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stuppid newb Q

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Old 01-20-2007, 07:30 PM
  #1  
tonywayne
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Default stuppid newb Q

I know what AM and FM is but why is one preferd over the other in this hobby. And what is PCM, QPCM and any other acronims that iv seen befor about radios but cant remeber them?
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Old 01-21-2007, 04:45 PM
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Default RE: stuppid newb Q

AM is more prone to interference than FM.

PCM is Pulse Code Modulation, I guess the best way I am able describe it is digital encoding of the FM signal.

Each manufacturer has their own versions of PCM. JR for example has ZPCM and SPCM,I think the only difference is that SPCM has a servo resolution double that of ZPCM, making it very accurate.
PCM is different between all the makes (JR,Futaba,Hitec.....ect) that's why there are so many different acronyms for it.

Hope that helps
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Old 01-21-2007, 08:02 PM
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Default RE: stuppid newb Q

Thanks iraiam. After some research and help from other rcu members im understanding better. But im still a little in the dark. Is PCM better than FM? How would i notice an improvement if i used PCM over my older FM radio. I gues what im saying is i just cant quit get the answer im looking for in PCM. The only answer im getting is ''its Pulse Code Modulation'' and digital encoding. I understand what that means is that theres a digital signal coded in the sign wave, But pretend im old old school that never even heard of sign waves and analog, digital, and descrete signals, what would PCM mean to me? Thanks for you info................. Tony
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Old 01-22-2007, 09:05 AM
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Default RE: stuppid newb Q

In my opionion the only advantages of PCM is the faisafe ability, and servo resolution.(for lack of a better term)

in the case of FM lets say you get some radio interference, the servos may go haywire and glitch all over.

in the case of PCM, you get radio interference, the servos will "hold" until a radio link is re-established, also most PCM failsafe systems have either the hold failsafe method OR you can select a specific positions for the servos during failsafe.

lets compare servo resolution, lets say FM has 512 servo positions throughout the stick movement on an axis, and PCM has 1024 making for more precise movements.
I have seen ads for futaba "2048 PCM" I think this would be even more precise.









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Old 01-22-2007, 03:51 PM
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Default RE: stuppid newb Q

I believe that in actuality, AM or FM systems, being purely analog would theoretically have an infinite number of "steps" available to them....limited only by servo resolution, gear/linkage slop, etc.

PCM systems are limited to a MAXIMUM number of steps...how ever many that may be owing to the bit count involved.

PCM systems are slower in response time to FM systems...sometimes noticeably so, but don't tell anyone that.

The major benefit to PCM (over FM) is the failsafe function. As was noted in another post, the PCM system will ignore invalid or corrupted signals by "holding" servo position until a proper signal is received. An AM or FM system will just go crazy.

I prefer the PCM system simply because it can fly through brief encounters of interference. The "failsafe" function is IMO...very worthwhile if for no other reason than safety. If the radio loses signal...it's going to crash. If it loses signal for some reason near the pits or specators? At least the model won't go wild on me unexpectedly. It'll sail calmy into the ground at low throttle.

Now, there are newer spread spectrum systems available that are quite affordable, and use a much more advanced technology to maintain a positive link between you and your model.

Have fun!





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Old 01-22-2007, 04:50 PM
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Default RE: stuppid newb Q

Hi RaceCity,
You beat me to the "punch" on the PCM resolution issue, 100% correct
Now as to the value of "crash safe" that debate will never be settled, as a sport flyer I see little value to PCM for myself, and feel it is not needed for the entry level flyer. It's kinda like the ultra luxury car dealer scenario, "if you have to ask the price, you can't afford it", so with PCM, if you have to ask what it is, you don't need it.
Now please let me make it clear that I don't in any way mean that as a put down on tonywayne, as his question was a very good question and anyone new to the hobby would be expected to have his head going in circles with all the accronyms we use
Regards,
Pete
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Old 01-22-2007, 05:57 PM
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Default RE: stuppid newb Q

Well...I don't disagree with you on the failsafe issue, but I'd still rather have it if for no other reason than the safety aspect of a low-throttle failsafe. Provided you remembered to set the throttle failsafe position. Many don't and effectively negate the benefits of the system entirely. The rest of it is pure guesswork, as it isn't possible to anticipate what attitude the model would be in if/when a signal loss should occur. A "last position hold" system for the flight controls is as good as any in that respect. You're going in if the signal loss duration is long enough. The issue I have with the pure AM or FM rigs is that they can/do go WILD in the presence of interference, and seemingly always wind up at WOT, inverted and headed straight at the ground. Go figure.

My first 1024PCM (Futaba) system was purchased in the late 80's. Then, it was commonly believed that the system could somehow magically "save" an airplane. I think we all know that's not true, but that wasn't really understood by the lion's share of hobbyists at the time.

In the final analysis...there's better available to us now in the form of S/S. Perhaps limited in selection at the moment, but entirely feasible and affordable enough so that anyone planning to remain in the hobby for awhile could easily get a good return on their investment in terms of enjoyment without the constant spectre of "glitches" looming over and threatening to end the life of their new airplane. Heaven knows we'll devise new and improved methods of destroying airplanes in the meantime.






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Old 01-22-2007, 06:28 PM
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Default RE: stuppid newb Q

My terminology may be incorrect when I say "servo resolution", but I went from FM to Spread Spectrum with a DX7, I found that the sticks were notecibly more sensitive and precise, enough that I increased the EXPO settings so I could fly as "neatly" as I could with FM ZPCM and minimal EXPO. I also noticed a difference between JR ZPCM and SPCM the SPCM was more sensitive.

but you're right the servos themselves don't have a number of "steps".
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Old 01-23-2007, 06:57 PM
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Default RE: stuppid newb Q

pilotpete2 TY for the consideration but im no where near being new to the hobby. Iv been flying for years and have only had a few FM radios for all my models. Being just a sport flyer, radio shoping has never been a big issue with me. All i ever looked for was good price, FM and 4 channels. Now that my collection of aircraft has grown somwhat iv been shoping for a computer radio for all my planes. Just wanted to get cleared up on what PCM really was and how i would benifit from it .
By reading through your guys replies I have now made my dicision '' im gonna stick with FM''. Now i just have to decide what brand and how many channels i want.
I have one more Q. I discovered the spectra module like on hitec radios and its got my intrest. Like iv said iv never used a computer radio befor and the main reason i want one is to store multible airframes on one TX. My Q is how do most ppl do this? Do u simply pick one channel and make all your models for that TX the same channel? My models r not all the same channel and i was thinking of getting the spectra module for this so i wouldnt have to buy all new crystals for my RX's. Or would i be better off not getting the spextra module and just going and making all my RX's the same, becaus it seems like it would be more of a headache in the long run changing the spextra module channel everytime i want to fly a different plane. Dont you have to take the TX apart to change freq. on the spextra or is it on the outsaide? Thanks alot for your help guys...... Tony
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Old 01-23-2007, 07:00 PM
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Default RE: stuppid newb Q

Oh yah i posted the topic '' stuppid newb Q'' to try and get more attention and i gues i am new to the PCM radios. Tony
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Old 01-23-2007, 07:11 PM
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Default RE: stuppid newb Q

All non Spread Spectrum radios are in the same boat - one person at a time on one channel.
There are some really good systems for this
In small models (electric powered foamies for example ) the RX available are countless it seems because of the short range needed.
All of em tho are subjest to glitching due to internal electrical noise or mechanical induced "noise".
All of em
If you are afraid PCM is complicated -don't be - it is a system which is almost glitch proof but not interference proof if the problem is "just right" and that may be impossible to determine some times.
Depending on how many people are around who use the same channel you have- one tx frequency and the same frequency fo rx is fine.
I am sold on SS- but that does not mean you should be . look at both BASIC approachs to control and then decide.
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Old 01-24-2007, 02:27 PM
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Default RE: stuppid newb Q

ORIGINAL: tonywayne

I have one more Q. I discovered the spectra module like on hitec radios and its got my intrest. Like iv said iv never used a computer radio befor and the main reason i want one is to store multible airframes on one TX. My Q is how do most ppl do this? Do u simply pick one channel and make all your models for that TX the same channel? My models r not all the same channel and i was thinking of getting the spectra module for this so i wouldnt have to buy all new crystals for my RX's. Or would i be better off not getting the spextra module and just going and making all my RX's the same, becaus it seems like it would be more of a headache in the long run changing the spextra module channel everytime i want to fly a different plane. Dont you have to take the TX apart to change freq. on the spextra or is it on the outsaide? Thanks alot for your help guys...... Tony


Where my brother and I fly we have some guys who have everything one one channel and some who use multiple channels.

I'm on 47 and 49. My brother is on 44, 47, and 49. We use a Futaba 8U transmitter with the Futaba synthesized module.

The Hitec Spectra module has to be removed from the transmitter to access the dials to change channels.

The Futaba synthesized module (for the 8U & 9C) does not have to be removed to change channels.

I do not know how any of the other brand synthesized transmitters work.
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