RunRyder - FM or PCM

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Radio - Servo - Gyro - Gov - Batt > FM or PCM

dpwatson
Heliman
Location: Arlington, TX

I have an opportunity to get a new radio. I have pretty much decided on the Futaba 8U. What's better the FM or PCM? Is PCM worth the difference in cost? Also I have a ham license. Is it worth getting someone to order one in the ham band? It just seems to me like that band would be much less crowded. I have noticed that the ham band radios have a much lower resale on Ebay although I have only seen a couple of them there. Any comments would be appreciated.

Doug

09-04-2001 Over year old.

RappyTappy
Elite Veteran
Location: Las Vegas, NV

FM for me

I like to use FM because it lets me know I have hits, while PCM just masks the problem. One time I had someone turn their radio on while I was flying, my heli started dancing around, but I was able to control it every now and then keeping it from crashing for about 2 min. He eventually turned his radio off and I was able to bring it down safely. If it was PCM it would have locked out and crashed.

Chris
Xero G

Forever Brothers
Mickey Tylo

09-04-2001 Over year old.

HOMEPAGE

charliex
Heliman
Location:

one time we switched our PCM transmitter on and the FM heli went nuts.. hey wait a minute..

but seriously we did a few tests and found that when both radios were on the FM bird went nuts, and the PCM ignored it and continued without problems. Ie no lockout.

Next time you're are the field we can experiment more, but this time i suggest we try an in air test for the FM bird.

09-05-2001 Over year old.

RappyTappy
Elite Veteran
Location: Las Vegas, NV

They were on the same channel right? I'll be out at the field after 5:00pm all this week flying, I've gotta see this.

Chris
Xero G

Forever Brothers
Mickey Tylo

09-05-2001 Over year old.

HOMEPAGE

charliex
Heliman
Location:

no, one was on ground frequency channel 12, and the other air frequency 59.

same channel, 11 IIRC

09-05-2001 Over year old.

RappyTappy
Elite Veteran
Location: Las Vegas, NV

So PCM overpowers FM?

Chris
Xero G

Forever Brothers
Mickey Tylo

09-05-2001 Over year old.

HOMEPAGE

charliex
Heliman
Location:

no i don't think so, i think either the PCM lockout is really really quick, or the stronger radio signal works (though they ought to blend)

We tried a couple of different people/craft, should try the 8u vs the 9z.

i've never been able to get anyone to give me a satisfactory answer, so i tried it, more reasearch is needed =)

09-05-2001 Over year old.

RappyTappy
Elite Veteran
Location: Las Vegas, NV

That would be interesting to see.

Chris
Xero G

Forever Brothers
Mickey Tylo

09-05-2001 Over year old.

HOMEPAGE

Dragon2115
Key Veteran
Location: New England

PCM vs. PPM (FM)

First off neither PCM or PPM are bulletproof. They both have their advantages and their disadvantages.

Advantages;

PPM - you can see RF hits to the system in the form of glitches. This may not seem like an advantage but it can serve as a warning that a bearing is on the way out, that there is a loose piece on the heli, or that there is some metal to metal contact somewhere on the heli that shouldn't be happening. Also, if you get hit with interferrence you usually have some degree of control.

PCM - you don't see glitches or minor radio hits because the receiver goes into a "hold" mode ( 1.5 second precurser to failsafe) and ignores the inteferrence by holding the sevos at the last commanded position.

Disadvantages;

PPM - when you do get a radio hit the model with react, sometimes violently, but it's usually of very short duration and can be flown though. More of an annoyance than anything else. If you loose signal to the receiver there's no telling what the servos are going to do.

PCM - If you loose signal or the signal is corrupted by interferrence for more than 1.5 seconds the receiver will go into failsafe and the servos will go to a preprogrammed location and stay there until either the signal clears or you hit the ground. You will have absolutely no control while in failsafe.

Two radios, PCM vs PPM, same channel. - I ran into this at a fun fly last year. The CD fired up one of his helis thinking it was on a clear channel. Unfortunately it was on my channel (I had the pin) and I was flying at the time. The funny part is we both flew for several minutes without seeing interferrence problems. We were spaced a good 100 feet apart on the flight stations. My Rappy (FM) suddenly started doing 3D without me, which for a rank beginner on his 5th or 6th forward flight was a real eye opener.

I would get full control then loose it for a second and then get it back again. After about a minute of this I got my bird back on the ground safely. About one second later we all heard the sickening thump of the CD's Xcell Graphite SE hitting the ground from about 100 feet up. My tx won and his rx went into failsafe mode. Everything from the mainshaft forward was at a 45 degree angle from where it should be.

The point here is that there's no telling which tx is going to win. There are tons of variables involved such as tx output power, rx sensitivity, tx antenna direction, rx antenna direction, distance between tx's, and distance between tx and rx, etc. All of these variables will play into which tx the rx is going to respond to. In the case of the the PPM rx, it doesn't know what to do with the PCM signal so it spaz's if it becomes too strong and overrides the PPM signal. The PCM rx doesn't know what to do with the PPM signal and sees it as interferrence and tries to ignore it until it becomes too strong at which point it goes into failsafe.

Neither system is bulletproof. You just have to decide what works for you.

09-05-2001 Over year old.

HOMEPAGE

charliex
Heliman
Location:

what happens if you don't have the failsafe switched on?

Also for the PPM interference it'll last as long as the interference is there, its not like it'll be a quick spaz then clear, and when it is there the servos freak out , at least on the 3 machines we tested, good luck controlling that ! =)

Whereas the PCM can sort out between good signals and bad signals and has a better tolerance and rejection than PPM, since the servos won't spaz out you have no way of knowing where they currently are with the PPM , so in order to gain control the PPM driven servos have to be reset back to where they were, or to be under control again.

whereas PCM will just ignore commands that are wrong, leaving them at the last position, or a predefined position, you could down the throttle at least, given theres no safe position on a heli, its less useful than for a plank.

your explanation is good, but it does make it sound like PCM will lock you up for longer than PPM, (at least to me) which isn't the case, they will both suffer from interference for exactly the same time, the only difference is one freaks out, the other does nothing or goes to a preprogrammed position.

The PCM receiver will recover quicker though, and if the interference is light you've got more chance of getting through the right signal.

RF is a bit like light, they don't override they blend.

09-05-2001 Over year old.

RappyTappy
Elite Veteran
Location: Las Vegas, NV

I'll say it again, I had a flew with another radio on for 2 min going crazy and I was able to control it and land it safely. Also, when there is interference from another PCM radio on the same freq, I doubt the receiver is smart enough to tell which one is the master and which signals are false, so it ignores both signals, no control. Either way you will be crashing if the signal last long enough. I just don't think that PCM is worth the extra money, when FM has got served me well for years now without any hassles, in fact saved me many times by warning me of impending disaster before I ever left the ground. Make your choice PCM or FM, both are fine and will fly your heli and one is more expensive than the other, make your choice on one and then become bias about the other

Chris
Xero G

Forever Brothers
Mickey Tylo

09-05-2001 Over year old.

HOMEPAGE

charliex
Heliman
Location:

It certainly makes sense that you can counter the interference on a PPM receiver more than you could on a PCM, I'd like to see you do it

we can setup a test

The reason i like PCM better is because it suffers from interference less, thats why battlebots etc use PCM, all that metal grinding around causesa lot of interference. Tap a dissimilar metal screwdriver against metal on a PPM machine and watch it jump around.

Hey mark, time to import more of those smileys from vBulletin/UBB =)

09-05-2001 Over year old.

RappyTappy
Elite Veteran
Location: Las Vegas, NV

My Ergo should've gone in a long time ago because that thing was a grind o matic after a while.

Go to rconline in the helipad forum and check out the thread entitled "Is metal/metal interference real?" Its a couple of pages back, I believe about 3, but there is 57 responses to the thread arguing their point each about half a page to a page long. It made me wonder if that screwdriver touching the heli is really a viable example of metal to metal interference. Very technical if you want to look at it, you might enjoy it, food for the brain as you would say

Chris
Xero G

Forever Brothers
Mickey Tylo

09-05-2001 Over year old.

HOMEPAGE

charliex
Heliman
Location:

I can't even get on rconline anymore, doesn't seem to work with IE6.

I'm suprised though, its basic physics. ( Well actually i'm not suprised

)

Two dissimilar metals at different temperatures form a thermocouple.

the voltage generated is proportional to the temperature difference at the junction.

Its basic EMF (electromotive force), thermoelectric voltage. Its called the Seebeck effect.

09-05-2001 Over year old.

RappyTappy
Elite Veteran
Location: Las Vegas, NV

If you say so cause I have no idea, I just fly em'

If it doesn't twitch and doesn't fall out of the sky, I'm one happy man!

Chris
Xero G

Forever Brothers
Mickey Tylo

09-05-2001 Over year old.

HOMEPAGE

charliex
Heliman
Location:

I've got to have something to do while Diannas flying!

09-05-2001 Over year old.

Dragon2115
Key Veteran
Location: New England

PCM vs. PPM

Hi Charliex,

As far as receivers recovering faster than one another. Can any of us tell the difference between an rx that recovers in 50 mSec vs. one that recovers in 150 mSec? Not that I'm aware of anyway. So really that is a moot point in the comparison.

You are absolutely correct about Battlebots using PCM and why. However, I'll be sure to keep that in mind the next time I decide to fly demolition derby with my heli.

But you're right, PCM won't see this, which can either be a good or bad thing depending on your perspective. It can serve as a warning that something is wrong in the heli, such as a notchy main bearing. Sometimes this is good because you see a problem and go check it out before it becomes major. With PCM you won't see this until it's so strong that the rx goes into failsafe and you crash.

Personnally I prefer to put up with the occassional glitch and have the ability to see what the health of my heli is. FM glitches typically don't last long and result in a quick kick of the tail or something like that. I've never seen it worse than this. Maybe I'm just lucky.

09-05-2001 Over year old.

HOMEPAGE

charliex
Heliman
Location:

Maybe you can't tell the difference but your gyro and servos can! (if they are any good) so its not really a moot point, plus you are forgetting the cumulative effect don't take such a narrow view of it, consider it happening many times in a row, soon adds up. you are also ignoring response time. Look at the frame rate of a GY401 and up. Gyros are all about speed and response time,. fine the throttle servo can live with it..

I knew someone was going to say that, I give in

Its got nada to do with the bots being involved in combat, its got to do with the interference generated by the robot. PCM receivers are used because they are less prone to interference in the first place because they can remove spurious signals, and the method used to get the information to the receiver is much much better than FM

Now why anyone would think that thats a worse thing is beyond me

Ground or air, there are airborne versions of the bot competitions using helis etc....

09-05-2001 Over year old.

Dragon2115
Key Veteran
Location: New England

Metal to Metal Interferrence

Hi RappyTappy,

Metal to metal interfenrrence most certainly is real regardless of whatever arguments you see. I've was in RF for over 10 years and have lots of EMI troubleshooting experience. I think one of the reasons people have a hard time believeing it's real is because tapping one piece of metal against another doesn't produce the same effect all the time. Therefore people have trouble reporducing it exactly the same each time so they think it isn't real. Unfortunately fot them they're wrong, it is real and if it's not taken care of in our helis you will eventually crash from the interferrence. That may be today, tomorrow, or next year. I get more glitches at one particular field I fly at more than any other. I found out that there's a welding shop fairly close by in the area. Every time they stike the rod to the steel it sings out. Sometimes it hit our band. Most of the time it doesn't. Interferrence like this is broadbanded and the power at any given frequency can vary greatly. If there is enough power on my particular frequency it can and will temporarily overpower my tx's signal. But like I said earlier, this is very short duration and usually results in nothing more than a quick tail kick.

09-05-2001 Over year old.

HOMEPAGE

Dragon2115
Key Veteran
Location: New England

Quote
Originally posted by charliex Maybe you can't tell the difference but your gyro and servos can!

Did you see the part where I said it usually results in a tail kick?

Quote
(if they are any good)

I don't know, that would depend on what do you consider JR digitals to be?

Quote
so its not really a moot point, plus you are forgetting the cumulative effect don't take such a narrow view of it, consider it happening many times in a row, soon adds up. you are also ignoring response time.

I'm not ignoring anything, I'm just not making a mountain out of a molehill over it.

Quote
Look at the frame rate of a GY401 and up. Gyros are all about speed and response time,. fine the throttle servo can live with it..

Refer to what I said about tail kicks again please.

Quote
I knew someone was going to say that, I give in Its got nada to do with the bots being involved in combat, its got to do with the interference generated by the robot.

Which is being generated by what Charlie? Metal to metal contact when the bots come into contact or when they're damaged and have internal metal to metal contact occurring. For this application I agree PCM is the way to go because it will ignore the occassional corrupted signal. (added) Plus they're running serveral electric motors which as they wear tend to become more and more electrically noisy and radiate more and more RF.

Quote
PCM receivers are used because they are less prone to interference in the first place because they can remove spurious signals,

Absolutely false, a PCM rx is just as prone to interferrence as a PPM rx is. The difference being the PCM rx will ignore a corrupted signal whereas a PPM rx will try to interpret the signal the best it can.

Quote
and the method used to get the information to the receiver is much much better than FM

Irrelevant to the point at hand. (edited) Both are actually FM when it comes to the RF transmission, the being the encoding of the data within the transmitted signal.

Quote
Now why anyone would think that thats a worse thing is beyond me

Then you just don't see the value of knowing when there is a problem with your heli before it turns catastrophic.

Quote
Ground or air, there are airborne versions of the bot competitions using helis etc.... [/B]

I haven't seen the airborn version yet. When are they on?

09-05-2001 Over year old.

HOMEPAGE

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