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HomeAircraftHelicopterBeginners Corner › PPM / PCM
10-11-2005 08:45 PM  12 years agoPost 1
FireStarter

rrApprentice

England

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As I'm kinda new to all this heli stuff can someone tell me the difference between PPM and PCM on transmitters cos it's all confusing to me,

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10-11-2005 09:07 PM  12 years agoPost 2
BarracudaHockey

rrMaster

Jacksonville FL

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PCM is more immune to interference than PPM and it provides a fail safe position for the servos if signal is lost.

It also costs more. I fly both and haven't had any problems with ppm that were not self induced. If your flying something with an ignition then PCM is the way to go.

Andy
AMA 77227
http://www.jaxrc.com

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10-11-2005 09:46 PM  12 years agoPost 3
Tron

rrApprentice

Morgan Hill, CA

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Just to add to the above, most mid to high end transmitters can transmit in either mode. It is the receiver that needs to be ppm or pcm capable.

Some people like the failsafe function while other prefer to be hit and have intermittent control in case of glitch in flight.

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10-11-2005 09:55 PM  12 years agoPost 4
BC Don

rrElite Veteran

Calgary, AB Canada

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Well, PPM will pick up interferance (noise) quicker and will have a more negative impact when it does. A glitch could cause your throttle to open wide or collective to go full negative or cause weird interactions between multiple controls. I've tried chasing them down and it can be tough.

PCM is faster and if you get a hit everything goes to the fail safe position (which for throttle should be idle), So, you won't suddenly get a glitch where collective goes full negative, cyclic goes one way, the trail rotor goes another etc. (not that you'd see a lot of that under PPM either).

However, if you are getting glitching, you'll pick it up pretty quick under PPM. You won't see anyting at all under PCM unless it is so bad you go to fail safe.

Got Money? Send it to me, I'm a Heli Addict.

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10-12-2005 03:28 AM  12 years agoPost 5
trevorz

rrApprentice

Calgary, Alberta Canada

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I'll add some thoughts on this.

PPM - Pulse Position Modulation. It is an entirely analog signal. So whatever 'noise' is in the signal it will get transfered to the servos. It is like the old analog cell phones. They work well, but you hear the back ground noise. PPM will not hide any RF interference. It is also the fastest system (from Tx to Rx). The frame rate at the servo is faster with PCM, but PPM is quicker overall.

PCM - Pulse Code Modulation. It is a digital signal sent on the FM carrier. Again, using the digital phone analogy, it 'hides' any noise. However, the drawback is a slower signal from the Tx to the Rx, because it needs to digitize the info and send it to the receiver. The other disadvantage of PCM is because it does hide any RF noise, you will not know about it till you get A LOT of it. Which means a lock out and complete loss of control. Just like a digital phone, it either works completely, or you got nothing.

For most applications either is fine. As was mentioned earlier the Tx is usually capable of both, and you need the Rx for it. You also cannot mix brands of receivers with a single transmitter. If you have a Futaba transmitter you must use a Futaba receiver.

For even more explaination I found this web site that does a pretty good job of explaining it.

http://www.rcfaq.com/answers/radio/

Trevor.

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10-12-2005 07:46 AM  12 years agoPost 6
trvo

rrVeteran

Bognor Regis,UK, Aurorra Ltd

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I have also flown PPM and PCM, I will not touch PPM now as it is not suitable for our flying site. We have underground pipes with running water under our field which causes huge interference. I once had a PPM Rx in an F1 Carbon, glitches were causing my heli to roll and loop on it's own as I had it rather twitchy...Never had a problem with PCM having lockout at the same field, although did have lockout once in an auto at a different site with my Sceadu which I found to be a faulty Rx...

In my opinion, your primary concern should be the safety of the other people there, the model and your wallet come second. With PCM there is a predictable flightpath in lockout but PPM it goes all over the place. A predictable flightpath allows people to get out of the way...

Just my opinion...

Trev

-----
Trex 600e
Trex 600n
Aurorra Ltd, Align UK, Align Factory, CSM, Quick UK, Heli-Lessons

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10-12-2005 04:56 PM  12 years agoPost 7
FireStarter

rrApprentice

England

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Cheers for the replies guys, It makes more sense now.

So much for getting into this hobby to pass a bit of time every now and then,

There's so much to learn!!! It's totaly addictive.

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