lownslo

On a previous thread of mine the discussion "drifted" to PCM receivers. Most all were negetive accounts. I would like to hear more, hopefully some good things,.... SINCE I just bought my first computer radio and PCM receiver!!
I had moved away from Futaba receivers and servos. All my planes have evolved to Hitec. I bought the Futaba 6XA with the PCM receiver.... because I did not want the 127 model rx. I pulled out a brand new Hitec receiver and slipped in this PCM jewel. It is in my winter project and it probably wont be flying yet for a couple weeks. I set the fail safe for idle+2 clicks and my elevator slightly up. Hopefully I will hear something good about these PCM marvels(?). What do you guys set your fail safe settings at??? Reasons?? ...thanks....lownslo..

Bob

rajul

My Feedback: (58)

We had many crashes since a telco tower was put up near our field. The affected frequencies were quite wide ranging (72Mhz FM and PCM). Don't know what kind of waves the tower was transmitting but the amount of damage was enough to make most flyers change field. I just don't see any advantage in PCM as they are all equally affected.........

HarryC

My Feedback: (1)

PCM is better. The faults are in some users, mistakenly using PCM to mask underlying problems which are still present, or in thinking that failsafe really does make the model safe.

H

amcross

My Feedback: (2)

In most scenarios, PCM will reject interference that would hit your PPM. PCM offers failsafe so your model's behavior in case of a hit is predictable and hopefully safer and less likely to be a simple fly away, whereas MOST PPM systems do not.

PPM systems take any information they receive on their channela nd try to do something with it.

PCM, like your cell phone, is a digital signal and is encoded. the receiver is looking for information in the language it understands, and does not obey commands not in its language.

Thud_Driver

My Feedback: (1)

Maybe take some time and research PCM and understand it. There's many links for this in other threads here on PPM vs PCM. The RF link from either system is of equal quality. What AnnMarie said above is right. Both effectively send a sequence of frequency shifted pulses, 1's, and center carrier freq, 0's. On PPM systems the RX measures the time between the 1's to get servo data. When you get noise, you get bad servo position data. On PCM Rx's, the receiver decodes the sequence of 1's & 0's to get servo data (just like inside the PC you're using does with it's data handling). When you get noise with PCM the receiver ignores the input data and does what you've told it to do.

The cell phone industry went away from analog to PCM to get cleaner voice data to the user and more users onto the system. You might notice the vast majority of the jet crowd uses PCM only on their high end models.

mglavin

My Feedback: (31)

PCM is considered better from a technical standpoint. It is possible to swamp a PPM or a PCM RX. It largely depends on the strongest signal being transmitted. How and when the RX reacts to these stray RFI/EMI signals is where one can realize the difference.

The strongest signal recognized by any RX will be accepted. With PCM as the communication medium, it's easier for the RX to lock in and act appropriately. PCM RX's enjoy approximately a 20db SNR advantage above other types of modulation... If you were able to transmit and observe PCM and a PPM signal and you slowly raised the noise levels, the PPM signal would become garbled and unusable approximately 20 dB before PCM. As the noise goes up, PPM becomes unstable, PCM keeps flying until such time the noise gets much higher.

PPM radio waves pulse positions can be shifted slightly by RFI/EMI noise (amplitude and or width) on it's way to the RX. Depending on the severity of the RF noise/noise's being realized by the RX, dither or glitching will occur. Intermittent or sporadic control is common in this scenario.

PCM radio wave pulses don't have to be perfect, they may change amplitude, width or both. The decoder is only looking for the presence of pulses, either they exist or they do not. This is what makes the difference. The pulse trains are recognized as viable code or rejected (no garbled stretched, compressed or out of time pulses are accepted). After several invalid pulse trains are rejected either FAILSAFE or HOLD is initiated. Once a valid signal is decoded and accepted normal control is resumed. This happens in less than a second in most cases, the determining factor is the strength and length of the interference. It's plausible that the radio link may never be re-established.

It's really a simple consideration, IMO. With PPM your in for a ride an uncontrollable ride at that, anything can and will happen. Theres no absolute occurence. PCM offers you the ability to select what happens in the event of interference swamping your system. Many user's program back to idle and pre-select some specific control surface deflection, hoping the model is in an appropriate attitude when lock-out occurs. Others kill the engine and program surface deflection to auger the model into the ground for safety reasoning. Others use the hold feature which will maintain the last valid control input until a radio link is reestablished. Which is the best is subject to interpretation and the models flight attitude.

NEVER utilize a PCM RX to fly a model that has RF interference which is obvious with PPM RX's. Many seem to think a PCM RX is a fix, NOT!!! A PCM RX may work through the interference and this is why they are considered superior, but the interference exist's and needs to be eliminated. Your flying on borrowed time if you ignore the obvious...

lownslo

Many thanks for the time taken on the answers you all posted. ......lownslo.....

Bob

sfaust

My Feedback: (11)

Michael said it eloquently. In the end, they are both more than capable of flying our cheapest beaters to our most expensive show airplanes. Users preference usually determines which is a better choice based on the failsafe features, price considerations, and so forth. Some like the wild ride because it makes them feel like they have some control over the outcome. Others like the fact that they can set the failsafe to their preferred configuration. In either case, the pilot has about the same amount of control during an interference event.

There are also lots of old wives tales that many are still spreading. Its worth some reading to fully understand exactly what happens during an interference event in both PPM and PCM encoded systems. Only then can you differentiate the old wives tales from engineering facts.

Here are some more relevant threads on PCM and PPM.

http://www.rcuniverse.com/showthread...hreadid=473062
http://www.rcuniverse.com/showthread...hreadid=355152
http://www.rcuniverse.com/showthread...hreadid=339931
http://www.rcuniverse.com/showthread...hreadid=246442
http://www.rcuniverse.com/showthread...hreadid=436492
http://www.rcuniverse.com/showthread...hreadid=730635
http://www.rcuniverse.com/showthread...698&forumid=27]

Crash_N_Burn

Astute post, Mr. Glavin!

Thanks!

ml3456

MGavin,

Best explanation I ever read. Thanks

mark

MO_Radio_Tech

I repost this from an earlier thread:

Every time PCM is brought up some folks step in and voice concerns, fears, or just plain condemn it outright. The truth is that the folks at NATS, turbine jet flyers, etc., here and overseas went to PCM years ago. When you get into this high end market, many of your folks are Avionics and Electrical engineers. They have technical knowledge and they fly PCM.

In Japan, where probably 90% of our best RC is made, almost no one flys PPM anymore. It is delegated to 40 size trainers and park flyers.

This can (and will) be argued, but our two biggest manufacturers, Futaba and JR, both have PCM receivers at he top of their product lines. Why would these two competing manufacturers choose to both say PCM is better?

Let's not get cynical and say it is just hype and marketing. I actually do think the manufacturers want to compete and make better products.

PCM (and dual conversion for that matter) are actually very similar to modern digital technologies in the communications field. Regular PPM (similar to FSK) is no longer the norm in "the real world". The world has moved on.

Of course, technology can and will get better in our hobby. I would love to see more error correction in PCM receivers or even digital spread spectrum type technologies incorporated into our hobby.

If our radio systems were two-way, we could have on-board signal monitoring. We could know if there was interference, low battery, or low fuel. Maybe someday.

locoworks

in the UK , it is ( as i understand it ) a LEGAL requirement for all models over 7 kilo to have a PCM receiver fitted with the failsafe set . at over 20 kilo's all models have to be inspected at stages during the build and certificated by the Civil Aviation Authority , and exempted to fly. the LMA ( large model association ) have people that are allowed to inspect and certify aircraft for the benefit of CAA exemption. from the outside it looks like too much legislation! but PEOPLE's SAFETY is the most important aspect of this hobby. its a very sad fact that two youngsters ,seperate incidents,one a bystander walking in a public place, have been killed in the UK in the past couple of years by impact from model aircraft. in one case had the PCM failsafe been set to shut the motor to tickover instead of left at the default HOLD possition, high throttle in this case, one tragedy "may" have been avoided.

ec121k

Back in about 1990 or so I attended (as a spectator) an old timers fun fly at our field. There was one flying that got hit hard. The plane went into such wild gyrations that the wing eventually folded and the plane was a total loss. This was one case where a pcm receiver properly programmed would have prevented a total loss since these models really don't need r/c anyway.

sfaust

My Feedback: (11)

Its fairly easy to come up with accidents that could have been avoided if PCM were used, and just as easy to come up with accidents that could have been avoided if PPM was used. Without the benefit of a study, its hard to say either way.

However, its very easy to say that a model plane flying without the control of the pilot, is ALWAYS better off with the engine stopped, rather at some random throttle position. This alone means PCM with failsafe is better than PPM without. Fitting a PPM receiver with a separate failsafe device for the throttle evens up the laying field quite a bit. One of the primary reasons I use PCM is for the failsafe features.

MO_Radio_Tech

Even if PCM did not have failsafe I would still use it over PPM.

HarryC

My Feedback: (1)

UK law does not require either PCM or failsafe be fitted. All it says is that where required by the CAA a failsafe must be fitted.

Remember that there are now PPM Rx with all channel fully programmable failsafe. Failsafe is not exclusive to PCM.

Harry

mglavin

My Feedback: (31)

What Harry said and more:

FAILSAFE is not exclusive to any specific manufacturer, modulation or use.

Hitec offers Digital programmable FAILSAFE servos, these can be used with any radio system on any frequency with any type of modulation...

HarryC

My Feedback: (1)

Indeed, and Hitec copy the Multiplex MC/V2 digital programmable failsafe servos, also all modern ESC's for electric models have factory set failsafe to shut off the power. None of these depend upon PCM, in fact a mis-programmed or unprogrammed PCM or PPM failsafe will prevent an ESC's failsafe from shutting off the power. All the more reason for users to really have an understanding of what they are doing which is too often lacking, viz the fatal accident in the UK a few years ago which exposed shortcomings in the user's knowledge and ability with PCM failsafe.

DavidO

I would like to clear one misconception stated in an earlier thread.

S/N or Signal to Noise is a loose term and tells us very little. Reference to the level of noise at the receiver input is generally termed SINAD, a measure of SIgnal to Noise And Distortion ie. 5micro volt @ 10dBi. S/N or SINAD can only be used to express changes or variance in amplitude of changing or varying analogue signal.

PCM and PPM receivers use the same RF, FM/IF demodulator IC and the same IF strip with the exception of a wider ceramic IF filter to accept and pass wider band information of PCM.


PCM is only as good as the PPM section fronting it !!!

BTW anything that can give us 1% extra safety has got to be the better option.

aksmithrc

My Feedback: (2)

I was just at Top Gun in Florida...amazing that practically all (if not all) of those guys use PCM in those birds !!!!
Just an observation ........

HarryC

My Feedback: (1)

If Woketman was there he certainly wasn't using PCM, he uses PPM with failsafe. Most people with PCM use it for the failsafe features rather than because they understand PCM, you don't want a short time glitch to ruin your competition flight, 200mph model etc.

H

klbass

My Feedback: (7)

I saw the benefit of PCM first hand today with my Hanger 9 33% Cap. I have two 4.8 batteries with two switches running into a single receiver. I was trying a new manuever (snap-roll to hover ala Jason Shulman 2002 TOC) when I noticed my throttle went to idle during the snap. I had no throttle control until I brought the stick back to idle, then throttle control returned. I brought the plane down to investigate. Could not find any problems. Went back up to do normal 3d, spins, with no problems. i tried the snap-to-hover manuever again and the engine went to idle with no throttle control. This time I figured out that the battery failsafe feature of my 9 channel Futaba PCM receiver was triggering with the huge control surface load of the snap to hover manuever. I took a few laps around the pattern and 30 seconds later, the throttle goes to idle. I returned the stick to idle and throttle control came back. I finally landed and called it a day. I read the Futaba manual and it said that the batteries must fall below 3.8 volts for the failsafe to engage. LESSON LEARNED...go buy 6v batteries for violent manuevers!!!! The failsafe rengages every thirty seconds even if power returns to normal. Works as advertised! I had no idea that this manuver could just empty the volts off your battery. PCM is a great feature to warn you of battery problems.

rajul

My Feedback: (58)

Klbass, this is a very good input and strengthens my move to 6V batt packs. Reminds me also that there are two types of failsafe, battery and interference........

sfaust

My Feedback: (11)

Its probably not the maneuver, but something else. There are many things that can rob you of voltage. Marginal batteries as far as current load handling. Using standard wiring instead of heavy duty 22ga or larger wires. Using the off the shelf Y connectors, too many connectors, all of which can suck up voltage faster than one might believe. It could be a combination of these things. And yes, you are correct that using 5 cell packs is worth it. Thats half the battle right here.

If you haven't seen this already, its worth a look.
http://www.rcuniverse.com/showthread...hreadid=424119
The info was gathered by another modeler on the IMAC list, and it's comprehensive as far as voltage depression is concerned. Lots of ways to avoid the issue.

While I wouldn't recommend it, I have flow a 40% with 4.8v packs and never had an issue even with hard/violent maneuvers. I was probably lucky, as I did have substandard wiring in that airplane (before I knew better), as well as lots of digital servos. Never saw a battery failsafe kick in though..

FLYBOY

My Feedback: (11)

I switched to PCM about 15 years ago and have never had a glitch. I have been flying on the boat when a guy behind me turned on his radio on the same freq and tried to start his plane. His went nuts, mine didn't even know his was on. He was FM. I have PCM in all my power birds. I like it.