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PCM vs FM

Old 04-11-2006, 07:49 PM
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Jimmy Hoffa
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Default PCM vs FM

What are the advantages of PCM? Less chance of a glitch?
Phillip
Old 04-11-2006, 07:59 PM
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redfox435cat
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Default RE: PCM vs FM

well to me it depend on weather everyone else at thefeild uses it. if your on PCM and some else is on fm on the same channel it won't effect your model. you'll effect him though. Glitch wise i thinks it's perception. that advantage is PCM has a fail safe that when set right will upon signal loss reduce engine to idle.
Old 04-11-2006, 08:25 PM
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Default RE: PCM vs FM

ive heard some bad news about those PCM's like locking up and freaking out. im sure others have no problem's so i'll shut up.
Old 04-11-2006, 11:44 PM
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Rcpilot
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Default RE: PCM vs FM

I fly 40 size glow and 50cc-60cc gas.

I use PPM. Never had a problem.
Old 04-12-2006, 01:56 AM
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PlaneHeli
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Default RE: PCM vs FM

Interesting Question. I just assumed everyone would use PCM.

My airplne kept going to idle with PCM faisafe on. No other interference. I just turned it off and it flew OK.
I had made my own reciever pack, soldering batteries. (couldnt buy one at LHS) Maybe my soldering was not the best or I heated the batteries too much.

I have since read up on PCM and Failsafe on the internet and found that it going to idle may have been due to a faulty reciever battery, so I have since bought a proper pack. Will have to reset PCM faisafe and see what happens.

Do you experienced modellers use PCM or PPM, also do you use the failsafe on.
Just thought everyone would use it as I havnt heard of other people having problems with it.
Old 04-12-2006, 06:59 AM
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exeter_acres
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Default RE: PCM vs FM

has it been 2 weeks already?

Wow....




Here are a couple sites:
http://www.aerodesign.de/peter/2000/...M_PPM_eng.html
http://www.torreypinesgulls.org/Radios.htm
http://www.modelaircraft.org/insider/05_11/03.html


Hope that helps...
Old 04-12-2006, 08:40 AM
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JohnW
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Default RE: PCM vs FM

I’m amazed at how little people know about this, and worse the incorrect responses and myths that surround the issue. There are tons of threads on this topic, but I’m bored so I’ll play along.

The question really is PCM vs PPM. Both are FM. The primary difference between the two is the encoding. PCM transmits a digital signal, i.e. move servo to the 512th position out of the 1024 available, etc. PPM uses a analog signal where the servo position is determined by the length of the signal pulse, i.e. longer pulse, move servo this way, etc. This is highly simplified.

Since both are FM and both use (or can use) the same frequency, i.e. channel 44 or whatever, both are susceptible to the same types of interference. Redfox’s post about PCM effecting FM (I assume he meant PPM) but not the other way around is flat out wrong.

What is different is how these two system react to interference, lost signal, etc. and the threshold at which the interference becomes a problem.

In general, PCM system have slightly better noise rejection, as per what the manufacture has published. But this diff probably isn’t enough to really matter.

As to reaction, PCM typically has two levels of failsafe. Failsafe isn’t directly a function of PCM, but PCM system lend themselves to easily checking the validity of the signal, hence every one I seen has a failsafe feature. PCM is not standardized across the industry and it varies slightly from vendor to vendor. With PCM, when the signal cannot be deciphered, the receiver will hold the last known good commanded servo position. If the signal is bad long enough (about 1 second), the system enters full fail safe. Depending upon how the radio was programmed, the servo will either be held at the last know good position, or it will move to a predefined position (such as idle for throttle.) Many call this PCM lockout, which isn’t exactly true. More properly it is failsafe which has nothing to do with PCM and it really isn’t a lock out… it is a holding pattern. The system hasn’t locked. It entered this mode because the signal was lost. It will instantly (or nearly instantly, like less than 1 second) give full control back to the TX as soon as it receives a valid signal. Many don’t get that last part.

Traditional PPM has no failsafe. If the signal get garbled, the RX will do it’s best to interpret the garbage, which results in the twitchy and sometimes violently random servo movement we’ve all seen. Some believe they have more control without the PCM as they believe they are getting some control with PPM where PCM would have locked them out. Hogwash. If the signal was bad enough for PCM to enter failsafe, you have NO control with PPM either.

PCMs major selling point is failsafe. Anymore, the lines are blurred with failsafe as there are PPM systems that now support failsafe as well as servos that can be programmed to support failsafe. There are pros/cons to each system, but I believe most will agree that any failsafe is better than no failsafe.

Cheers
Old 04-12-2006, 09:05 AM
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Jimmy Hoffa
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Default RE: PCM vs FM

Thanks JohnW. That is a very good explanation and the kind of info I was seeking.
Phillip
Old 04-12-2006, 07:09 PM
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redfox435cat
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Default RE: PCM vs FM

Since both are FM and both use (or can use) the same frequency, i.e. channel 44 or whatever, both are susceptible to the same types of interference. Redfox’s post about PCM effecting FM (I assume he meant PPM) but not the other way around is flat out wrong.
I've done hours of trial and error on this. I set my radio to pcm on my plane. buddy on the same frequency set to PPM both on futaba 7C radios. Both models running anchored to the ground. I could control my model antenna extended to 500 yds on the ground. He could not do anything from any distance. Perception run rampent here. Right wrong or indifferent yes there all FM. PPM is a digital representation of FM with a digital time pulse ridding the carrier and PCM is a digital code representing a voltage output just like digital music with of course alot less resolution riding an FM carrier then the receiver read and decodes it and puts out the digital pulse to the servo , yes way oversimplified. PPM is generally called FM right or wrong and PCM is PCM
Old 04-13-2006, 10:23 AM
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Jimmy Hoffa
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Default RE: PCM vs FM

After reading all of the replies and the 3 links that "exeter_acres" provided, IMHO, PCM is not the absolute best system. Without a doubt it does have some advantages over FM. The main problem that I have with it is that if the RX does lose the signal, the controls will go to a predetermined position. This would be great for a helicopter but would result in a crash of a fixed wing. For a heavy model that could possible kill bystanders, you do want the plane to come down promptly and with the throttle cut. A glitch in a FM system will not be masked like with PCM and there is the possibility of maintaining some control. The best setup may be the FMA with the infared auto stabilization and it operates on FM. I'm sure some of you will respond that PCM doesn't glitch. My response is that my FM also doesn't glitch, at least not in the past two years since I started flying.
Phillip
Old 04-13-2006, 12:58 PM
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Default RE: PCM vs FM


ORIGINAL: Jimmy Hoffa
<<snip>>...my FM also doesn't glitch, at least not in the past two years since I started flying.
Phillip
That (and the price difference) are probably the reasons that PCM capable transmitters and receivers have not taken over the market.

Best wishes,
Dave Olson
Old 04-13-2006, 02:28 PM
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Gremlin Castle
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Default RE: PCM vs FM

My direct experience with Futaba PCM 9 Channel receivers has been from 1991 forward in both gas and glow. I fly them with conventional, Deans or Hayes whip and Stealth Strip antenna installations.

They are in balsa, conventional fiberglass, and molded carbon fiber airframes. All except the Whip antenna are internal. They all exhibit range to the ability of the pilot to see the attitude of the airplane.
It is my personal belief that Futaba PCM has a better ability to reject noise than a PPM receiver and that anything that takes a PCM to the ground will do it quicker to PPM receivers with just more drama as they flail around prior to the crash.
If the PPM will respond enough to gain marginal control then the PCM will probably do it quicker and better.

On a glitch versus a hold there is absolutely no way of knowing which position the aircraft will be in when it goes into hold or which control surfaces will move which direction in a glitch.

You can look in my Gallery to see the range of aircraft that I am flying on PCM. This is less than a third of the fleet that is on PCM.
I have played with PCM and PPM transmitters set side by side on the same channel and found that the PCM will ignore the PPM signal most of the time but I would never suggest that you should knowingly try to operate in that environment.
Also there is a misconception among even some PCM operators as to how long it takes a PCM receiver to restore when it receives a valid signal. It is in milliseconds, the slow response that most people see is the result of turning their transmitter off and on to test a fail safe setting. The slow response that they see is the transmitter re-booting, not the receiver responding.

Again my only experience is with the Futaba 9 channel PCM receivers so I can speak to no other PCM system

Old 04-14-2006, 12:43 AM
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JohnW
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Default RE: PCM vs FM

Ditto with Gremlin; however I have gone into failsafe before, while in the air, because someone turned on a PPM TX on my freq. I was running Futaba PCM 9ch RX non-synth with a 9ZAP TX with synth. The offending TX I don't remember but it was some 4ch entry level PPM. Like Gremlin said, PCM may be better at rejecting PPM than the other way around, but PPM interference can and will put PCM into failsafe. Maybe not every time under all conditions, but it did happen to me.

Jimmy, I'd suggest you have it backwards. The main reason why PCM is "better" is because of the fail-safes, predetermined positions or hold last known position, etc. You say if PCM enters failsafe, it would result in a crash in a plane... what do you think would happen to PPM... some control? Actually less control. If there is enough signal with PPM for control, there is probably enough for PCM not to go into full failsafe. Under these conditions of partial signal loss, the PCM system would have intermittent control with periods of holding the last known position. Remember it is not in full failsafe yet because some signal is getting thru. Plane is controllable and remains on predictable and commanded path, but controls will be sluggish. With and non failsafe system, such as standard PPM, you would have periods of intermittent control and periods to total chaos... you'll "control" your plane in a random spiral of death right into the ground.

I have only gone into failsafe while in the air once, and it happened in a plane (see first paragraph.) I was in full fail safe for about 8 seconds. The plane remained on a very predictable path, a slight arc at low throttle, and once I regained control, I actually finished my flight without any problems. No crash. I guarantee that if I was running PPM no failsafe, the random servo movements caused by the offending TX over 8 seconds would have created a smoking hole. I guess you can believe what you want, but I think this is pretty clear empirical evidence that some of the PCM vs PPM arguments, or more properly fail-safe vs no fail-safe arguments you saw in other threads are off base.

What do you consider big? A 25 sized plane could easily kill someone. The ability to cut throttle alone is a big plus. You don't need PCM for that, any failsafe system will do.

Great for heli...LOL. Just curious... Ever fly a heli? Failsafe is better than nothing, but if I'm doing anything other than a hover at 3' and enter failsafe, I'm calling heliproz for a crash kit. Even in the hover if I go failsafe, I'm probably a gonner. Helis just don't stay stable as they violate the laws of nature. Maybe a short hit when upright and you'll be fine. But the same could be said for a non failsafe PPM as you'll probably survive if the hit is short enough.

PCM does not glitch IF you mean random servo movements. That is by design, specifically the failsafe feature. But as I posted earlier, any failsafe system I know of, including PPM RX with built in failsafe, also won't suffer servo movement glitchs.

Auto stab? Is that the fail to autopilot idea? That has both pros and cons. The pro is it may save your plane. The con is it may cause your uncontrollable plane to fly off miles into the distance and cause property or personal injury.

Cheers.
Old 04-14-2006, 07:42 AM
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Jimmy Hoffa
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Default RE: PCM vs FM

Maybe the ultimate system will be the new 2.4 GHz DSM technology that Spektrum is now offering for park flyers. Hopefully they will eventually offer a system for larger planes. The issue of two flyers being on the same frequency would no longer exist. The receivers are actually two seperate units that utilize two frequencies for redundant backup. I have heard and read nothing but praise for these new radios.
Phillip
Old 04-14-2006, 04:56 PM
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JohnW
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Default RE: PCM vs FM

What seems to happen a lot in the PCM vs. FM, or more properly PPM threads is there is a lot of confusion as to failsafe, how they work, etc. and that failsafe is not the same as PCM. Going to 2.4GHz spread doesn't by itself address the idea of failsafe. Of course there is no reason why a failsafe feature couldn't be added to 2.4GHz radios. The two freq RX you mention was required to get the link reliable enough to bring the product to market from my understanding. Yes, it's redundant, but I wouldn't read too much into that. There are technical issues on that band that will need to be addresses before you will see many 2.4GHz radios for planes. I'm sure they will probably be solved in the next several years... at least I hope so. Cheers

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