding. Developed and implemented exclusively in FMA Flight System receiver products, DSR works with any standard FM transmitter, is affordable, and provides interference rejection far beyond any other method of decoding used in the R/C industry to date. On startup, Flight System receivers check for valid data. DSR locks on to your unique transmitter characteristics such as FM shift, channel count, pulse widths, and frame length. During flight, DSR compares the received data against stored DSR variables. If an anomoly is encoutered, DSR invokes 4 levels of error correction that can actually repair damaged data and re-construct frame information. This approach is only possible through the advancement of powerful microcomputers used in Flight System receivers. The method is faster and smarter than PCM or any other method for secure data decoding. In addition, all DSR based receivers from FMA support fully programmable failsafe on all channels. Failsafe is simple to setup and uses the controlling transmitter as an interface during the process. Enter failsafe setup mode by holding down the programming button as you apply receiver power. Move the transmitter channel you wish to program. Put the servo where you want it for failsafe condition and press the button again. That's all there is to it. You can read more about DSR technology by visiting the support section of the FMA Direct website and downloading the "FMA Flight System White Paper".

RE: RE: Jet radio modulation
 

Even an IPD 7 is relatively big to fit in a Pico Jet, are you quite sure it is an IPD and not a standard Multiplex receiver?

If it is an IPD then it's probably not radio interference down the aerial. I use a real cheap PPM Rx in my TwinJet without ever once having had a glitch, and the TwinJet is a worse environment than the PicoJet. The more likely causes for your problem are things like the ESC voltage to the Rx is dropping too much, or the ESC is feeding back some nasties into the Rx probably via the powerbus. When you switch on an IPD it sends the servos off to one side for a moment while its processor boots up. If your voltage is dropping seriously and going below some critical point the Rx might be switching off, the motor ESC does its failsafe of shutting down which brings back the voltage and the IPD fires up sending the servos off to one side for a moment. It could happen fast enough to look like an interference glitch.

RE: Jet radio modulation
 

Well i think we can see from the results of the poll which system people DONT trust!
1 vote for PPM .... ummm i bet that was Stigs!
No of fence Stig you have your views, and yes i remember our conversation, but since then i have had NO crashes due to lockout(only one before) i trust PCM, it works for me, and i would NEVER fly a jet with PPM, it would seem, is true for most jet pilots.
I don't know of any current, display pilots in the UK that fly PPM, and the truth is that if you go to a UK jet meet or public display, there are almost NO radio problems. I have flown when the sky has been full of planes, running 10kc spacing, (mostly Futaba). No problems!
Lat year we had a situation at the British Nationals, where a Jet on PCM reported 'holds' and landed. When he switched off, the monitor was checked and a solid signal was found on his frequency. It turned out to be some idiot that had brought a set of radio at a stand and 'tested' it in the car park! The key here is that the pilot was able to fly and control the model, land it safely without incident. I can tell you now, without fear, that if this had been standard PPM that model would have been in a bag! Or even worse, someone would be dead or in hospital. I can relate many stories over the years of similar situations.
One that sticks in my mind is a case at my local flying field where i was flying with another pilot, i was on Futaba, he was on Multiplex. He was on 76 i was on 77. I did a pass with 2 consecutive rolls, the model then did a 3rd on its own! Hold had kicked in, i throttled back and landed. when we checked, this guys tx was bleeding all over adjacent frequencies and Had to go back to the manufacturers.
I know , without PCM my plane would have been dead! These are just examples of problems i know about. How many times has PCM rejected crap, without me knowing?

I love it, and would fly nothing else, until a PROVEN better system is 'out there'.

RE: RE: Jet radio modulation
 

Gray, as I said: at least in my experience there already IS a PROVEN better system out here. The MPX 9 & 12 channel IPD receivers. They have already proven more reliable than any Futaba PCM stuff I have owned, with twice the range. To each his own, but I would not trust my jet to anything else (until something is PROVEN to be better).

RE: RE: Jet radio modulation
 

Woketman,

What do you mean by twice the range?

An antenna down range test comparison could just be showing that Multiplex transmitters leak more RF.

If you did full power range tests, did you use the same test range? Were the ambient RF conditions the same?

RE: RE: Jet radio modulation
 

No, I can not say definitively. The twice is a guesstimate. But it is based on full antenna range testing (ala Lee Demary). Unfortunately, it was not the same place. But if anything, it was a conservative test as the place that I tested the MPX (Lumberton, MS) has a crown in the runway. In other words, when I get to the far point in the test (never really have found the limit of the MPX system as there is a farmer's fence that limits how far I can go), I can no longer see the aircraft by direct line of sight (I am shooting the signal through the runway, so to speak). With the Futaba 9Z, it was direct line of sight. Both tests were with the plane up on a non-metallic table. Range was roughly double for the MPX and, as stated before, I never really found the limit. Ask Eddie how much range he could get on his Profi 4000!

RE: RE: Jet radio modulation
 

Because the RX has no way of knowing which is a valid pulse if the "valid" pulse is within the margins specified. Even if it does go outside there is no way to know if the previous "good" pulse was valid either, so the failsafe hold hold could be anywhere... It is beyond me that R/C mnf can ignore the many years of theory and practice on this subject. PCM is the only way to have a reliable failsafe. PERIOD.
Having said that it is quite possible to under bench conditions demonstrate to the uninformed that the two systems are equal, which is probably why there are a few believers.
If you had to make PPM more "failsafe" ( the bandwith issue is a good reason to do this) there are a few schemes that would do a better job than the what the IPD claims. I have had experience with Multiplex in years past and their RF design is nowhere near as good as any other brand. They may have improved since but at jet meets we have a few crashes locally. And the R/C systems were....
Andre Baird

RE: RE: Jet radio modulation
 

Hi Andre!

That is what we call a "fart in the phonebooth"!! I have heard from other old timers than things like this IPD have been around for a long time, but I never heard it described like you have.

No flames or bashing, I just have never understood even the CLAIM that IPD could perform the same as PCM.

RE: Jet radio modulation
 

I confess:D But there are also votes for PPM Dual conversion. So I'm not alone in trusting PPM modulation. Thanks guys.;)

I think this is a very positive thread, with some good reading and information. Although the vote is very much against my personal points of view, I am glad that some people have pointed out the benefits of the various modulations. As we can read from some of the posts, PPM and PCM failsafe is worth nothing, if it isn't set up properly. That is valuable information.

In my 22 years with RC modelling, I have owned all the major brands of radios. I have only experienced major problems with one brand. But that was many years ago. I think that todays manufacturers know their stuff, otherwise they wouldn't be on the market. As it is with turbine, U/C gear and other products, we all have our favorites. And thats fine too.;)

RE: RE: Jet radio modulation
 

Because an error pulse overruns into the following pulse so even if it would have been within limits the merged pulse isn't and is rejected. Since you showed us very clearly in a previous thread that you didn't have the slightest clue how the PPM error detection works and you were just making it up in order to be able to rubbish your own speculation, I can't give any credibility to your claims.

Woketman and I are not uninformed twits taken in by snake oil salesmen as you insinuate. I have used IPD since it came out and know from years of experience that it has never once failed to deliver.

H

RE: Jet radio modulation
 

Gray, you are still not comparing like with like. I could equally say that without PPM failsafe my plane would have been dead. We can make PCM radios without failsafe and then your PCM set would be like PPM without failsafe. Interference would cause it to go berserk. It's not PCM or PPM that makes the big difference, it is having failsafe. Until recently it was only PCM that had failsafe fitted, so a lot of people have confused PCM and failsafe as being the same thing. They are not, they are separate entities. Failsafe is now available on PPM and it is that system that you have to compare against PCM failsafe, not a PPM without failsafe.

H

RE: Jet radio modulation
 

Mick Reeves uses Multiplex.
Away from jets, top scale modellers like Duncan Hutson and Len Gardner use Multiplex, Duncan Hutson adores the PPM failsafe.

No-one bought PCM radios. What they paid for was a sytem that did not glitch and lose points in a competition flight, or could be failsafed for public demos. None of us really care two hoots if the radio is PPM or PCM or morse code or smoke signals, if the end result does what we want. What people bought was failsafe because that's what gives them the result they want. It just happened to be on PCM so that is now very widespread. Failsafe on PPM is very new so naturally you will see very little of it. That doesn't make it any less good than PCM failsafe.

H

RE: Jet radio modulation
 

This thread referenced Jet pilots, not others. Mick may fly PPM i do not know, but Mick is also regularly crashing Jets. I don't know that it is the radio to blame, but it does happen.

I still disagree with the statment that PCM would be the same as PPM if it had no failsafe. PCM does not pass errors to the servo's, this happens continually and before failsafe engages. So if it had no failsafe surely what would happen was that the last good frame would hold the servo's in the last good position. This is not the case with standard PPM, crap gets through to the servo's and cause the plane to 'go all over the place'.
If you leave a PCM rx turned on at the field without turning your TX on (to upload failsafe data), provided no one is on your frequency it will sit quiet. Do the same with a PPM, it will buss and flick about, just from picking up spurious signals, or splatter from other TX's. I can not comment on your 'new' system as i have not seen it in action but as i say, PCM works well for me!