FM or PCM Whats the diff
 

Im looking to buy a new radio and don't know if the pcm is worth the extra money. Whats the pros or cons of either one or does it matter. Thanks Terry

FM or PCM Whats the diff
 

Try http://www.torreypinesgulls.org/Radios.htm

I don't think PCM is worth it. Plenty of people think otherwise. Get the facts and decide for yourself.

Steve

FM or PCM Whats the diff
 

Whats the diff eh'?

The spelling, or the price. Pick one.

pcm or fm
 

I know the price difference but is it worth the extra money. A lot of people say don't buy anything but pcm and some it doesn't seem to matter. Is thier any advantage to the pcm over fm or it just something new that everybody wants. terry

FM or PCM Whats the diff
 

I beleive all JR radios from the 783 model and up come PCM/FM selectable, unlike Futaba radios.... I have received several noticable radio hits lately on my funfly, If You have a more valuable plane, I would definitely use PCM. I have a DP Ultimate I'm using a JR PCM receiver in, I havent received a hit so far. Good luck.

PCM
 

Coulter_Dean? O'm not sure what you mean by "unlike Futaba." Every Futaba radio I have owned was PPM/PCM selectable.

Helihead: The main advantage to PCM is that the signal is digital. PPM signals are analog. With a digital signal the encoding is more prescise. PCM also offers several stages of fail safes in the case of signal loss or signal corruption. IMO, PCM is superior in all aspects to PPM with the possible exception of frame rate.

I disagree with the "Achilles heel of PCM" from the web site posted by Steve. Simply stated, the achilles heels is supposed to be that when hit with PPM you still have some control even if eratic while when hit with PCM you have no control. From my experience, the eratic PPM control during interference just means you do some unplanned aerobatics before you crash.

When PPM is hit, the servos will jitter and move all over the place. When PCM is hit, the rx will hold your last commanded position for about 1 second and then go into failsafe.

If the hit is short, say less than a second, PCM will hold the last commanded position so if you are pulling out of a loop you will continue pulling out of the loop while being hit. With PPM you will do some unplanned aerobatics.

A long hit of over one second: With PPM, by the time you get a good signal your plane may be in a tumble, inverted flat spin, full throttle pointed at mother earth... who knows. With PCM, while you may not have any direct control, you can at least set the failsafe servo locations to something reasonable... like neutral on control surfaces and a low throttle setting.

If you are serious about the hobby/sport, I'd suggest getting a tx that supports 1024 PCM. IMO both Futaba and JR make some really good 1024 PCM transmitters. If pressed for cash, you can always use an el-cheapo PPM rx with a PCM tx, but why? Isn't it worth spending a few extra bucks to put a good rx in your bird? While you still may crash if hit, I believe you have the best chance of surviving with PCM.

FM or PCM Whats the diff
 

Realistically speaking, if you encounter enough interference to send PCM into failsafe you'll most likely lose the model. Some folks think PCM is more resistant to the electrical noise generated by a poorly set-up gasoline engine.

If you're an average glow/electric flier PCM is just more money than PPM. Some people think they're buying peace of mind.

Average Flier
 

MikeL: I do agree PCM is no more resistant to interference in the respect that both PPM and PCM use similar FM transmissions. If your getting hit on PPM you'll get hit on PCM too. The big difference is how the Rx responds to the hits.

PCM is not cost effective for the average glow flier? About six months ago I saw an "average" flier (good friend of mine) that refuses to buy PCM becasue of the cost. He was shooting touch and go's in a nice piper cub and was just on climb out when a fellow flier on the same freq turned their radio on. His plane went full throttle and full down elevator and smacked the asphalt hard, destroyed the plane, busted the engine, shattered servo gears, etc. Now I'm not saying PCM would have saved the day, but I doubt the plane would have been destroyed if his rx was PCM. If his fail safe was set to neutral surfaces and low throttle the cub might have landed itself a little more gently than full throttle on the nose. Also, a PCM fail safe glide might have also allowed enough time to yell out and get the offending Tx off in time to save the plane. It is impossible to say, but I think he could have saved the plane if he had popped an extra $50 for a PCM rx... as it was, he lost hundreds in gear not to mention all the time to build the plane.

I personally have been hit a few times. While hits are rare, I like the fact that the plane will be well behaved during the hit. Another nice feature of my PCM (Futaba) is that a low rx battery pack will put the throttle channel into fail safe until you move the throttle stick. I had this happen in the air once on my $1,500+ 25% CAP... has a cell go bad in the air. I'll never now for sure but that feature probably saved my 25% CAP.

Each person should evaluate their own risks. I'm not saying PCM is best for all. But for me being at a busy field flying $1K+ planes with yahoos turning on Txs without checking the freq board, I wouldn't even consider flying without it.

Re: PCM
 

Before switching to JR, I owned a Futaba 8UAF. It was only FM, it was NOT PCM selectable. I wad to spend an additional $150 to get the PCM version. Now if you got the 8UAP, then you'd have selectability.

After a while, I switched to all JR stuff, and bought the JR 8103DT, which is PCM/PPM selectable. There is only one model of the 8103, so you dont have to spend an additional $150 or so to get the PCM version, (cause its built in). See?

I just Prefer PCM, I can tell a difference, and I dont want to risk my planes. (I dont have time to rebuild them) :p
I also fel JR has the better 8 channel, Futaba's 8UA's small screen was hard to understand, and a little more difficult to program. It also looked and felt like a cheap toy. I Much more prefer the 8103. ......MHO.... :D

FM or PCM Whats the diff
 

I have too jump in on this one. It is pretty simple: PPM is pure analog, no brains no checking of the data to see if it is correct, just send and respond.

PCM encodes the data into digital bits and send the data along with what is called a checksum, when the RX gets the data it compares the data with the checksum, and if it is wrong it tosses it out and waits for the next one.

With PPM if some noise or interfearance gets mixed in the servos will do what they are told no matter how wrong the data is.

With PCM if the data is corrupted it will hold the last position and wait for the next valid data packet. this happens extremely fast so you will usually not notice if it happens.

With PPM if the interfearance is bad enough and long enough it will crash you whatever in a random fashion, that is: the servos could be anywhere, full throttle, no throttle, full collective, no collective, you get the picture.

With PCM you can program in failsafe positions for all sticks, and for me I would rather the helo hit the ground as slow as possible, so I have it set to idle the throttle, and neutral the controls, at least it will give a chance of limiting the dammage.

You must also take into consideration the people around you, with PPM it could crash into a car at full bore, with PCM I know that the throttle will be at minimum, while it will not absolutely prevent damage or harm to someone else, it may help lessen the impact.

People always tlak about the added expense, but look at the helo as a whole, the extra 50-100 is worth every penny.

As far as masking problems, it does not mask them. Since I have my failsafe to set throttle to idle I would notice the throttle dropping out, and that would be an indication that something is wrong. With PPM I see most people just fly through the radio hits, and shrug them off. With PCM if I get interfearance that is strong enough to put me in failsafe I land it whereever it is.

It is an extra added piece of insurance for me, and people will spend hundreds on gyros when the basic piezo one will work fine, but balk at a little extra for a RX, which is the life of the helo.

I see no cons to using PCM. I used PPM for several years until I switched, and I will NEVER go back.

PCM!
 

I just started this hobby and had a Futaba T6XAS with PPM, this range checked at about 90 feet on my seniorita. Now, I sold the 6XAS which was new in Sept. and purchased a Futaba 9CAP, used the same receiver (same freq) in my Seniorita, range checked again at 90 feet on Sunday. Now I range check my new Ultra Stick 40 with the PCM receiver, and radio switched to PCM and I walked out to over 250 feet and never had the servos twitch, the control surfaces were always nice and smooth. I didn't bother to walk any farther away............

In my next new expensive project, I'll have PCM. :D

WOOD

FM or PCM Whats the diff
 

"As far as masking problems, it does not mask them. Since I have my failsafe to set throttle to idle I would notice the throttle dropping out, and that would be an indication that something is wrong. With PPM I see most people just fly through the radio hits, and shrug them off. With PCM if I get interfearance that is strong enough to put me in failsafe I land it whereever it is. "

PCM will definately mask glitches. It is designed to do so. When a PCM receiver recieves a corrupt frame, it will hold the last known good servo positions until it receives a valid frame. Or if you have failsafe selected until a predetermined amount of time has passed (usually around a second). It will then go to the fail safe positions. As soon as it receives a valid frame, either in hold or failsafe, it will go to those positions. When it receives another corrupt frame, it will hold again and start the failsafe clock from 0.

"Since I have my failsafe to set throttle to idle I would notice the throttle dropping out, and that would be an indication that something is wrong. "

You will only notice your throttle going to idle if you are receiving hits that are longer than the failsafe time. Most noise induced hits (metal to metal contact) are very brief in nature. With a PPM system you will see it, with a PCM system you will fly through it and never know it was there. This is good and bad. Good cause your flying doesn't suffer, bad because whatever is causing the noise is probably going to get worse. Some people have reported that they have been able to "feel" the sluggishness of the controls when they are getting continuos hits on a PCM system. If you get to this point, then you are getting hit very frequently and are very close to lockout. There are pros and cons to both systems. PPM has kind of an early warning system that something is wrong on small hits. But PPM can really go crazy, even full throttle on big ones. PCM handles the big hits best because of the failsafe, (throttle to idle is the biggie!) but masks the little ones that are the warnings. The best would be a PCM system that has a built in glitch counter. It would have to talk to the receiver, and count how many time the receiver goes into hold. Current glitch counters cannot do this, they can only count how many times a PCM system goes into failsafe. This is because the servos are always being fed "good" information when the PCM receiver is in hold. To count failsafes you have to have an unused channel, and program that channel to change state when in failsafe. Manufactures know how to make this glitch counter, but have not. Some people feel that the they (the manufactures) think that it would cause confusion among the general flying population (ie some people would think that they should never get any glitches. In actuality the rf link between transmitter and model is not perfect, and you will always see some glitches, you want to look for a dramatic increase in the # of glitches per flight, and that would tell you something is wrong).

Alan Angus

FM or PCM Whats the diff
 

Coulter_Dean:



"Before switching to JR, I owned a Futaba 8UAF. It was only FM, it was NOT PCM selectable. I wad to spend an additional $150 to get the PCM version. Now if you got the 8UAP, then you'd have selectability."



I have a 8UHF. Just about every model I use it with has a PCM receiver. Most of them are planes, so the 8U is in ACRO mode.



The difference between the PCM and PPM tranmitters is the label, some default settings and the receiver and servos it is bundled with. I don't think you looked hard enough to find the PCM. It's in the PARA menu, down the end. You also have to re-power after changing the setting for it to take effect.

Re: Re: PCM
 

Check out the 9CAP. I got mine last week and love it! The interface blows the the 8UAF/P away. :p
And yes, it's PPM/PCM selectable. I fly at a rather slow field, no more than 10-15 guys on a very busy day, so I feel comfortable with PPM for now. I can fully understand what these guys are saying though.

FM or PCM Whats the diff
 

GMPHeli,

Maybe my choice of words is lacking in this matter. "Mask" seems a little decetfull(spelling?). Maybe a more appropriate term would be "Filter". I like to think that the PCM RX filters out the junk that I do not want to get to the servos. I find it pretty un-nerving to see my helo twitch here and there, and anything to get rid of it is good stuff in my book.

Think of it like the filter in modern FM radios in your car. The tuners have gotten a lot better than in the old days, and most of us prefer that the background noise and static are removed from the data stream.

CD players also use a filtering technique to fill in the gaps caused by minor defects in the media. I for one would not want them all to get through, I like the clarity that I get from the "Filtering"

So it can be compared to cassetts or CD's I for one prefer CD's

thanks...

FM or PCM Whats the diff
 

PHIL COLE:

No arguments from me, I guess I didnt get to know that radio very well. It's been so long since I got rid of my 8UAF, I wouldnt be able to remember exactly what it did:p

Fly safe:D

FM or PCM Whats the diff
 

"Maybe a more appropriate term would be "Filter". I like to think that the PCM RX filters out the junk that I do not want to get to the servos. I find it pretty un-nerving to see my helo twitch here and there, and anything to get rid of it is good stuff in my book."

Exactly, This is the good side of PCM, but don't you wonder where this junk is coming from? If I had a heli that was glitching on PPM, and put PCM into it and all seemed fine. I would not have a warm fuzzy feeling. I just went through a glitching scenario with a PHI Huricane. To make a long story short, the tail rotor pulley (it was not the stock pulley) was rubbing on the outer race of its bearing. It took a while to find, and I was tempted to go to PCM. I am glad I didn't. I stuck with it and found the problem.

Alan

PCM
 

Just to add something , Failsafe is not intended to save the AIRCRAFT, it is intended to save PEOPLE. Any failsafe system set for anything other than crash is no longer safe.

failsage on PCM
 

Dave,

Good point, I have heard of people losing some control on expensive aircraft at a show and the pilot put the aircraft in for safety reasons ans lost a few thousand dollars. safety first, fun second.

WOOD

FM or PCM Whats the diff
 

I agree, I would hate to be responsible for an injury to somebody else. That is why my failsafe is programed to drop the throttle, and neutral the control. It is not so much to try to save the craft, but minimize any dammage to person or helo.

One one hand I do wonder where the interfearance is comming from, and maybe it would be neat if the PCM RX had a switch to change from PCM to PPM so that you could do a check. But, noise is everywhere, and some of it does not come fro the helo itself. If the PCM can filter most of the little anoying interfearance I am all for it. Maybe it would be good to keep a PPM rx around and do a periodic check for interfearance.

I am a Preventave Maint freak, so I go over my helo with a fine tooth comb at least once a month, and change things like the main shaft bearings on a regular schedule, I mean they only cost a few bucks, so why not. This is a carry over from my military days working on Helicopters, we changed things on a regular basis weather they were broken or not.

People mostly talk about the added expense of the PCM rx, but somethings are worth the extra cost, and if you maintain you helo properly the chances of having problems should be lower. While I will not say that having PCM makes me imune to interfearance, it sure does help smooth things out.

FM or PCM Whats the diff
 

What you're asking for does exist Brian, it's called a 'glitch counter'.

FM or PCM Whats the diff
 

Yes, I know they exist as an add on, but I do remember hearing at one time in the past that a manufacturer actually embedded it into the RX and after a flight you could download the information. That is more what I want.

FM or PCM Whats the diff
 

The one's I'm familiar with have an LCD counter that display the glitch count. No downloading required.

Still, I'd prefer to see the number of rejected frames rather than the number of fail-safe occurrances. I think you would then have an earlier indication of something slowly going wrong before getting near the "point of no return".

Do you remember the manufacturer with the built in glitch counter? I'd like more info on this, at least it sounds like a move in the right direction.

FM or PCM Whats the diff
 

I would like to point out that none of the preceding posts has dealt with the question of is PCM better? The reason is that failsafe and PCM are different things. I only use PPM yet I have fully programmable failsafe. Yes, all of you using PPM even non-computer transmitters can buy a PPM receiver with fully programmable failsafe using 1/2 second hold of last good position before going to pre-programmed servo positions. It is available in 7 channel single conversion or 9 channel dual conversion and is about to be available in 12 channel synthesised frequencies dual conversion, all in PPM.

Now that we have got rid of the red herring regarding failsafe, what is the advantage of PCM over PPM?

Harry

P.S. It is Multiplex's IPD receivers that have failsafe on PPM.

the diff is......
 

The manufacturers say you get a cleaner signal, that is the diff.

That must mean that less interference gets through your receiver, and doesn't make your servos twitch, because,

If you know how FM and PCM work, you will know that PCM stands for Pulse Code Modulation, which is a series of ones and zeros, instead of an analog signal like FM, which stands for Frequency Modulation. Ones and zeros can be recognized easier than an analog signal, which is a constantly changing sine wave.

That is one reason it is different, and I think better because more information is processed correctly and less intereference is passed through the filtering of the receiver. :D

WOOD