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FM vs. PCM

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Old 07-17-2003, 03:10 AM
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Marc_Haase
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Default FM vs. PCM

Hey All:

So I am purchasing a new radio, of the 8 (maybe 9) channel type and need to know the difference between PCM and FM. To be used on 4 engine bomber.

What are the major differences and is the PCM worth the extra dough?

Thanks in advance for any response....

Marc
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Old 07-17-2003, 03:55 AM
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Default FM vs. PCM

http://www.rcuniverse.com/showthread...376&forumid=27]
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Old 07-17-2003, 08:37 AM
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Default FM vs. PCM

Basically, PCM takes your radio's FM signal and 'scrambles' it. Then the PCM receiver descrambles it and utilizes it. Random noise from other items are not going to be mistakenly read as proper servo instructions as can happen with an FM.
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Old 07-17-2003, 11:38 AM
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Default FM vs. PCM

In my opinion, generally not worth the extra cost, especially for additional Rx's. One of the primary benefits is the ability to pre-program the Rx to send the servos to a pre-determined postion if there's a major radio "hit".

From the stuff I've heard and read, the majority of people with PCM gear have never programmed their gear, so when the "hit" comes the plane goes in anyway.

As for "random noise"; in my limited experience (8 years), I may have been "hit" once, and I have NOT seen more than one or two other crashes that I think might have been caused by radio noise.

Maybe if you fly in an area known to have problems it would be different, I don't know.

For what it's worth.
Dennis-
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Old 07-17-2003, 06:38 PM
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Default PCM

Can't say much about people who don't program the PCM features. It only takes a few seconds.

Luckily I've only experienced it once. Had a Kadet Sr with a full tank of fuel at about 400 ft. Transmitter fuse blew. PCM shut off the engine and gave me 1/8" of right rudder, 1/8" of up elevator. After a period of big lazy circles it landed about 3/4 of a mile from the field and the only damage was to the landing gear.

Let's see, 4 engine bomber, running at full speed, transmitter fuse blows, it takes off without control...
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Old 07-18-2003, 10:35 AM
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Default FM vs. PCM

Staggerwing,

You were fortunate. Next time, next aircraft, next flight the situation may be different. There is no position to which you can program your flight controls that will save your bacon even a majority of the time. We have a lot of guys that like to fly low inverted passes. Most default control positions you would program would kill anything if control communication was lost while doing so.

Short of auto-pilot, there isn't a way to save your plane from an extended loss of signal. I am just not sold on PCM default programming as an answer.

Bedford
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Old 07-18-2003, 11:11 AM
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Default FM vs. PCM

Originally posted by beepee
There is no position to which you can program your flight controls that will save your bacon even a majority of the time....
I was hoping to stay out of this round of ppm vs pcm but here we go again.

fail-safe is not about saving the plane, it's about saving potential victims of the plane by preventing a flyaway and reducing impact (throttle to idle/cutoff).

If I'm not in control, I want my plane on the ground, within the safe confines of the field, not in the windshield of some poor sole on a local highway.

my fixed wing fail-safe settings:
throttle to idle, sticks to the corner
reasoning
reduce impact and distance travelled

my heli fail-safe settings:
- throttle to idle, all other controls on hold
reasoning
best chance of decaying headspeed (inertia) before impact while limiting distance travelled.

I am just not sold on PCM default programming as an answer.
If you choose not to decide, you still have made a choice. If you fly pcm and don't program fail-safe, you've chosen HOLD (default). This is the most likely decision to result in a high speed straffing of innocent bystanders.
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Old 07-18-2003, 04:41 PM
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Default FM vs. PCM

Another feature of the Futaba PCM receiver is the low battery warning. It will move the throttle to idle if the battery runs low. I have seen several situations where this could have saved the plane.

Tom
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Old 07-18-2003, 09:02 PM
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Default PCM vs. FM

Well needless to say there are several opinions that go both ways. I believe the theory that the fail-safe option will help reduce chance of horrible accident versus saving the plane. At that point in time the plane becomes secondary and other people and their property scoot to the top of the list. I actually witnessed an unfortunate incident where a man died from an rc plane, had the pylon racer had a fail-safe option, that man would be alive today. The racer pulled out of a race, lost signal and it came straight down at FULL speed and impacted with the man. Not a pretty sight.

Well I think I will shell out the couple extra bucks, get the PCM Tx and Rx and feel better about it. Seems as long as I range test it properly I should be no worse for the wear.

Thanks all for your opinions.....what a great forum for us Rc'ers.

Happy flying.....


p.s. I found a Futaba 8uhfs (NIB) which is a PCM TX, all I gotta do is get the PCM Rx to go with it and I am ready to go. Only drawback is that the TX is setup for Helis, but I dont think that will matter much. 8th channel is wheel, which should be perfect for flaps. And for $299 NIB with all servos etc, cant go wrong..

Any thoughts on this model TX? Should I go ahead and get the 9CAP?

Thanks again,.
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Old 07-19-2003, 12:11 AM
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Default FM vs. PCM

I am slowly going to PCM because of a situation that happened at the field. My transmitter started powering down intermittantly due to a loose battery connector. My plane took off and hit a flight station. People could have been injured. With a PCM radio the plane would have set still and ran that way until the tank was dry.
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Old 07-21-2003, 10:29 AM
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beepee
 
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Default FM vs. PCM

Jim,

I applaud your willingness to immediately put you airplane in the ground in the event of a radio hit. For the circumstance above where there was a permanent lose of signal that would be appropriate. In the more likely event that you experience a short term lose of signal it must be very disconcerting to have your plane jump into a snap roll or similar gyration suddenly every time there is a glitch. At least you would know when it happened. At any rate, I expect VERY FEW other fliers subscribe to your solution.

Bedford
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Old 07-21-2003, 11:58 AM
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Default FM vs. PCM

Originally posted by beepee
Jim,

I applaud your willingness to immediately put you airplane in the ground in the event of a radio hit....
Don't applaud too quickly, the aircraft will only be lost if signal loss is of long duration...

This is a common mis-understanding with how PCM behaves. A short duration glitch will likely go unnoticed to most unfamiliar with pcm. To those familiar with PCM, intermittent or short duration interference is experienced as a general "notchy" feel as the servos fail to respond to dropped (corrupted) frames.
Fail-safe is not engaged until after a long (in radio terms, several frames) duration of lost signal. Even then, when good signal is received, the receiver responds almost immediately (~2-3 frames I understand). I've been "hit" before and managed to save the aircraft, truly scary but less drastic than an out-of-control situation I would have had with ppm. I believe the superior selectivity of pcm saved me that time as I recovered control and managed to land to seek out the offending signal (transmitter turned on in the parking lot).

BTW, battery-low fail-safe (as mentioned above) is a common feature on must radio brands including my brand ... JR.

RE: Heli configured Tx go for it. I fly with the heli layout and prefer it.
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