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FM or PCM??

Old 02-10-2004, 12:20 PM
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Tony Gag Jr.
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Default FM or PCM??

I hate to ask this question but what is better FM or PCM? I am going to be buying the new JR6102 and not sure which one to buy. I have always flown FM and have not had any problems. Is the PCM receiver worth the extra money? Is PCM less likely to be affected by interference?

Thanks,
Tony
Old 02-10-2004, 12:30 PM
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tadawson
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Default RE: FM or PCM??

I think you mean PPM vs. PCM - both transmit thier respective signals via FM. Persoanlly, I prefer PCM, but this is reopening a BIG can of worms (again). Allow me to kindly suggest that you do some searches - this topic has been beaten nearly to death very recently.

- Tim
Old 02-10-2004, 05:47 PM
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dirtybird
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Default RE: FM or PCM??

To sum up all of the information the answer to your question is yes- PCM is better on both counts. Only you can decide if it is enough better to justify the extra expense. If you fly in pattern competition you have to spend the extra money, but otherwise as you say PPM has worked for you before. PCM is not going to make you a better pilot.
Old 02-10-2004, 06:57 PM
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Lynx
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Default RE: FM or PCM??

PCM is also more likely to hide catastrophic interference from the user's abilitiy to figure out what's going on and react to it in time... It's the price you pay, period.
Old 02-10-2004, 07:22 PM
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Geistware
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Default RE: FM or PCM??

I prefer PCM or any system that will take the throttle to idle with loss of control
Old 02-10-2004, 07:50 PM
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AMA-69405
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Default RE: FM or PCM??

Tony,

Admittedly ad copy, I think the following quote from a well known distributor of RC equipment says it best:

"And as far as the tired old PCM vs. FM debate goes, do any of us really believe every TOC competitor in recent memory flew with PCM just to be fashionable?"

Despite the naysayers, the best fliers in the world seem to think there are advantages to flying PCM. In your specific case (the 6102 system and R700 PPM vs R770 PCM), there is exactly 20 bucks difference. IMHO, it's a no-brainer... buy the PCM receiver.

Gary L.
Old 02-11-2004, 12:47 AM
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JohnW
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Default RE: FM or PCM??

Ditto... PCM may not be a perfect system, but overall it is better than PPM without error checking. As with any system, you must fully understand how to use it or risk problems. Be sure you understand how to range check a PCM system and fully understand failsafes and how to set them up. If you can afford the few extra bucks, use PCM or a similar error checking encoding such as IPD... one day it may save your plane, or more importantly a life.
Old 02-11-2004, 07:17 AM
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Highflight
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Default RE: FM or PCM??

Then there's the "writing on the wall".
Multiplex is the mfgr. that INTRODUCED the PCM technology for R/C radio use many, many years ago. All the other's followed.

The ONLY advantage to PCM is failsafe (and I know of no one who actually programs it at my club field even if they DO have PCM equipment. How about you.. how many of you DON'T even bother to program in failsafe if you have PCM?). Claims of greater range are anecdotal at best and usually in the range of 5 to 10%, and like, who took a tape measure to THAT one?
With either PCM or PPM, if you can see your plane, you can control it. So who cares?.

The disadvantage of PCM is that the RX can kick into failsafe repeatedly and mask a frequency glitch problem. If you see that you get hit a couple of times witn PPM, you land and figure things out. With PCM, you don't know what's going on until it's too late. Failsafe will NOT land your plane for you; it just crashes it in a pretty way.

Now fast forward many years from the time that Multiplex introduced PCM, and note that the same company, Multiplex, has developed a PPM failsafe receiver they have termed IPD. Again, Multiplex leads the industry. So now we have (Multiplex, anyway), PPM failsafe receivers if you so desire.
Then note that while the Multiplex flagship radio, the Profi 4000, is a PCM radio, it was introduced way back when PCM failsafe programming was considered "important".
But now note that their NEWEST high-end radio (the EVO) is NOT a PCM product. If PCM had a future, then the EVO would have been introduced with PCM capability since it was only brought onto the market a couple of years ago.

That's the writing on the wall. IPD has made PCM obsolete taking away the only (perceived) advantage of PCM so I've already sold off all my PCM stuff (at high prices while I could) and went straight PPM. And now, I can fly ANY manufacturer's receivers with my EVO as opposed to HAVING to match brands with PCM.

That's just my take.

Highflight
Old 02-11-2004, 10:38 AM
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dirtybird
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Default RE: FM or PCM??

PCM has another advantage. It covers up any minor transmission errors. If you have a minor glitch on the take off or landing( where glitches are likely) The PCM system will not respond. If you are being judged on your flying quality you would get downgraded for this if you used PPM. If you are serious about your flying you cannot give this advantage to your competition.
In the begining every PPM system came with a failsafe system. It was deemed to be unnessesary and dropped. In fact one of the manufactors, PCS, told me that it caused more crashes than it saved.
Today its more a safety factor consideration in that it can be used to shut down the motor in case of transmission failure.
Old 02-11-2004, 03:51 PM
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Default RE: FM or PCM??

> In the begining every PPM system came with a failsafe system.
> It was deemed to be unnessesary and dropped. In fact one of
> the manufactors, PCS, told me that it caused more crashes than
> it saved

"In the beginning"?

When digital proportional gear first began appearing on the scene, I was involved in the design of several systems and also serviced quite a bit of commercially made RC gear.

I don't recall *ever* seeing a PPM system that had any kind of fail-safe system.

Back in those days, all the encoding/decoding was done with discrete transistors and there was no such thing as "memory" -- so how would such a failsafe system have worked?

To the best of my knowledge, there was no "failsafe" assocaited with PPM until the advent of microprocessor-based decoders -- which only appeared 20 year so after the advent of PPM.

I expect the reality is that the PPM/PCM debate will continue virtually for ever -- just as there are still audiophiles who swear that valves and vinyl are superior to today's latest digital audio discs and solid-state amps -- and who knows, in some cases they may be right.

The reality is that the quality of today's radio gear is very high and 99.9% of the time, people won't notice the difference between PPM and PCM in everyday flying.

I expect that the only time we'll get rid of this argument is once we've all switched to spread-spectrum equipment that should, for all intents and purposes, significantly reduce the problem of interference from external sources. By that time, all systems will be PCM (or a more advanced variant thereof) anyway.
Old 02-11-2004, 04:25 PM
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JohnW
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Default RE: FM or PCM??

Highflight: Dude, you need to reeamine you facts, your point are totally off base and in some cases totally wrong. IPD is a slick system and I have nothing against Multiplex, but you obviously don't understand how IPD works becasue every argument you gave against PCM would apply to IPD too!

"The ONLY advantage to PCM is failsafe"
Not true, PCM offers error checking on every frame. This means that minor glitches are smoothed out. IPD does somehing very similar but in a slighty different way. PCM also has better range than PPM systems and yes I have personally checked it and it is a lot more than 10%. Range equates to noise rejection. It is not the fact that I can now fly 2 miles away as opposed to one, but that my RX is better able to reject interference. This means PCM is less likely to have problems in the first place.

"The disadvantage of PCM is that the RX can kick into failsafe repeatedly and mask a frequency glitch problem." Dude, what do you think IPD does during a "glitch problem" as you put it. Same thing as PCM. IPD first masks the problem and then goes into failsafe.

"If you see that you get hit a couple of times witn PPM, you land and figure things out. With PCM, you don't know what's going on until it's too late." What happens is BOTH PCM and IPD get mushy, and eventually you have no active control as failsafes are engaged... this applies to BOTH systems. If you pay attention, it is very easy to detect PCM and IPD glitches. But I suppose you are right that it is very easy to see PPM glitches... I typically watch for a sudden snap that rips the wings off, or a full throttle spin into the ground, or an out of control full throttle taxing plane eating planes and people as it races thru the pits.

IPD has NOT made PCM obsolete... that is why JR and Futaba still use PCM. Why? Becasue IPD offers no functional gain over PCM becasue they are basically identical. The diff is IPD works with a standard PPM signal, which means you can use any PPM TX with them. This is IPDs strong suit. It also means Multiplex opened thier RX up to a larger market... hum, looks to me like it was a marketing decision, not a superior tech decision.

" I can fly ANY manufacturer's receivers with my EVO as opposed to HAVING to match brands with PCM. " What? You know IPD is in the RX, not the TX. I can fly any RX (assuming futaba shift) with my PCM radio too...I just reset the TX for PPM by pushing a button.
If you don't use the Multiplex IPD RX, you don't have IPD so why would you want to use another brand of RX anyway with your Evo?
Old 02-11-2004, 05:27 PM
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Highflight
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Default RE: FM or PCM??

ORIGINAL: JohnWillman

Highflight: Dude, you need to reeamine you facts, your point are totally off base and in some cases totally wrong. IPD is a slick system and I have nothing against Multiplex, but you obviously don't understand how IPD works becasue every argument you gave against PCM would apply to IPD too!
That's my point. PCM offers no significant advantage over IPD/PPM. I've yet to see any anecdotal evidence (which is the kind that shows up first when there is even a "perceived" problem of this nature) that activities at any flying site by PPM equipment is any different than activities by PCM equipment. I've owned and flown PCM, and I've owned and flown PPM (both for years) and I have never had even one instance where either technology performs less well than the other.
That's why I've dumped my PCM equipment; it just doesn't matter in the "real" world. You can play with scopes and meters all you want, but out there in the air, there is no difference.
I have nothing against PCM and it's a slick system; it's just no better than PPM.

ORIGINAL: JohnWillman
" I can fly ANY manufacturer's receivers with my EVO as opposed to HAVING to match brands with PCM. " What? You know IPD is in the RX, not the TX. I can fly any RX (assuming futaba shift) with my PCM radio too...I just reset the TX for PPM by pushing a button.
If you don't use the Multiplex IPD RX, you don't have IPD so why would you want to use another brand of RX anyway with your Evo?
Yes, I'm aware that Multiplex is the only IPD receiver at this time. My reference to mixing brands of TX's and RX's points out that PPM users simply have lot's more options because they DON'T have to stay with the same brand of TX and RX if they don't want. As far as IPD, I couldn't care less about it myself because I don't care if I crash my plane, or the radio does it; it's still junk.

Highflight
Old 02-11-2004, 05:34 PM
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tadawson
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Default RE: FM or PCM??

Since IPD is PPM, and most PCM systems set the failsafe points in the TX, which loads to the RX via the PCM data link protocol for that MFG, how, exactly, do the failsafe positions get set in IPD? Since there is no inherent protocol to transmit them to the RX, I assume that this would then require the annoying step of having to set them with a button on some such on the RX?

- Tim
Old 02-11-2004, 08:43 PM
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Forgues Research
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Default RE: FM or PCM??

You set Failsafe on the IPD directly on the Receiver, you do it once and then forget it unless you want to change your settings.

and by the way, you also have FMA that has introduced one of the best PPM receiver on the planet with the copilot 8. Not only failsafe, but with different sensors mounted on the airplane, you get hit and it will even turn an upsided airplane right back up and circle the field until you regain control, that is if you program it properly.

PCM is out for all practical purposes.

When the IPD starts getting hit, it gets mushy on the control and it tells you to get back while with PCM, it locks out period. So as I said PCM is out.

Also for the one that said that the Multiplex MC 4000 is PCM, false, it has the capacity to transmit in PCM but also PPM. That's what I'm using and I don't use PCM. All my Multiplex PCM receivers are gathering dust.

Roger
Old 02-11-2004, 10:10 PM
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dirtybird
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Default RE: FM or PCM??

ORIGINAL: XJet

> In the begining every PPM system came with a failsafe system.
> It was deemed to be unnessesary and dropped. In fact one of
> the manufactors, PCS, told me that it caused more crashes than
> it saved

"In the beginning"?

When digital proportional gear first began appearing on the scene, I was involved in the design of several systems and also serviced quite a bit of commercially made RC gear.

I don't recall *ever* seeing a PPM system that had any kind of fail-safe system.

Back in those days, all the encoding/decoding was done with discrete transistors and there was no such thing as "memory" -- so how would such a failsafe system have worked?

To the best of my knowledge, there was no "failsafe" assocaited with PPM until the advent of microprocessor-based decoders -- which only appeared 20 year so after the advent of PPM.

I expect the reality is that the PPM/PCM debate will continue virtually for ever -- just as there are still audiophiles who swear that valves and vinyl are superior to today's latest digital audio discs and solid-state amps -- and who knows, in some cases they may be right.

The reality is that the quality of today's radio gear is very high and 99.9% of the time, people won't notice the difference between PPM and PCM in everyday flying.

I expect that the only time we'll get rid of this argument is once we've all switched to spread-spectrum equipment that should, for all intents and purposes, significantly reduce the problem of interference from external sources. By that time, all systems will be PCM (or a more advanced variant thereof) anyway.
If you were involved with the design of the first digital systems you will recall Doug Springs Digicon and Bonners Digimite and the first Logictrols. They all had a failsafe system. It consisted of limit switches built into the servos and a delay system in the receiver. If the transmisssion was interferred with and the last pulse did not appear when expected the delay circuit would switch to the other line to the servo and cause the servo to drive to where the limit switches stopped it. That was at the neutral point on all of the servos except the MC servo which was driven to one end. The result was very similar to the failsafe of today
Jerry Pullen and Clif Weirick produced the PCS system that appeared in 1964 and was one of the first to eliminate the failsafe system. PCS was later absorbed by Kraft
There are many ways to implement a "memory".
I had a very effective failsafe in the pulse systems I flew in the late '50s. It used a relay and a rubber band.
Old 02-11-2004, 10:50 PM
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JohnW
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Default RE: FM or PCM??

Highflight, your still making no sense. You claim that PCM offers no advantage of IPD/PPM... that I'd 99% agree with. However, PCM is indeed better than PPM. You keep making unfounded leaps in logic and keep confusing a PPM ssystem with IPD/PPM as well as implying a PCM TX can only talk to PCM Rx's... this is not true. Every PCM transmitter I've seen is backwards compatible to PPM, for that matter, a PCM transmitter would work with IPD. And it does matter in the air... I've had several planes saved because of PCM. I can't say 100% they would have died with PPM, but I'm pretty sure they would have because the problem was another TX on my freq.

Aerographics... sheesh... talk about propaganda. PCM DOES NOT JUST LOCK YOU OUT! If you really knew what you were talking about you'd know that. PCM and IPD feel almost exactly the same under intermittent quick interference. PCM drops bad frames and IPD reduces resolution... the net result is both feel mushy in flight. With PCM this happens because the good frames show up less often, causing sluggish response. With IPD, no frames are refused, but resolution is reduced as it absorbs bad frames and the result is sluggish response. If either PCM or IPD has a major hit, they both failsafe, in which case both the IDP and PCM plane are doomed. I've been hit before with PCM, it gets mushy... I flew back and landed, the plane never went into failsafe and never locked me out. PCM DOES NOT JUST LOCK YOU OUT! PCM is out? I think Multiplex makes great radio equipment, but as best I can tell, every Multiplex user totally misunderstands the basic laws of the universe.
Old 02-11-2004, 11:41 PM
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TLH101
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Default RE: FM or PCM??

This always good entertainment. Just doesn't come up as often as it used too.
Old 02-12-2004, 01:59 AM
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JohnW
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Default RE: FM or PCM??

TLH101 - I agree the PPM/PCM/IPD posts often turn into a circus. I remember the once last year that happened about this time of the year - I believe it was four pages long. However, the question is a legitimate one. When I first started RC, I had no clue what PCM was. What is a new pilot supposed to do? Ask RCU of course! Now I know everyone has their opwn opinion, which is great, but there is a lot of disinformation about PCM. Opinions are fine, Ilike PCM because... I dislike PCM because... That's fine. But when an experienced member intentionally spreads disinformation intended to look as fact on a serious subject someone has to say something. I'm used to having my head on the chopping block so I figured I'd stick my neck out again. Aerographix was deeply involved in the PCM-IPD thing from last year and spread the same disinformation, which is why I took a little jab at the end, all in a little fun of course. But, TLH101, the good news is, my guess is your entertainment has just begun.
Old 02-12-2004, 04:11 AM
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HarryC
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Default RE: FM or PCM??

ORIGINAL: JohnWillman
PCM drops bad frames and IPD reduces resolution... the net result is both feel mushy in flight. With IPD, no frames are refused, but resolution is reduced as it absorbs bad frames and the result is sluggish response.
That is not correct. IPD rejects bad data, it does not reduce resolution, it does not absorb bad frames. As soon as one servo's data is invalid the remainder of the frame is rejected and replaced by failsafe data.
Perhaps you should learn a bit more before accusing others of not knowing how the system works.

H
Old 02-12-2004, 04:28 AM
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ZAGNUT
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Default RE: FM or PCM??

ORIGINAL: aerografixs



and by the way, you also have FMA that has introduced one of the best PPM receiver on the planet with the copilot 8. Not only failsafe, but with different sensors mounted on the airplane, you get hit and it will even turn an upsided airplane right back up and circle the field until you regain control, that is if you program it properly.

and don't forget the FMA's signal recognition feature.
it supposedly learns the features of your tx like + or - shift, number of channels and so on and rejects signals coming from a tx that doesn't match.

dave
Old 02-12-2004, 12:45 PM
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tadawson
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Default RE: FM or PCM??

But, with IPD, the systen will have a hell of a lot harder time figuring out what a bad frame is, versus PCM. On IPD, any signal which has servo position data is the realm of reality (the normal range of pulse times) will be accepted, valid or not. With PCM, the checksum makes a much more distinct go/no-go decision on a frame possible. Perhaps the net effect is the same - there can't be too many cases where a bad signal still looks good (but in error) to a receiver. Everyone seems to be ignoring the inherently greater S/N ration of PCM (greater usable range) and I don't know about you guys, but having to set failsafes at the RX would piss me off royally! I have some planes/helis that the receivers are buried in pretty deep, and if I am trying to setup a failsafe to behave a certain way, taking the *#&Y$()*# thing apart every time I need to make a tweak would just plain suck! IPD sounds like a very viable option, but I would position it between "traditional" PPM and PCM. I don't think it can (or will) ever be able to totally equal PCM, but if they get 90% of the way there for, say, 50% of the cost, it sure will sell! As much as I have been attacted to the Evo line, the lack of PCM is really a great deterrent to me, since I just plain refuse to fly my helis on anything BUT PCM. (Yes, I have flown PPM in the past, and promptly moved up - the interference handling of PPM (or lack thereof) STANK! And for those who say you can't feel interferecnce in PCM, you apparently are not trying hard enough - to
me, it is obvious, far before taking a lockout, which will most likely be INTERMITTENT and recoverable. I think too many folks mistake the LOCK in lockout to mean that once you go there, that it will stay locked. In a heli, where there is a lot of potential for on-board generated RF noise (bad bearings, etc.) a lockout will drop the throttle, thus (frequently) removing the source of noise, allowing you to try to keep flying, or to auto the bird safely.

- Tim
Old 02-12-2004, 01:43 PM
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Default RE: FM or PCM??

There are pros and cons of PCM.

PRO
PCM will reject data frames that aren't received with valid checksums. If you are just getting a bad frame once in a while, only the bad frame is rejected and the last known good frame data is applied to the servos. If you get a number of bad consecutive data frames in a row, the PCM receiver will drive the servos to failsafe position (ASSUMING FAILSAFE POSITIONS ARE PROGRAMMED BEFOREHAND).

CON
Because PCM will reject sporadic bad frames without giving any indication to the user (servo jitter) when you range check a model that has a marginal interference problem (RFI from a Gas Ignition System or some Glow Drivers), you would never know you had a problem.

I fly primarily with PCM receivers BUT when I've done major work on a plane or am setting up a new plane (especially Gas),
I do my initial range checks (with and without the engine running) with a FM (PPM) receiver. If everything checks out good, I then install my PCM receiver.
Old 02-12-2004, 02:47 PM
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tadawson
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Default RE: FM or PCM??

As an addition to what AirBearMA said:

"If you get a number of bad consecutive data frames in a row, the PCM receiver will drive the servos to failsafe position (ASSUMING FAILSAFE POSITIONS ARE PROGRAMMED BEFOREHAND)."

As well as come back OUT of failsafe the instant a single valid frame is recived. Keep in mind, that if the signal is bad enough to drive you into failsafe, PPM would be usless. Those who claim "but PPM will still give some control" - well, PCM will do that with every valid frame it sees - it just does not "thrash with the trash" like PPM.


"Because PCM will reject sporadic bad frames without giving any indication to the user (servo jitter) when you range check a model that has a marginal interference problem (RFI from a Gas Ignition System or some Glow Drivers), you would never know you had a problem."

Myth. As PCM starts taking interference, control response gets sluggish due to the discarded frames, and you know that you have interference, WITHOUT having the model go insane, as would be the case with PPM. If you don't notice this in PCM, then you aren't paying attention!

- Tm
Old 02-12-2004, 02:51 PM
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Forgues Research
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Default RE: FM or PCM??

You know until you have actually tried the IPD's and the new FMA copilot, how can you be so sure of yourself.

I remember when PCM came out and nobody would touch them, it took a while for people to change, and now we are seeing the same thing over again, its called evolution.. Some people don't like driving front wheel drive because they are scared of change.

Roger
Old 02-12-2004, 03:39 PM
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Default RE: FM or PCM??

For what it's worth, I've just ordered an FMA M5 receiver and, when it arrives, plan to do some side-by-side testing against regular FM receivers such as the Hitec 555. I would also test it against the new Hitec QPCM receiver but I don't have one :-(

I'll be testing for susceptibility and handling of impulse noise (as you'd get in a gasser), general noise (as you'd get at the edge of useful operating range) cross-mod/intermod handling (as you'd get when flying very close to another transmitter on a different frequency) and capture capability (as you'd get if someone turned on a transmitter using the same frequency while you were flying).

The testing will be conducted in a scientific manner, without prejudice and although I'm sure the results will produce much discussion and debate, I'm hoping they'll cast a little light on just how the receiver-side failsafe and noise rejection systems on the FMA receiver work in comparison with a regular FM system.

Of course if someone wants to donate a PCM transmitter/receiver I'll gladly include that in the comparison too :-)

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