logo
View Poll Results: A system recommended to a friend with low budget, keeping it safe, on his new gasser?
At least run PCM to keep it safe. 28 56.00%
Run your old PPM system and take your chances. 2 4.00%
Lie to the wife and drop big bucks on a new 2.4 gig sys. 16 32.00%
Experiment with PPM to see if it will work. 3 6.00%
Sell plasma to get enough cash for a system that do it all. 1 2.00%
Voters: 50. You may not vote on this poll

Thread Tools
Old 04-29-2007, 11:40 PM
XJet is offline
Find More Posts by XJet
Uber Contributor
XJet's Avatar
New Zealand
Joined Feb 2006
886 Posts
Re: PCM, PPM, or 2.4 gig

I would strongly advise PCM on any aircraft that is large/heavy and/or fast, simply because it will be safer.

Although there are some who say (including FMA apparently) that PPM gives you some measure of control whereas PCM goes into failsafe, this has not been my observation.

The problem is that when encountering interference, a PPM receiver might command your servos to move to their extremes (or beyond) for 90% of the time. The 10% of valid frames that get through the noise will undoubtedly be commanding exactly the opposite movements (as you try to correct the unwanted deviations from the expected flightpath).

Unfortunately, the 90% of bad pulses will overwhelm the 10% of good pulses, so your servos will still remain hard-over or very close to it.

Now consider PCM.

It takes a *lot* of inteference to cause your average PCM system to go into full failsafe mode.

If 10% of the frames a PCM receiver gets are valid, that's probably going to avoid failsafe and still give you control -- albeit there will be a greater latency because your servos won't respond at all to the interference, moving only when a valid frame is received.

This means that, for a given level of interference, PCM will always provide you with more control than the PPM system will.

By the time things get so bad that a PCM set goes into failsafe, a PPM system is getting virtually no valid signal and those few valid pulses that get through will have virtually no affect, because servos simply cant react that quickly.

Now comes the issue of the smarter PPM receivers such as the Berg, Multiplex, some FMAs, etc.

These use a processor to vet each set of pulses that are received, with the idea of ignoring obviously bad data generated by interference.

This provides a useful measure of reduction in the receiver's susceptibility to brief pulses of very strong interference, but won't prevent glitches in the way PCM does.

The reason these smart receivers won't reject all interference is because they use a fairly simple set of rules to work out what's good and what's bad. Those rules aren't always adequate to detect inteference and therefore much of it will still get through.

For example, a valid servo pulse will be somewhere between 1 and 2mS. The receiver can therefore assume that pulses shorter or longer than this are interference and reject them. What happens however, when a pulse of 1.9mS is received? How can the receiver tell if this is valid or not?

Well providing it arrives at the right time (the time-relative position of the pulse is also used to help sort the wheat from the chaffe), it can't.

With a smart IPD/DSP receiver, the magnitude of the glitching may well be reduced, but it will still glitch when there's enough interference around.

And, if there's enough noise to push a PCM receiver into failsafe, an IPD/DSP one will do the same.

If people are still skeptical, try range-testing a PCM receiver alongside a PPM one.

As the signal gets weaker (while you walk away with transmitter in hand), the noise level begins to rise. This produces a situation in the receiver which is very similar to that which is encountered when there's interference around.

Note that the PCM receiver will usually provide *much* greater range than the PPM one does.

Why is that?

The PCM receiver will only respond to valid frames that weren't distorted or corrupted by the noise. This means that while the PPM system starts shaking and jumping (as the servos respond to noise which is now being interpreted as valid commands), the PCM remains stable.

Both systems are receiving servo data around 50 times a second but, in the case of a PPM set, there might be 40 of those frames that are corrupted and cause the servos to move to uncommanded positions.

In the case of the PCM system, the 40 "bad" frames are completely ignored and only the 10 valid frames are used, so the servos will still move to the requested position, albeit they may move more slowly.

Both the PCM and the PPM receivers are seeing exactly the same amount of noise/interference but the PCM is *still* giving you control long after the PPM one has started having convulsions.

Walk further and they'll both stop -- that's what happens when the interference is too great.

Now down to the practical aspects of this.

I fly PPM and PPM/DSP receivers in 90% of my aircraft because:

1. the receivers are much cheaper (under $30 for a *good* 8-channel DC receiver with DSP)
2. PPM is faster (lower latency) than PCM and this *is* noticeable on my profiles when hucking low and hard.
3. my smaller models are light and slow enough that they're unlikely to cause a problem if they get badly hit by interference.

However, I *do* use PCM on my gaser and on another plane that weighs over 14lbs. If there's enough interference to trigger the failsafe then I'd rather that it went in without the engine running at what could otherwise be full throttle.

However, some folks fly their gassers with PPM and have no problems, some fly with PCM and suffer lockout/failsafe incidents that destroy them.

A lot of what happens is down to "luck" and all we can do is try to mitigate the effects of "bad luck" as much as possible through chosing the most appropriate technology for the task.

If I flew at a remote field with little chance of interference and no bystanders to get nailed by an out of control model, I'd probably consider using PPM in my gaser too.

As for 2.4GHz, well I'd recommend sitting on the sidelines a bit longer and just see how that all pans out.

Right now we only have a relatively small number of 2.4GHz filers and systems out there and although things seem to be working just fine, we don't really know for sure exactly what's going to happen when we have larger numbers of systems all contesting for a few channels of spectrum at the same time.

Sorry about the length of this reply.
XJet is offline Find More Posts by XJet
Reply With Quote Quick reply to this message
Sign up now
to remove ads between posts
Old 04-30-2007, 03:37 PM
crhammond is offline
Find More Posts by crhammond
Registered User
crhammond's Avatar
Edmonton, AB, Canada
Joined Mar 2006
979 Posts
Re: PCM, PPM, or 2.4 gig

First and foremost nothing will work unless installed correctly and properly range tested.

I recently went 2.4GHz. If the equipment is mechanically reliable then it is a good way to go. My experience with 2.4GHz was initially good until the equipment (TX) failed.

I have run PCM, PCM G3 and PPM. I have experienced PCM lockout twice and both times it meant crashing as I never had control long enough to do anything. I had a PPM issue once (plug cable rubbing on muffler causing noise) and was able to land (badly - but hey I was able to have some control.)

I like the Multiplex IPD RX's myself and that is what I have gone back to until the 2.4GHz modules are available. The IPD RX's allow you to set a throttle failsafe and they hold. They also do very effective filtering.

In bench testing PCM and PPM using an aeroperfect meter it is clear that some (not all) PCM RX's are poor resolution. The Multiplex IPD PPM RX's give me the same locked in feel as 2.4GHz.

The nice thing with PPM in a gasser is you will 99% of the time find interferrance problems on a range check if there are any. Once you sort those out unless something fails/moves etc it will fly without issue.

My 2c.

I can't wait for the higher end 2.4 systems/modules.
crhammond is offline Find More Posts by crhammond
Reply With Quote Quick reply to this message
Old 04-30-2007, 03:44 PM
Kiwi is offline
Find More Posts by Kiwi
Registered User
Kiwi's Avatar
Chile
Joined Jan 2006
5,983 Posts
Re: PCM, PPM, or 2.4 gig

XJET,

Dont aplogise please for that. You have probably given the best ever description of the whys and whens of 72 Mhz ever seen on this or any other forum.

Thanks a million for an excellent explanation in terms that even I can relate too.

Kiwi
Kiwi is offline Find More Posts by Kiwi
Reply With Quote Quick reply to this message
Old 04-30-2007, 08:35 PM
Jcarte is offline
Find More Posts by Jcarte
It's a beautiful RC day!
Jcarte's Avatar
Jacksonville Fl
Joined Jan 2006
1,768 Posts
Re: PCM, PPM, or 2.4 gig

Very well put
Jcarte is offline Find More Posts by Jcarte
Reply With Quote Quick reply to this message
Old 05-02-2007, 12:27 PM
crhammond is offline
Find More Posts by crhammond
Registered User
crhammond's Avatar
Edmonton, AB, Canada
Joined Mar 2006
979 Posts
Re: PCM, PPM, or 2.4 gig

Quote:
Originally Posted by Kiwi View Post
XJET,

Dont aplogise please for that. You have probably given the best ever description of the whys and whens of 72 Mhz ever seen on this or any other forum.

Thanks a million for an excellent explanation in terms that even I can relate too.

Kiwi
Kiwi - do you still have/run your Weatronic systems? I just ordered two of them as I am planning to stay on 2.4GHz a while yet. Just wondering if you have any more feedback. I have heard nothing but great things myself.
crhammond is offline Find More Posts by crhammond
Reply With Quote Quick reply to this message
Old 05-02-2007, 04:52 PM
Jcarte is offline
Find More Posts by Jcarte
It's a beautiful RC day!
Jcarte's Avatar
Jacksonville Fl
Joined Jan 2006
1,768 Posts
Re: PCM, PPM, or 2.4 gig

I have a weatronics in one of my Wildhare 41% 300s and is is great, I have never had an issue. Some don't like it because of the computer interface, but I love to look at the data it collects.
Jcarte is offline Find More Posts by Jcarte
Reply With Quote Quick reply to this message
Old 05-02-2007, 05:06 PM
3ddd is offline
Find More Posts by 3ddd
tissue ? cry baby jack wagon!
3ddd's Avatar
United States, AZ, Tucson
Joined Jan 2006
2,886 Posts
Re: PCM, PPM, or 2.4 gig

i fly nothing but gassers and have 2 pcm recivers and they are both in the top drawer of my desk.no problems with them i just don't like lock out compared to glitches that i can most times fly thru and safely land
3ddd is offline Find More Posts by 3ddd
Reply With Quote Quick reply to this message
Old 05-18-2007, 07:20 PM
babflyer is offline
Find More Posts by babflyer
Gettin' Lower!
Delaware, OH
Joined Aug 2006
73 Posts
Re: PCM, PPM, or 2.4 gig

I ran ppm futaba recievers for years in my gas planes and at one time had some issues with what turned out to be a faulty spark plug cap on an older 3w. I had bought a pcm reciever but never installed it as I thought It would crash for sure when it went into hold, luckily I finally was given the correct advice and was able to figure out the problem before something bad happened. Well this year on my 42% extra I have had some problems. I was running futaba ppm recievers, yes a dual setup like I had done in the past. Thought everything range checked fine but I was getting hits when I was just about ready to start the roller on this years advanced sequence. Basically cross box staring at the tail of the plane and I tend to hold the antena pointed right at the plane in this attitude. Did some extensive range checking and from directly behind the plane there was a significant drop in range with both recievers on at the same time. So I decided to go with the better of the two recievers into a smartfly power expander. Renge checked just as it did with one reciever on, flew a couple flights just fine, get to the contest and start into my roller with what the judge described as a 4 of 8 entry into the roller, lol. Made the antena snake back and forth instead of a straight shot down the tail and the unknown went fine although no cross box roller which is the only time I have gotten hit. Said the heck with it and put in a futaba pcm 149 and have done maybe 10 rollers glitch free, NOCK ON WOOD! So I wrote all of this as to why I finally switched to pcm, couldn't feel any difference in the way the plane flew, as of now I am happy. The thing that I would like to say though is how things on the internet hit peoples hot buttons and you get lopsided responses. While at the Clinton imac a couple weeks ago I got to looking at many peoples setups, I think I was the only guy using ppm! Talked to many people about my problems and had a couple state that basically unfliable planes on ppm were ok on pcm. Also for those that remember our poll on single reciever, dual reciever, or power distribution device this past winter, the poll showed that more than half used some kind of distribution device. I was the only guy there that I saw using one, lol. Most were using dual reciever setups. for those that use a single reciever and mathboxes that is fine but I consider that to be to some extent a way to distribute power so techincally atleast the way I think those setups would fall into the single reciever with power distribution device catagory. This last topic was brought up just to show that the polls online might not be as accurate as one might expect. If anybody has had the same problems with lack of signal when the antena is pointing directly at the tail and found a way to fix it besides the obvious dont point the antena that way, please let me know. See you all at Columbus.
babflyer is offline Find More Posts by babflyer
Reply With Quote Quick reply to this message
Old 05-18-2007, 08:04 PM
luv2flyguy is offline
Find More Posts by luv2flyguy
Life Begins at 200 mph!
luv2flyguy's Avatar
Gastonia, NC
Joined Aug 2006
1,346 Posts
Re: PCM, PPM, or 2.4 gig

There are plenty of good pcm recievers for sale on the swap shop that one can pick up at a budget price, it's not that we need to try and keep up with the latest and greatest but safety comes first! Just my 2 cents
luv2flyguy is offline Find More Posts by luv2flyguy
Reply With Quote Quick reply to this message
Old 11-05-2007, 12:31 PM
Vendman is offline
Find More Posts by Vendman
Flyin' Around
Joined Oct 2007
29 Posts
Re: PCM, PPM, or 2.4 gig

What about just regular old FM transmitter and receiver? I have been flying glow planes for 20 years and NEVER lost one to a glitch or interference. But, I recently acquired an ACE 33% Extra. It is my first "giant" gas powered plane. Most people I have talked to are recommending that I NOT fly it with just my 7 Channel HiTec FM transmitter and receiver. Most say the ignition system on a gas engine may cause interference. What do yall think?
Vendman is offline Find More Posts by Vendman
Reply With Quote Quick reply to this message
Old 11-06-2007, 02:17 PM
luv2flyguy is offline
Find More Posts by luv2flyguy
Life Begins at 200 mph!
luv2flyguy's Avatar
Gastonia, NC
Joined Aug 2006
1,346 Posts
Re: PCM, PPM, or 2.4 gig

Absoultely PCM! Although today's ignition systems are pretty much free of emitting interference such as DA,3W and so on still I would not take a chance on it. My first gasser was a Brison 3.2 which I tried to fly on fm and had glitches and swithched it to a PCM reciever and after that had no problems at all. After switching I will never go back to just FM, you may even want to try the new 2.4 however I'm going to wait a while longer myself.
luv2flyguy is offline Find More Posts by luv2flyguy
Reply With Quote Quick reply to this message
Old 11-06-2007, 10:40 PM
Vendman is offline
Find More Posts by Vendman
Flyin' Around
Joined Oct 2007
29 Posts
Re: PCM, PPM, or 2.4 gig

Quote:
Originally Posted by luv2flyguy View Post
Absoultely PCM! Although today's ignition systems are pretty much free of emitting interference such as DA,3W and so on still I would not take a chance on it. My first gasser was a Brison 3.2 which I tried to fly on fm and had glitches and swithched it to a PCM reciever and after that had no problems at all. After switching I will never go back to just FM, you may even want to try the new 2.4 however I'm going to wait a while longer myself.
Your probably right. I did a little more research myself and decided not to risk it with such a big, expensive plane. Even though I got my Ace 33% Extra 230 for next to nothing, I still would not want to take any more risks with it than I have to. I am working a deal right now to buy a good used Futaba PCM transmitter and receiver.
Vendman is offline Find More Posts by Vendman
Reply With Quote Quick reply to this message


Quick Reply
Message:


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
FlyingGiants Exclusive! Introducing The Futaba 12FG, Modules & More! madmax General Discussion 473 01-24-2010 11:01 AM
9303 PCM to 2.4 conversion Stainless Skills Radios 14 01-10-2008 11:51 PM
2.4 Gig Antenna??? Cobra1 Radios 5 06-21-2007 04:29 PM
Spectrum DX 2.4 gig Bulldog Helicopters 4 04-14-2006 06:10 PM
PCM or PPM 8iggles General Discussion 8 01-27-2006 01:40 PM