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FM or PCM?

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Old 02-14-2003, 05:28 PM
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Wooki3
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Default FM or PCM?

I have been away from R/C flying for about 10 years. I am looking at getting a new radio so I can do 3D. My LHS is saying that I will need a PCM radio. Do I really need PCM? I never had a problem with any of my FM radios. I am looking at a futaba 9 or a JR 8. I want to know if the $100 plus for the PCM is worth it?
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Old 02-14-2003, 09:04 PM
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sfaust
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Default Re: FM or PCM?

You do not need PCM at all. It sounds like your LHS is looking for additional sales volume, or is severely misdirected.

You can run PPM or PCM in anything currently being flown in the hobby. From .25 trainers to 50% or larger gasoline powered airplanes. If you get a good range check with either PPM or PCM, you should have absolutely no reason to second guess the receiver that you installed.

Here are some posts for additional background info if you want to research the differences between them. But, in a nutshell, you should have full confidence in PPM or PCM.

http://www.rcuniverse.com/showthread...hreadid=473062
http://www.rcuniverse.com/showthread...hreadid=355152
http://www.rcuniverse.com/showthread...hreadid=339931
http://www.rcuniverse.com/showthread...hreadid=246442]
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Old 02-14-2003, 09:17 PM
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Phil Cole
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Default FM or PCM?

Generally the PCM radios also come with better grade servos, so the higher price does get you more than just the PCM.
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Old 02-14-2003, 09:52 PM
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sfaust
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Default FM or PCM?

Thats a good point Phil. The difference in a package deal for PCM with the transmitter might just be offset by the better servos alone, or be drastically reduced. I think you also get a higher capacity battery as well. Its worth looking into the savings, to determine what you are really paying for.


You can use the same transmitter for PPM, so your receiver purchased later can be either. In the end, the PCM receiver might be a wash in the package, and you can then choose PCM or PPM later when buying receivers for new airplanes. There you can save $70 or so going with PPM, or determine that the PCM is worth the extra.
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Old 02-15-2003, 03:55 AM
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Wooki3
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Default FM or PCM?

Thanks for the posts. I guess im wondering if I fly .40-.60 planes if there is much chance of problems with FM. I was told you need to have PCM if you fly any of the gassers because, of all the vibrations etc that can cause failure.
Also I looked into going FM and purchasing a PCM Rx later. It is cheaper to just go PCM right off if I'm planning of going that route.
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Old 02-15-2003, 04:19 AM
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sfaust
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Default FM or PCM?

The short answer;

If you look at what people are flying day in, day out, without any issues, it doesn't make a difference either way. Gasser or Glow. PPM or PCM. You can use either type of receiver with either type of engine, and it will work fine.

The long answer is buried in the references to many posts on the subject in my earlier reply.
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Old 02-15-2003, 04:23 AM
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Default PCM vs PPM

I was told you need to have PCM if you fly any of the gassers because, of all the vibrations etc that can cause failure.
If you have electrical noise anywhere in the aircraft, PCM will not eliminate it for you.
PCM works on a square wave signal with a modulated carrier imbedded in that signal.
Basically it sends a signal to the receiver which it then cross checks and compares to the second carrier. If they match, the signal goes through to the servos. If it does not match, the signal is rejected.
If the signal gets rejected for more than a pre set amount of time, the Fail safe (if programmed) will kick in.
It is kind of like putting a square peg in a square hole. It fits and goes through. Try putting a round peg in and it won't go through.

In helicopters I have found fail safe to be useless. It means crash over here, or crash over there. Either way the outcome is the same. You can get away with it in an airplane because low throttle does not mean an imminent crash.

Peter
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Old 02-15-2003, 05:18 PM
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Rodney
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Default FM or PCM?

Unfortunately the statement above about PCM eliminating noise is totaly false. PCM is just as suceptable to noise as PPM is, it just masks it until to late to correct the problem. Note: most modern PCM is FM as is most PPM or PWM although you can have any of them on AM as well. Most of the first PCM radios all used AM rather than FM. I fly lots of big gassers, most with magnetoes and all are on PWM/FM and never have a noise problem.
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Old 02-15-2003, 05:48 PM
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Default FM or PCM?

I have a PCM radio and I really like it. I don't fly gassers, but just the fact that you can program your radio not to send your plane sprialing to the ground at full throttle in the event of interferance is enough for me. Although I like mine, I firmly say NO you do not NEED one. It's a personal preference. Use either one you want. Hope this helps.
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Old 02-15-2003, 05:49 PM
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Default FM or PCM?

If I'm not mistaken, PCM is only standard FSK modulation (RC equipment is not true FM it uses FSK which is a little brother to FM but more appropriate for sending the data involved) And it's just serial data, no special carrier frequencies are used. The error correction is obtained by sending CRC information along with the data.
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Old 02-15-2003, 07:04 PM
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jbflier
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Default Mine does both PPM & PCM

I don't know about Futaba or Hitec, but my JR 8103 does either. I"ve got 5 airplanes on PCM, and 3 on PPM. I would suspect most other manufactures would do likewise to stay competitive. I am having second thoughts about PCM though. I see more and more gas planes with PPM; and as long as the set up is right they don't have any problems. Hmmm.......
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Old 02-15-2003, 07:41 PM
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Default FM or PCM?

No you do not need PCM.
No one has an application where PCM is manditory. (IMHO)
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Old 02-15-2003, 07:53 PM
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Flyfalcons
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Default FM or PCM?

Any modern radio capable of PCM is also PPM selectable. You could buy a PPM version of a PCM radio to save some cash now, and if you decide to get a PCM receiver later, the radio will be compatible with it. This goes for Futaba, Hitec, JR, and Airtronics.
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Old 02-15-2003, 10:54 PM
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HarryC
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Default Re: PCM vs PPM

Originally posted by Climate
PCM works on a square wave signal with a modulated carrier imbedded in that signal.
Basically it sends a signal to the receiver which it then cross checks and compares to the second carrier.
I think you have been mis-informed. The carrier is FM. The PCM data is embedded in the carrier FM, not the other way around. It certainly does not cross check the data with a second carrier. There is no second carrier. It just does basic error detection using a checksum bit. Some systems send full position data, followed by change data so that if the full position data is detected to be corrupt the change data can be checked and used if it is not corrupt.

Error detection and failsafe programming is now available on some PPM receivers. Since PPM and PCM use the same FM carrier they both experience the same interference. Nevertheless, PCM remains superior at getting some usable data through to the Rx after the point at which the PPM signal has become useless, because of the way it handles data packets, and the importance of the time location of the signal edge in PPM. This is not a radio matter, it is a data matter and would be equally true if the PPM or PCM signal were being sent via wire rather than radio. But PCM should never be used to mask problems like spark ignition.

Harry
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Old 02-15-2003, 11:02 PM
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Default That is what I meant to say

The carrier is FM. The PCM data is embedded in the carrier FM, not the other way around. It certainly does not cross check the data with a second carrier. There is no second carrier
Thank you. That is what I meant. I guess it just came out wrong when typed out.
I was thinking of my JR ABC&W receivers.
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Old 02-15-2003, 11:10 PM
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Default FM or PCM?

JR's ABC&W Rx don't get second carriers or do error detection comparisons with other data either. The ABC&W is JR's patented method of blocking image frequencies, which everyone else uses double conversion to do. It has nothing to do with PCM, PPM, or the data!

Harry
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Old 02-15-2003, 11:48 PM
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Default FM or PCM?

RC FM is not true FM it's FSK. The transmitter is either transmitting at the carrier frequency or the shift frequency. The difference in timing between the shift in pulses is where the data is encoded. The PPM signal is encoded this way, using one frequency for 0 and the other for 1. True FM is designed for audio and micro fluctuations in the carrier frequency are where the encoded signal is stored. RC isn't variable, it's full on, or full off. Yes, to a lot of people this is considered splitting hairs, but it is how they actually function and it's best to refer to them that way. It's 'technically' FM because the frequency is shifted from one to another to get the modulation, but it's actually FSK which is a subset. True FM is a little harder to tune because it's easier to accidentally over modulate the signal because it's purely analog in nature. FSK is binary in nature, and because you're dealing with full on or full off it generates harmonics a lot easier. The further away from being a sine wave the more the harmonic generation, and FSK is square wave so it's definitely as far from a sine as you can get.
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Old 02-16-2003, 03:20 AM
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Phil Cole
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Default FM or PCM?

Originally posted by HarryC
JR's ABC&W Rx don't get second carriers or do error detection comparisons with other data either. The ABC&W is JR's patented method of blocking image frequencies, which everyone else uses double conversion to do. It has nothing to do with PCM, PPM, or the data!

Harry
JR's way of blocking image frequencies is to not sell crystals for channels with images within the RC band. This only applies in the US, where the 72 MHz band is wide enough for the problem to occur. If you look on Horizon's web site you'll see that crystals for single conversion JR receivers are not available for the bottom few channels. Even if the crystals were available, they'd probably get away with it most of the time since the images are on the 10 kHz slots between the RC channels. The 10 kHz offset means that the IF will be able to partially reject the interference.

ABC&W is just their way of saying that they used good design practices (strong front end, AGC, and good IF filters). These things can be applied (or not applied) to all receivers, no matter how many frequency conversions they have.
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Old 02-16-2003, 07:30 AM
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Default FM or PCM?

It's called harmonics not images. It's unavoidable in the RF world, you can help reduce it, but it can NEVER be eliminated (The only way to have a 0 harmonic wave form is to have a perfect sine wave) A perfect sine wave carries no data. I wish more companies did have good design practices. Just think of the cost of a modern day cell phone. Think of how many channels are crowded on their bandwidth and how well (considering traffic) they perform. R/C equipment manufactures are out to make a buck, not break new ground in functionality. Or we'd all have high speed bidirectional data links to whatever you fly. They keep just enough innovation in the market to keep people buying new products. (With a few exceptions) Then again the markets tough too, so it's hard to blame just corporate America.
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Old 02-16-2003, 08:53 AM
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Default FM or PCM?

Quote by Lynx.
------------------------------------------------------------------------------------
"square wave so it's definitely as far from a sine as you can get".
------------------------------------------------------------------------------------

Whoops !! But a square wave is made up of Sine Waves, therefore they are one of the same.

Harmonics are multiples of a fundamental frequency. Each harmonic generated from that fundamental has less power and does not become a significant interfering factor unless amplification and/or tuning takes place for that specific harmonic.
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Old 02-17-2003, 05:01 PM
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Default FM or PCM?

I have flown in Utah for 50 years, AM,FM,PPM,PCM and have had no interferance on any. I would buy PPM just because of cost you can add a PCM reciever any time. Where are you located in the state. I fly in Salt Lake and St George.
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Old 02-18-2003, 04:08 AM
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Wooki3
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Default FM or PCM?

I'm in cache valley. I fly at the logan airport.
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Old 02-24-2003, 03:38 PM
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Default FM or PCM?

Hi Wooki3
I live in Wellsville and fly at the Logan Modelport at lease 5 time a week with PCM and PPM I have never had a problem with interferance. You do not need to have PCM
I would love to hookup with you and fly
Please let me know when you will be there and I would be happy to help you
Thanks
Rcman
AKA David Olsen
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Old 02-24-2003, 04:50 PM
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Default FM or PCM?

Originally posted by trailingedge
I have a PCM radio and I really like it. I don't fly gassers, but just the fact that you can program your radio not to send your plane sprialing to the ground at full throttle in the event of interferance is enough for me. Although I like mine, I firmly say NO you do not NEED one. It's a personal preference. Use either one you want. Hope this helps.
But you don't a PCM receiver for anymore. Look at the Multiplex IPD receivers (PPM with fail safe. And I beleive the FMA M5 has similar caracteristics.

PCM is slowly going the way of the dinosaur.

Multiplex introduced it to radio control and now they have eliminate it for better equiplement.
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Old 02-24-2003, 05:17 PM
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Default FM or PCM?

not to get off the sine wave topic or anything but i have a really silly question. am i understanding people correctly when they say you can get a ppm TX (say the futaba 9caf) and still use a pcm receiver? if that's the case why would anyone buy the 9cap? or does that just mean you can use a pcm receiver but it will only function as a ppm receiver? which begs the question why would anyone spend more money on a pcm receiver without getting pcm functionality.

sincerely,

obviously confused
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