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PCM or Fm

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Old 08-15-2003, 01:54 AM
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t8jones
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Default PCM or Fm

Hello everyone, I am just a beginner and I was wondering if anyone knows of a good website that explains the differences in AM, FM, and Pcm? I am just wanting to learn for future reference because when I go to buy another plane, I will probably get a new radio that will last me for years. I have heard that the futaba 9c is a good transmitter, but I have also heard about PCM- whatever that is? If anyone has any info let me know.
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Old 08-15-2003, 02:20 AM
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GalenB
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Default Re: PCM or Fm

Originally posted by t8jones
Hello everyone, I am just a beginner and I was wondering if anyone knows of a good website that explains the differences in AM, FM, and Pcm?
This one! Do a search for FM and PCM and you will find a wealth of information...
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Old 08-15-2003, 03:22 AM
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randy41
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Default PCM or Fm

PCM = Pulse Code Modulation
PCM systems use digitally encoded signals to minimize interference and provide today's most advanced R/C control.

PPM
Pulse Position Modulation. Another term for FM.

FM
Frequency Modulation. This describes the mode of transmission of radio signal from transmitter to receiver.

AM
Amplitude Modulation. This describes the mode of transmission of radio signal from transmitter to receiver.
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Old 08-15-2003, 06:55 AM
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Default PCM or Fm

Originally posted by randy41
PPM
Pulse Position Modulation. Another term for FM.
Commonly used as interchangeable terms but utterly wrong. PPM has nothing to do with FM. Your PCM radio is also an FM radio.

PPM and PCM are ways of encoding information, like Morse code is a way of encoding info. They are about data, not the means of transmission. They have to be sent then from one place to another. We could send PPM and PCM by smoke signals, flashing lights, honking a car horn, as electrical signals along a wire etc etc etc. You can transmit PCM but not PPM just by clapping your hands. PPM and PCM are absolutely nothing to do with radios whatsoever.

To send that PPM or PCM data by radio, the radio can choose AM or FM. We actually have 4 choices: PPM AM; PPM FM; PCM AM; PCM FM. AFAIK no-one bothered making a PCM AM and very few now make PPM AM.

So the radio part of your radio is most likely FM unless you still use AM. The data part of your radio is either PPM or PCM but the radio part of your radio knows nothing about that, all it sees is an on-off signal that has to be put into FM or AM.

H
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Old 08-15-2003, 10:36 AM
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The 9C does both so you can try them both and make your own choice. I have a 9C and fly mostly (all but one) FM PPM. One plane is PCM and I can't tell any difference in the controllability.
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Old 08-15-2003, 01:12 PM
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Originally posted by CsteveC
I have a 9C and fly mostly (all but one) FM PPM. One plane is PCM and I can't tell any difference in the controllability.
You will the first time someone turns on a radio in the pitts on the same freq. If you are on the PPM plane, it will tuck and go straight in, if you are on the PCM, you won't notice it most likely. That is why I fly PCM. I have been shot down before, but have had guys turn on when I am up with PCM and never got a glitch.
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Old 08-15-2003, 01:39 PM
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Rodney
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Another good choice is Hitec's Eclipse. We have at least 6 in our club (all with Spectra modules) and all have been very good. You can run most any brand FM radio with them and; if you want PCM it is also available. However, all PCM radios are usually only compatable with whatever brand the receiver is and not even then all the time as that technology is constantly changing and there is no general agreement between the different manufacturers as to how to encode their PCM signals.
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Old 08-15-2003, 01:44 PM
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HarryC
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Originally posted by FLYBOY
if you are on the PCM, you won't notice it most likely.
I appreciate what you are saying, and your own experiences, but beware of giving less experienced people a false sense of the security of PCM. They might think this makes PCM almost infallible and safe, which can lead to slack frequency control, as you have unfortunately experienced but got away with it. A few years ago a boy was killed in Britain when a PCM model went into failsafe, the reason was never proved but is suspected to have been someone else in the pits switching on. The only death in Britain caused by interference was to a PCM model, not a PPM model. PCM can sometimes be better than PPM, but it is never a safe system.

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Old 08-15-2003, 04:52 PM
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Default PCM shot down?

We have a gentleman at our club who will no longer fly his really expensive planes at our field. He has only PCM stuff and has been shot down twice. FM is FM and two transmitters on the same freq. WILL enterfere with one another. If it hasn't happened to you then you are lucky. Personally, I count on the freq board to keep me out of trouble. That and common sense.

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Old 08-15-2003, 05:17 PM
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Default PCM or Fm

The key to flying PCM safely is to SET UP YOUR FAIL SAFES!!!!!!!!!

Most all PCM receivers have a default fail safe of; retain the last position.

The modeler in Britain that was flying the plane that killed the boy HAD NOT SET HIS FAIL SAFES, so the model went in at full throttle.

My personal opinion is to set the throttle channel to close the throttle and leave the rest of the channels at the default setting of retain the last position. With these settings if the receiver goes to fail safe, the engine will go to idle thus alerting you to the problem, and hopefully giveing you time to possibly do something about the problem.

With PPM if there is interference the receiver will respond randomly and will ramdomly drive the servos, if you are low the plane will be into the ground before you have time to yell "I ain't got it".

With propery set fail safes the engine will go to idle and the plane will continue in the same direction it was going.

You cannot fly through an interfering transmitter with either PPM or PCM. But PCM does give a little additional margin of control.

Nuff said, this is a never ending discussion!

Jerry
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