PPM or PCM ???
 

Hi Guys,

I'm in the process of researching for a computer radio system and am in kind of a dilemma.This will be my first computerized radio after the 4 YBF that came with my Nexstar.Now, I understand that PCM (Pulse Code Modulation) goes into a lock out mode where it goes to a pre set programmed position when it encounters some sort of interference and that PPM (Proportional Pulse Modulation) gives you some control with violent movements of the servos when interference occurs. I understand that PCM sends an exact code while PPM might be a little bit off.
I got a lot of information from this site http://www.torreypinesgulls.org/Radios.htm, but i still can't decide which one is right for me. I have also found out that PCM is more expensive.

So which one?


Thanks,
Sean

RE: PPM or PCM ???
 

I can't say what's best for you. I've personally never felt the need to spend the extra $ for PCM. However, I fly mostly smaller, less expen$ive planes. If I were flying larger, more expensive, planes or turbines, I'd definitely think about PCM. A good compromise might be to buy a PCM capable transmitter and hold off on the PCM receivers for a while.

RE: PPM or PCM ???
 

i got both in my inexpensive jr 6102 and the tx if bought in the flight pack is only $10 more than ppm(fm).

i have two planes on pcm and one on fm on the one transmitter and really cant tell the difference while flying;)

good luck!!

RE: PPM or PCM ???
 

I would buy a Futaba 9C or the JR equivalent depending on what most at your club fly. Either can take you from beginner the the ultimate in advanced flying, heli, jets, 3D, sailplanes,, whatever. . You can pick one up for about $300-400 depending on what package bundle you choose. I fly the Futaba 9CAP, its a killer transmitter and really easy to program once you get the basics which is you have any computer skills at all,,,, it will be easy for you.. There are lesser computer radio alternatives for less money, but if you have the money to spend trust me.. You will not be disappointed. And unless you just want to upgrade,, short of a major technology change you will never NEED another transmitter. And remember, with a CAMPAC (included with most 9C pakages) you can hold about 16 planes. All you need to to is buy the receiver for each additional plane you want to fly... And it will accept either PCM of FM (PPM).

RE: PPM or PCM ???
 

Thanks for the replies. I either going to go with the Futaba 7c or 9c depending on my budget. This radio system will all go in my second plane so that's why I'm asking PPM or PCM. I am aware that they can transmit in both PPM and PCM, but which radio system for my second plane? The 7C and 9C come with either a PPM receiver system or PCM system. I kind of worried about the PCM fail-safe. I don't like the idea of a plane flying on its own with no response for might be a few critical amount of seconds then again I like the idea that it sends out an exact signal.


Thanks a lot!
Sean

RE: PPM or PCM ???
 

the 7c does not offer the same software mixing ability as the 9c. the 9c is a superior radio. with the 9c you can have dual channel elevators that respond to the trim control and dual rate ail rudder and elevator. the 9c is the least expensive version of a computer radio that is the most versatile and should suit the most needs. the better much more expensive are the 9z and 14 which are very expensive compared to a 9c. sorry about not mentioning jr but I do not know the models too well. The 9c has a selectable transmitter that allows pcm transmission of ppm transmission. For pcm you need a futaba pcm receiver in the plane. For ppm you need a ppm receiver in the plane with the proper shift for that radio I think it might be a negative shift for futaba but you can use hitec receivers or futaba receivers.

RE: PPM or PCM ???
 

In your situation I would go for the 9c with the ppm reicever over a 7c with a pcm, if the price is close you will realize more from the 9c. I fly them both and really can't tell the difference.

And like Mike said, unless they come out with mind control, the 9c will be all the radio you need for a long time.

RE: PPM or PCM ???
 

Sounds like someone has been feeding you some info on PCM receivers. They do not possess the plane and do what they want. I have thousand and thousands of flight with Futaba PCM receivers and have yet to have a problem much less a frequency failsafe that possessed the plane and crashed it... I have seen folks flying PPM on the exact same channels as we fly PCM at the same field repeatedly get hit. I cant give you all of the science of why PCM is less prone to be interefered with, it just is by design. There is a difference.

FM is fine,,, and it works great most of the time, there are a lot of people that use FM out there. But there is absoutely no question about the fact that there is an increased risk of getting hit with PPM FM.


Also, as far as the plane doing what it wants because the signal is lost you have two choices. You can fly PPM, and if the signal is lost the plane is going to do whatever IT wants ( actaully it will do whatever then interfereing frequency drives it to do.) or you can go pcm and setup the failsafe to tell it what YOU want it to do if the signal is lost. Typically I set my PCM failsafe up for safety. which is all that matters if I lose control.. Engine to idle and all controls level is where I set mine up... An even safer sacrificial thing to do is set it for engine idle, full up, full left rudder, full right aileron and just let it snap into the ground.. That way you know its not going to fly off away from your flying site. It pretty much going to go straight into the ground.

RE: PPM or PCM ???
 

Sean-I tnink you'll wish you had PCM later if you don't get it now. I don't know at what level you want to go in the hobby, but the other thing to consider with PCM over PPM is failsafe operation, which is built into PCM but has to be added on to PPM. Joe Nall requires a failsafe on the throttle for all participants, and some of the talk I've heard at the event leans toward requiring it at many more of the larger fly-ins.

The 9C radio is a great one, that's what I use and I love it. But if budget is a problem, you might want to look at the Tracker III that Polk Hobbies sells. Not quite the programmability of the Futaba or JR top-end Tx's, but it is PCM, has frequency synthesizing (no crystals, fly on any available channel), preset mixes like the 9C, digital trims, 99 model memory, automatically sets to the proper shift if you're using different brands of servos, and more. The set comes with Tx and Rx battery, one servo, and the Rx, which is also frequency synthesized and will also work with any brand of Tx. The whole system is $200. A couple of guys in our club use it, they both like them a lot.

Maybe it's all moot; once DSSS is approved for all r/c airplanes, most of the other stuff will eventually go the way of the dinosaur.

RE: PPM or PCM ???
 

Im patiently waiting on DSSS,,, its working great on the park flyers.. I havent heard of a single problem yet. Its only a matter of time and probably a short matter of time until frequency control and FM signal interference will be a thing of the past. :D

I just hope they dont try to run the prices up a lot.. I dont think the technology is all that complicated or unique.

RE: PPM or PCM ???
 

If you program your reciever to go to idle on failsafe, your plane will telegraph the fact that you are being hit with interferance to you and you may be able to land the plane. Sometimes interferance comes and goes.
Has anyone ever programed the failsafe to kill the engine and pop a parachute?

RE: PPM or PCM ???
 

Hmmm.....

With the power to weight ratios of our model aircraft, this is certainly a viable concept.

RE: PPM or PCM ???
 

PCM is a digital signal that encodes the voltage level and sends a sequence of 1/0 signals to represent it. Not only is it more accurate, it is more proof against interference. If the price isn't much different, you would be crazy to take PPM over PCM.

RE: PPM or PCM ???
 

Don't be confused into thinking PCM (Fail-safe) can save your airplane. It's my understanding when the receiver locks into "Fail-safe" you are not going to regain control of the aircraft. However, there is some likelihood of regaining control of an FM PPM airplane because many times interference goes away, or is intermittent. The ability to program what will happen in failsafe would really only keep a stable trainer or sailplane from flying away if fail safe is programed to do so. Our locked-out and fail-safed 0-0 sport and competition airplanes are going to crash. Low throttle will help and putting the airplane into a spin could mitigate the damage. Any crash, be it a fail-safed PCM or a an interfered with FM-PPM is dangerous and to think one is less so than the other would be difficult to quantify. I will give you low throttle being better!

I think it is better to espouse PCM due to its' far better selectivity and interference rejection abilities, then to pose slim hope of failsafe causing safe crashes or worse; less damage to the airplane in the event of an interference incidence.

RE: PPM or PCM ???
 

Sorry Mode One but yours is a popular misconception about PCM. If you get interference on a PPM system its true that you can "fly through it". With PCM each frame is transmitted as a string of numbers with a check sum tagged to the end of the frame. A simplified explanation would be if the numbers when added up would be even then the check sum would be 0, if when added up they would be an odd number then it would be one. (this is a simplified explanation but the same principal applies to computer network frames) The reciever the performs the same known calculation on the frame and compares the result to the checksum digit(s), if its the same it passes the frame for processing, if its different it discards the frame. If its some sort of intermittent interference you would probably never know if it dropped a few frames here and there whereas PPM may give a small glitch. Now then, if a given number of frames in a row are bad then PCM goes into lockout and drives the servos to their fail safe point. Here's where you are incorrect in your assumption, as soon as the reciever decodes a good frame you instantly get control back if or until you get another preset number of bad frames. So.....if the interference is bad enough to cause ppm glitching or loss of control all the way to a crash then its likely that pcm will stay locked out all the way to the ground, an example of which is someone turns on and leaves on another transmitter on the same channel. The only difference is that the PCM one, if the person took the time to program the fail safe (not always the case), will hit the ground with the throttle at idle. If its intermittent interference a pcm flyer may never know, such as a bad bearing, until its bad enough to cause constant interference and lockout.

RE: PPM or PCM ???
 

""It's my understanding when the receiver locks into "Fail-safe" you are not going to regain control of the aircraft. " - FALSE

Ditto with Barracuda et. al., that is totally not correct, yet it seems to bounce around this forum and at the fields. Barracuda explained the technical bits, so I won't go there. But I will say that PCM, or more specifically PCM's failsafe, did indeed save me a $3K plane.

I believe I mentioned this is another thread, but I went into failsafe while in the air because of an offending TX that was turned on and transmitted on my freq (I had the freq marker.) I went into full failsafe, which I had set to engine at idle and controls neutral. I was somewhat lucky in that I was finishing an outside loop from the top and was pointed up at about 20 degrees up when the offending TX was turned on. While in failsafe, my plane followed a nice arc, at idle.

As one can imagine, I yelled some not so pleasant words pretty loud and the offending TX was turned off. I instantly regained full control of the plane and finished my flight without incident. The time I was in full fail safe was only when the offending TX was on, about 5-10 seconds.

With PPM, the offending PPM TX would have caused my controls and throttle to go nuts, probably put me in some funky spin. Even if I regained control before I hit the ground, I'd probably be pointed straight down and spinning, which takes altitude to recover from.

With the PCM, the plane was very well behaved and recovery once the failsafe releases is a no brainer.

Without flying under the exact same conditions with PPM, it is hard to say exactly what would or would not have happened. However, I am 100% convinced that there is NO way without the failsafes I'd still have that plane. Notice I said "failsafe." You don't need PCM to get the failsafe anymore. But as pointed out, there are some signal quality issues inherent in PCM that make it a bit better than PPM even without considering fail safes.

RE: PPM or PCM ???
 

Barracude, JohnW. Im glad you guys chimed in... I thought I had already repsonded to refute the fallacy about PCM failsafe being a permanant lockout, I guess I didnt.

RE: PPM or PCM ???
 

Sorry guys, I don't follow every thread and don't hang on your every word, anyway. Even so, I am glad to have had my missunderstanding, remidied. However, I did make myself clear that even with what I missunderstood about PCM Failsafe, I thought PCM to be the better choice!

RE: PPM or PCM ???
 

lol... its not about "who's" right or anything like that.. Its just about making sure that accurate info gets out there so that we can ALL learn from one anothers experiences. I have been wrong on here more than I care to admit, and Im almost sure that I will be again real soon:D, but thats okay. When I make an incorrect statement and I learn that I was wrong, but more importantly learn what is right/correct, its okay.

RE: PPM or PCM ???
 

all 9c radios have both pcm and ppm modes in their transmitters. It takes about 3 seconds to shift from one mode to the other. the part that is not easy to shift is the reciever. they are built for pcm or ppm and can not be changed. The 9c radios that come with pcm default to pcm mode when you set up a new plane in memory. ppm and pcm are both equally reliable under optimum conditions. If ppm was no good then it would not be on the market. If you are experiencing problems in an area while using ppm then there is no garentee that pcm will fix the problem. It might only be masking a potential failure. It is better to solve the problem by determining the cause of interference and avoiding it by changing frequencies or repairing defective equipment. The pcm provide a higher level of reliability and predictability if you get jammed. What I mean to say is if you are flying a large airplane I beleive it is safer for all concerned that if you get shot down that a pcm failsafe lockout will provide better predictability of where the impact zone will be so that people can get out of its way

RE: PPM or PCM ???
 

These replies have really helped me out in making my decision.It sounds like PCM is often used for pilots who fly large scale planes. Remember, this is for my second plane, but I know I will use the radio for years to come. So, when PCM goes into fail-safe, it does not just lock out and gives you back control right after the receiver gets the right encoding. Am I right? I'm saving up for the 9C, but because of the cost, I might go for the 7C.

I appreciate all your guys' help!


Thanks!

RE: PPM or PCM ???
 

Hi Frankie, I'm writing to you from Australia, while it is true that PCM is more expensive, if it saves just one plane, even a basic trainer, you are way in front fiancially. My girlfriend & I both fly with PCM, & we will not be going back to PPM, forget about failsafe, for a moment & think about a 40% increase in range, when range checking my girls trainer, which is a Mk1 Skyraider, by the way, she got about 40 good paces before any surface started to shake around. After I changed to a JR 770s PCM receiver she now gets at least 70 paces, with the arial down, & we have had no glitches at our field since, & we would know if we did because failsafe is set to reduce power to just above idle & give a small amount of up elevator & hold ailerons at neutral.
Good Luck & enjoy this fantastic sport.