Hi ..
Anyone can tell me what is the difference between FM and PCM trasmitters ?, i'm planning to buy 6 channel radio system soon.

Mishal

Cost and method of transmitting the signal. PCM costs more. Unless you fly in a HIGHLY congested area, PCM isn't worth it. The fail-safe usually provided with PCM systems MAY save your plane, and it may not, too. Most planes can fly through intermittent interference. Constant interference will shoot down any radio system. I've been flying FM for over 25 years, and never been shot down.

Dr.1

So the FM trasmitters practicaly are better than PCM ones, well .. I think i will go though Futaba 6EXAS 6-Channel FM with four S3151 digital ball bearing servos , do you think it is better to replace those servos (1.48oz (42g) each) with micro servos to reduce the overall plane weight ?

mishal

search is your friend


to answer your question...

Nothing...

PCM is FM....its just a type of

and as for the servos, it depends on the plane and how much torque you need

Not to get off subject too much, but DR 1, I have a question. From the thread entitled nextstar......., you wrote:

"My Instructor hates the plane.....says everyone he has trained on has had problems like this.......
Most pilots who fly the Nextstar don't like it. It does have problems, including the LG and that stupid autopilot. Even with the autopilot turned off (which I heartily recommend) it flies like crap. There are trainers out there that are a LOT better.

Almost ready to scrap it and go to something else.
I'd agree with that. Get yourself a Tower Hobbies trainer, or the Hobbico Airvista or Avistar. Frustration is NOT what you need while learning.

Dr.1

This doesn't sound like you've been flying for 25 years..... What gives?

The primary purpose of failsafe is not to save your model - it's to try to save someone's life. Having the ability to close the throttle when the aircraft is otherwise out of control means that you substantially reduce the killing power of the aircraft.

I've been shot down on PPM once, and watching the model scream towards the ground at full throttle and dig a hole 18 inches deep was a sufficiently scary experience that I immediately went out and bought a PCM system and programmed the failsafe for throttle kill.

Be careful what you read or get told about PCM - there are a whole bunch of ludicrous old wives tales around, because being totally ignorant of PCM's real behavior unfortunately does not lessen some folks' enthusiam for spreading their 'expertise" around.

See http://www.rcuniverse.com/forum/fb.asp?m=199627

Gordon

PCM fail-safe for throttle is a handy feature to have. However the older crowd seems to have a general dislike for PCM. Might be the cost, might be a general dislike of change, or might be a misunderstanding of the merits. PCM is a newer technology and the error checking is better than PPM. PCM will not save the airplane from continuous interference but might alert you to minor interference. I have and fly both.

Bill

I'm going to jump in here for a minute and clarify something. The original post wanted to know the difference between FM and PCM. There is no difference. PCM is a method of encoding the information being transmitted, but it is still transmitted as an FM radio signal. The real question should have been what the difference between PCM and PPM is.

Just wanted to clear that up.

Ken

Off the top of my head, besides failsafe, PCM is twice the resolution. Right? I wonder if that is translated through to the servos?

2slow2matter - back up a little bit on Dr 1. He was quoting the original poster and merely forgot ti indicate it in any way. Go back and check it out and you will fine the statements in question imbedded in the orig post. I believe an apology is in order. Just MHO.

Bruce

RCken is correct. Ptulmer is not. PCM's max resolution it the number of steps, usually 1024 max. PPM is infinite resolution limited only by the resolution of the pots in the transmitter encoding and servo feedback.

The masking problem stems more from a misunderstanding than anything else. The difference is in how the units behave under interference. PCM rejects bad frames, this results in hesitation or jerky response that is less obvious when compared to ppm units that twitch all over the place trying to move servos to bad position information. What is often not discussed is the fact that this hesitation will not even be noticeable until interference is large (amplitude and duration) to render the PPM unit unflyable. You'll often read of pilots preferring to "fly though" interference with PPM than be "locked out" of control

This PCM 'lockout' myth is another common misunderstanding. Failsafe is engaged sufficient back-to-back bad frames (frames dropped due to being scrambled by interference) are received to enter failsafe (varies by manufacturer but, typically ~30ms). This mythical 'lockout' is the radio entering failsafe mode.... You literally have lost all potential for control due to interference. Control is returned when a number of good frames (again, varies by manufacturer but is typically less than 10ms) are received.

It's also significant to note that the fact that PCM has a better ability to reject noise.
In fact, the signal to noise ratio tolerance is so significant that PCM units will continue to operate under conditions that would render a ppm (often misreferred to as FM) system useless. In other words, you would experience complete loss of effective control with a PPM unit before any noticeable impact on a PCM system. This is why many people suggest switching to PCM in conditions where interference is too great for PPM units (e.g. Gas/ignition engines). This is not good practice as interference has a tendency to increase over time. You might get by for a while but, eventually you'll get bit.

HTH, Jim.

Another myth.

The fact is that the input pots must be converted from a physical position (your thumbs on the sticks) to a voltage. These days this is done by comparing voltage drop across the potentiomenter to a reference voltage. This is commonly referre to as ADC (Analog to digital )conversion. Resolution for ppm is determined by the accuracy of this ADC conversion and accuracy in the broadcast signal.

This is why PCM is also preferred where repeatability (eg. always return to the same locations) is required. Since the position is sent as a digital number (the one from the ADC), there is no pulse position interpretation of a modulated signal (PPM) required at the receiver.

ahh, now I understand. My bad.
I don't know everything about this hobby--I'm sure no one does. I try not to give advice on things I know nothing about--I'm still learning. However, we all see people all the time give poor advice on RCU, and act like they know what they're talking about. That just tears me up. I was trying to figure out where DR1 was coming from. Now I understand.
Sorry 'bout that, DR!

I'm not an expert but I know a little bit about radio so here is some information that may help someone:

FM is frequency modulation, it's how the radio puts servo information inside of a radio transmission. There are two ways to modulate servo data used today, PPM and PCM. PPM simply takes the servo information and stores it in the pulse width, while PCM takes the servo information and stores in a binary format using a protocol.

In a nutshell it's basically the difference between an analog and digital cell phone. With the analog phone you get static, but sometimes you can talk though it, where digital phones you don't get static, instead you don't get anything if enough data is lost to make the protocol stop working.

I have never used a PCM system so I am speaking without experience, but my next system will be PCM simply because I would rather have my equipment use my last good known commands until the signal is completely lost, in which case do some programmed action, instead of do whatever random thing the interference is transmitting.

PCM advantages:
Servo movements without glitch, even if the model is far way.
Holding of the servo position during short glitches (Hold).
Moving the servo to a predefined position in case of a longer disturbance or
even complete failure of the transmitter (Fail-Safe).
Servos are not damaged by pulses that are too long/short, which could happen
with PPM.

PCM disadvantages:
More expensive.
Sensitivity to adjacent channels is usually worse comparing with PPM receivers.
Care has to be taken when flying near to a transmitter from an adjacent channel.
Due to different protocols, only receivers from the same brand or even type of
the transmitter can be used.
Checking the transmission quality can be difficult, because the hold-mode
smoothes out small glitches.
The lack of early warning signs may cause trouble.
Control problems that build up gradually, e.g. of a technical nature, get noticed
only when the connection fails completely, which may lead to a crash.

PPM advantages:
The PPM system is cheaper.
There should be no problems using different brands of receivers with different
transmitter manufacturers.
Transmission is fast enough to operate even the quickest of servos.
With PPM, the end of the transmission range is shown by the servos starting
to glitch. When the pilot notices this, he/she can probably still get the model
back home safely.

PPM disadvantages:
Due to its simplicity, PPM system cannot detect errors, the receiver does not
see the difference between valid and invalid servo pulses. When the range
boundaries are reached, pulses get slightly longer or shorter because of noise.
Servos start to glitch. This may happen when antenna orientation is not optimal,
when the projection of the receiver antenna is nearly down to a single point, the
signal breaks down and the servos get false pulses.
These short glitches go unnoticed most of the time because they are smoothed
out by the servo's and the model's inertia (response time).

Nothing is more convincing that this story which details my PCM PPM story.

I was flying my Cub on my Futaba PCM and shortly after take off the plane started to numb to my controls. I knew immediately that I was in the process of being shot down. Because it's a Cub and has self righting capabilities, when it wouldn't listen to me, at least all it did was to return to failsafe, which was idle and neutral controls. With just intermittent control, I flew her down. I walked all the way to the beginning of the runway to make sure I was the closest to my plane when it touched down. After cutting the throttle and retrieving my plane, I stomped over the frequency board and found that someone did have the exact frequency I was flying. I whipped out my mobile phone and called the culprit. We immediately got yelling at each other when I saw him walking up the outside of the fence, one hand carrying his phone and the other, his engine connected to the fuel tank. His plane was a total loss.

He was running PPM and actually even after he crashed, he didn't even turn off his radio. He left it on at the flight line, antenae fully extended. I was pretty pissed off that someone would be that EVIL as to do that but even with that, I was able to save my plane. While I still run many planes PPM, I save my best planes for PCM since failsafe can save a plane. Had it not been for failsafe (which I believe is now available separately) I would have lost the plane and may have injured someone.

Interesting....

So the question is how did you both end up with the same frequency? Who was the rightful 'owner' of the frequency at that time?

akschu - actually PPM stands for Pulse Position Modulation as opposed to PWM = Pulse Width Modulation. Now in our PPM system I believe we DO actually vary both the position and width of the pulses, but I could be wrong. I will throw my transmitter on a scope when I get a chance. I did this once before maaaaany years ago with an AM set but can't even remember the results of that test.

Opps, got my facts wrong, I hate when that happens. The rest of the post should be fairly accurate though....

There are some technical differences between PPM and PWM but on all recievers, including PCM receivers, the signal sent from the receiver to the servo is PWM, i.e. a positive going pulse that varies from 1 to 2 milliseconds in width depending on input command occuring once every frame of transmitted signal, typically every 15 to 25 milliseconds. In general the frame rate of PPM is faster than PCM, i.e. the servo is updated more often using PPM than it is using PCM although the difference is to small to worry about.

Sounds right because it would take more time to encode/transmit the PCM data word.

akschu - i was the rightful owner. our frequency board is pen and paper based. you put down your frequency and your phone number. those that come later have the responsibility to call those that have already posted should there be overlap. he had scanned the board too quickly and failed to see mine (i was like the fourth on the list under 36mhz)... so after all that yelling at me he sheepishly admitted that he didn't see it. although he didn't offer an apology, i didn't push it, i figure a wrecked plane was all the guy could handle at that point.

It is going to be great when that forty-year-old concept of pin board is not necessary. Please encourage the manufacturers to make it happen sooner rather than later.

Bill

Someone at your club should make a board. It would be pretty cheap and only take an afternoon....