SnappyRolls

will someone let me know what good the pcm mode does other than allow the fail safe options....is there a better reception or less chance of getting radio interference with the pcm or is it just simply the fail safe options that you are getting....if i understand it right it is still fm signal with an additional pcm signal to take over once signal is lost....

Thanks

transmission_dr

My Feedback: (1)

Suggest you do a search on PCM, this subject has been debated numerous times and there are many many threads on this.

Jerry

JohnW

My Feedback: (6)

I too suggest a search in this forum. This was beat to death in the spring of this year. However, I'll quickly answer your questions.


Yes, PCM is just a different signal that still uses a FM carrier wave. This means interference that effects PPM will also effect PCM. However, the digital encoding of PCM does help PCM reject some types of interference better than PPM, making for a slightly better over all link from TX to RX. For example, PCM will rage check fine out to distances further than PPM. For the most part, you hit the nail on the head...The main selling point of PCM is that with interference, your model will act in a controlled manner, either hold last commanded position with light interference or more to failsafe position with extended interference. With PPM, any interference, light or extended, will cause totally random and uncontrolled flight.

HarryC

My Feedback: (1)

But PPM is now available with programmable failsafe.

Harry

SnappyRolls

Thanks for the replies...i can see where there has been many debates....i have been modeling for nearly 20 years and haven't really understood this debate fully yet....haven't tried the range check on the two....but it makes sense to me that if you get better range on the the check something must be positive for the dollar amounts you pay for it....other than the fail safe options...
i have read of this new reciever that came out that will perform even with another transmission on same channel turned on...it is advertised in the latest RCM.............

thanks

p.s. is there a big advantage flying on the 50 MHz band.????

Rodney

Since both PPM and PCM are using FM as the carrier, you will not find any difference in range. Their are advantages and disadvantage to both. For most of us, PCM is not worth the extra money but I'm sure you will find some who disagree with that. I have used both but still have all but one of my planes on PPM and have never had any problem, even with magneto ignition on many big engines. Range and reliability have been good on both.
As to your question on 6 Meter flying, there is no difference in performance. You do need to have a valid HAM license to operate on 50MHZ and one major downside is that, if you ever decide to sell your system, there are far fewer potential buyers. A big plus is that you rarely have a frequency conflict at the field.

JohnW

My Feedback: (6)

Rodney, just because both PCM and PPM ride on a FM carrier does not mean they have the same range. Granted, most everything is the same between the two (Power, antenna, etc) but one is an analog signal and the other is digital. If the digital signal can be reliably decoded at a higher signal/noise ratio compared to the analog signal, then the net effect is better range. I haven't done any controlled tests, but from my experience, I always get a better ground check range with PCM, typically twice that of PPM. This leads me to believe that the digital signal can be decoded at a a higher S/N ratio. Maybe I'm wrong and I'm just seeing a difference in quality between PCM and PPM RXs and it has nothing to do with PCM vs PPM per se, I don't know. But for their most part the point is moot as both PPM and PCM will allow you to fly well beyond what you can see.

Snappy, the positive from PCM (when set properly) and all the extra dollars, is that you still have "control" over your plane during interference. Granted, it is preset control via the fail safe setting, but it is still control. This should allow one to crash a plane in the over fly area when hit with interference, which should minimize the chance of personal injury or property damage. A large part of what you are paying for is added safety.

There are other ways to accomplish this as HarryC points out, such as Multiplex IPD RX's, and I'll add programmable servos that include a failsafe to the list.

Rodney

Monkey boy, I think you are right when you explain the difference in range as to receiver design, not type of modulation. There can be a difference in receiver sensitivity, stricktly front end design, not modulation type. As far as PCM versus PPM being safe, there is little difference; in some cases PPM outperforms PCM and in other cases the opposite is true. The biggest problem with PCM is that interferance can be masked until it is to late, you either have a flyaway or crash. With PPM, interferance is usually quite evident prior to take off.

JohnW

My Feedback: (6)

"The biggest problem with PCM is that interference can be masked until it is to late, you either have a flyaway or crash. With PPM, interference is usually quite evident prior to take off." - Rodney

That is very true, PCM is kinda a double edged sword. However, it is possible to detect intermittent issues with PCM both on the ground and in the air, it just takes a better check than one has to do with PPM.

I however, disagree about the safety issue. Any method that allows one to throttle back and spin into the ground in the fly-over area when being hit is a lot better than random something else. Consider a hit when on landing approach. With a failsafe system, be it PCM, IDP, etc., your model can be programmed to throttle back and spin, which would dump it at the end of the runway. With PPM, you might go full throttle, turn and crash into the pits with the same interference. I realize this is probably a minor point because injury causing crashes due to interference are rare, but it is still safer if used properly IMO.

khodges

My Feedback: (1)

I just happened to be skimming through and this discussion on PCM/PPM is very interesting to me, as I am relatively new to r/c and was looking for a second radio and also wondered what really made PCM better; my questions are answered! I do wonder though when you talk about better range; if I am much more than 1/4 mile out, I can hardly tell what my plane is, much less which way it's pointed or its attitude (.40 size model), and since our transmitters can reach a mile or more, what's the point of better range, unless you're flying large scale, or have 20/10 vision or on-board video? (did I just answer my own question?)

MikeS

My Feedback: (37)

Real engineers know that in a baseband comparison (the only valid one in encoding comparisons) PCM has an average 14 Db advantage over PPM. That is very significant. That is a mathematical calculation backed up by over 30 years of practice, and is taught; or wa taught; in communications engineering classes as a valid, repeatable, testable fact.
Mike

Lynx

Mike, real engineers don't fly planes (please excuse me all you engineer fliers out there <G>) and that 14Db advantage is on paper, which means one thing it will never actually manifest itself in the real world. It's a theoretical advantage. Not a practical one. Put a transmitter on the ground a receiver in the air and then compare THAT. I will bet you any amount of money, that outside of a lab you can't replicate that advantage in a baseball field somewhere. The paper is just for reference and design, not a practical consideration. All that aside at the end of the day people will pick one based on personal choice and their experiences.

MO_Radio_Tech

Most PCM systems have a built in Data correction feature that chooses to ignore small glitches and "hold last instruction" for that glitch intead of PPM, which will simply interpret the interference as instructions.

This causes the double edged sword of PCM. You have a greater flyable range, but when the interference finally overpowers your signal, you get sudden loss of control without much warning because you never had glitches for warning.

Thus you could say that PCM range is like a smoothe river with a waterfall at the end. PPM will glitch increasingly worse until you loose all control. It would be like a river with incresingly worse rapids with a waterfall at the end.

PCM has the longer flyable range, but less warning of end. PPM can give more warning, but will have less flyable range because glitches are misunderstood as instruction.

PCM systems incorporate a failsafe system that allows a preset instructions to override in the case of no signal/ bad signal. This allows throttle to idle or kill, aileron defelction, etc. Whatever you preset. Now PPM failsafe systems exist within receviers which mimic this.

I have used Dynamite Race Guard failsafes with my PPM receivers for several years to achieve failsafe one channel at a time.
The problem with much of the thought on RCU about interference is that it seems people have the misconception that interference will be visible on a channel for a long time. Often times the interference only lasts a moment. If your receiver still ranges a moment after the crash that does not prove you were not shot down. It only proves you have no interference at this moment.

Another misconception is that receiver design/brand A masks all intererence while design/brand B does not. Everything can get shot down if there is enough power being broadcast by the enemy.

Often times interference is only momentary and very powerful. Much interference is so powerful that communcation will be lost no matter what. At these times, we don't care about PCM/PPM or about dual conversion/single conversion. We only care about, "can I save this plane?" Only failsafe will give you this chance.

Contrary to popular faith, dishonest or careless people can and do swap crystals at the flying field and at events without warning. People can and do power up their transmitters on your frequency without warning. If they are closer or more powerful than you, you will be shot down.

Failsafe, PPM or PCM, is a must for safety's sake. Not all agree with me on this.

If I have any $$$ in the air I fly PCM. It's my money. Spend yours on whatever you want.

I have heard on RCU of these receivers that can reject interference, even on the same frequency. This is interesting. I have yet to see one first hand. I would like to test it.

50 mHz does not have much advantage over 72 mHz. While it is true that most other flyers do not use it and are liable to step on you, ham radio operators miles away can step on you with powerful equipment. You have less than a watt of power, communcating with your aircraft only a few hundred feet away. They may have hundreds of watts of power, sending messages to friends across the world in Africa or someplace.

50 is also much more subject to harmonic intererence than 72 mHz.

Some people have flown 6 meters for decades with no problems. Good Luck.