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Radio PCM or FM

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Old 10-31-2006, 10:05 AM
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aerospot
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Default Radio PCM or FM

It's been several years (14) since I have purchased a new radio.
I put the wife on notice that I want a new computer radio for christmas this year...
Now I'm on a hunt for which one to request... They have changed a bit since my last Futaba 6 channel FM...
What is PCM and how does it differ from FM?
What about choices in different computer 6 channels? What radio do you prefer? What is the difference between "digital servos" and the standard ones I use now?
Seems I'm learnin this stuff all over again! I used to have a radio for each plane, each on its own frequency. Now I have the option to program several planes into one transmitter?? Do I need to have crystals for each frequency channel to insert into the transmitter? Are the frequencies pre-programed??
Christmas is approaching and I need to have the right information else she will just get what she thinks I want!

Dave
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Old 10-31-2006, 10:25 AM
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Default RE: Radio PCM or FM

Wow lots of questions.

Start here: http://www.torreypinesgulls.org/Radios.htm
as for digital servos: http://www.futaba-rc.com/servos/digitalservos.pdf

As for the ppm/pcm debate most modern computer radios will do either on a per model basis so even if you started out with a ppm setup to save some money you can always add a pcm reciever for another model if you wanted or needed one.

As for synth modules, they let you dial in any of the legal rc channels, you either need a synth reciever or a receiver crystal for any channel you want to use. IMO its only worth it if you goto a lot of fun flys or are at a busy field with lots of freqency conflicts.
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Old 10-31-2006, 12:53 PM
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Default RE: Radio PCM or FM

The above post is correct.but about changing crystal in your transmitter.That is not permitted by the fcc in the usa.rx crystal can be changed but not the tx.you can chane the rf module which can be on a different frequency or go with the syntheziser module..I have one tx with 6 rx tuned to it all on the same frequency.I prefer pcm over fm as I feel that if I am flying a mutli engine aircraft that took along time to build and the cost I have a bit of protection from being shot down from interference or some one turning on a stray tx.as pcm in the event of aan unregonized signal will keep the last known good signal and if no good signal it will go to a fail safe setting which is set by the flier.I have mine set for engines off and all else at neutral.least I won't have a fly away and may get some equipment back after the touch down .
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Old 10-31-2006, 04:08 PM
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Default RE: Radio PCM or FM

If I were in your position I would not hesitate to get the new Spektrum 7 channel radio announced recently by Horizon Hobbies.

http://www.horizonhobby.com/Products...ProdID=SPM2710

Hold out for this one! It will blow all the others away. Digital Spread Spectrum. Latest and greatest in radio technology. No more concern whatsoever with channels. This one is going to revolutionize the r/c hobby.

I have been using the earlier Spektrum 6 channel (which is supposed to be used only for park flyers) on electric helicopters. Not one single hint of a glitch or any problem of any kind. Four of us fly together and all use the Spektrum. I can't tell you how nice it is to be completely unconcerned about what channel someone else is on!
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Old 10-31-2006, 04:13 PM
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Default RE: Radio PCM or FM

One thing I forgot to mention is that there will never again be a concern over antenna routing. The Spektrum receiver has multiple antennae but they are only about 2 1/2 inches long.
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Old 10-31-2006, 04:17 PM
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Default RE: Radio PCM or FM

G'day Mate,
All the above posts are true, just one point FYI, both types of receivers are FM, the type of modulation determines whether they are PCM or PPM. Pulse code modulation, or Proportional Pulse Modulation.
If you want to read the pros & cons of both, with lots of technical stuff, check out this site.
http://www.torreypinesgulls.org/Radios.htm
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Old 10-31-2006, 04:40 PM
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Default RE: Radio PCM or FM

What is PCM and how does it differ from FM?

PCM is a "clearer" signal. PCM also lets you choose a "failsafe" position for your servos should you get some sort of interference. Usually most people set this to engines off. AMA mandates jets to be failsafed to turbine off. This will not work in prop planes if the rx battery is disconnected or you have a switch problem.

FM=PPM is still not a bad choice but for the money pcm has better resolution (no jitteryness with servos eg)

PCM and FM(ie PPM) are based on the 50-60 channels available to us on the 72, 27 and 50 mhz band. Here in the usa that is. We all have to share these. Spektrum is a new technology that runs in the the 2.4 Ghz range. Much like our cell phones. Until recently, this technology was limited to park flyers and R/c cars. There is a 7 channel system that is now available to "us" (the people that fly REAL airplanes J/k, its a joke, laugh come on...) Unfortunately, jets and large airplanes, chew through channels and 7 channels is really tough to make work in jets....[&o]

What about choices in different computer 6 channels?

What choices? FM(ie PPM), PCM, Spectrum??? I believe all the above are available in 6 channel variants.

What radio do you prefer?


Well, THATs a loaded question....With my props and electric stuff- futaba...With my jets JR. Why?? Props=futaba, turbines=JR I don't know why, it just is...


What is the difference between "digital servos" and the standard ones I use now?


In a nutshell....hmmm....digital servos center better and will put out maximal torque at minimal deflections.....Oh and they cost more!![sm=greedy.gif]

Seems I'm learnin this stuff all over again! I used to have a radio for each plane, each on its own frequency. Now I have the option to program several planes into one transmitter??

Isn't technology great??!!! My advice is to get a radio that lets you name your airplanes, not just asign a number to it. It's hard to remember over time which number corresponds to which airplane....

Do I need to have crystals for each frequency channel to insert into the transmitter?

Well that depends. You can also buy synthesized radios (only on PCM and FM(ie PPM)) Synthesized means you can program the transmitter to transmit at the freq YOU want! Great huh? Now your recievers should also be synthesized to maximize this feature.(I believe only PCM recievers have a synthesized option ~$200 a pop) Your rx can also be programmed to accept only a certain freq that YOU decide!! (It should match your transmitter output freq...)

If you buy spektrum, then this is not something you EVER have to worry about.

Are the frequencies pre-programed??

If synthesized, you program them (regardless of PCM or FM(PPM)), if spektrum, then nope!!

Hope that helps.
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Old 10-31-2006, 04:46 PM
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Default RE: Radio PCM or FM

Alan, thats an awesome link, I'm going to have to bookmark that one!
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Old 10-31-2006, 06:42 PM
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Default RE: Radio PCM or FM

Great info guys. Thanks!
Looks like my options weigh a lot on how much $$ I have to spend...
But even on a budget I seem to have way more bang for my buck over my old radios!

Looking over my stock of Attack 4 and 6 channel radios I find many with failing TX batteries. And the replacments for them are going to cost me pleanty. So it makes since to ebay some of my old stock and replace em with one computer type multi-model type...
I dont think I can afford $200 per RX and will most likely opt for FM. I've never owned anything but Futaba except for a 7 channel Kyosho that came with my first trainer.

The links have been valuable in getting me up to speed and educating me on the new technology. Like I said, learnin all over again!
So many options!! But then, Christmas is still a couple months away so I have time to decide...

Thanks Again!

Dave
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