I have several radio receiver with PCM. I really like them. But today I flew a plane with FM receiver. It was a different plane that I'm not used to but the receiver seemed fast and efficient. Exactly what do you gain with PCM over FM?
thanks,
Barry

Fail safe/hold function but more importantly (in my opinion) far better noise rejection as the signal is digital, not analogue.

Personally I would not fly a plane unless it had PCM. The only exception being if it was a wee foamy model.

Angus

Barry,

Glitch protection.

Most people fly FM and don't have any problems. On some plane-engines-installations, you can get glitching from a variety of reasons. I recently test flew a Great Planes Pitts Special ARF with Zenoah G-62 power for a club member. It range checked OK, but right after take off I got a nose down hit. Once at altitude and out in front, the plane was OK. On landing at about 10 feet, I got the same blip down. I added power as people docked for cover and made a second approach, steeper and shorter with the plane more in front all the way and got it down.

Here are a few things I caught after landing that I probably should have before I flew it. His tail bracing wires and connectors were all metal, making a nice 1 meter long loop, about the same length as an antenna. His antenna ran right down the middle. He also has long leads from servos mounted in the tail, running parallel to, and close to the antenna. Finally, he had the ignition switch of the G-62 mounted next to the radio battery switch.

All of these can be fixed, and should be, but a PCM receiver would have ignored these minor glitches unless they were repetitive enough to cause the system to go into fail-safe. He had close to a thousand bucks in the air, so an additional $20-$50 for a good PCM receiver doesn't seem to be out of line and is good insurance. The little JR 770 PCM receivers I mostly fly are only $99.99 compared to the FM 700 at $79.99, so for 20 bucks, I put these on my "good" planes.

He is buying a PCM receiver and making the other fixes before we fly it again.

Thanks for the input. So the PCM has a more pure signal recgonition? I'm using PCM receivers on all 3 of my planes and have two more for the two planes I have on order. I use a Futaba 9CAP radio. Is the signal received and faster that FM or can the input from the sticks be felt any more precise or is the only benefit, no glitchs?
Thanks,
Barry

some top show pilots ive talked to dont use PCM - principally because of the lockout function. Theyd rather have a glitching plane and have some control over it than a model thats gone into failsafe.

i'm however sitting on the fence here when it comes to this debate - thats why i use the multiplex IPD receivers, which combine all the plus points of both at a cheaper price than pcm. the only problem is crystals cost more.

The model will only go into failsafe as long as there is interference around. After the interference has ceased normal operation returns immediately. At least it does with JR and more than likely with other manufacturers as well.

Plus as PPM receivers aren't top of the range (correct me if I’m wrong) they don't come with all the other stuff the top end rx's do such as narrow band ceramic filters and all that gubbins.

I thought it was compulsory that models at shows had PCM and if it isn't then it bloody should be. You can’t argue that PCM is not far safer than PPM. When you go to competitions they check you have a failsafe i.e. that you have PCM. Should be the same at shows.

Angus

PPM receivers are top of the range too and they are narrow band and they are digital.

PPM is not digital. It varies the length of a pulse, the length of that pulse dictates how far the servo moves. The length of the pulse can be affected by noise and seeing as PPM rxs don’t have any software to recover the signal they have no way off telling if this is the intended signal or not. Even if they did, how would they know if this new signal is correct or not as it can vary infinitely between the two extremes.

PCM is digital as it uses binary code like 011001110. This type of code can be recovered as it is either on or off, nothing else, so the intended signal is seen at the servo end. Or something along those lines anyway.

Angus

Another important quality of PCM is 1024 bit operation, versus 512 with FM. This means that, for a given control input, the PCM divides it into 1024 steps, meaning smaller units of movement which means smoother operation.

Here is a question then. I am looking to buy a new radio as my futaba 6x is at about the capacity for which I can use it for. I have 3 FM recievers. I am looking at the 9Cap or 9Z and want to know if either will work with the FM recievers or is it time to sell them too and invest in all PCM. I understand that if I buy a PCM/FM transmitter I need to switch it to FM every time I am going to fly one of my FM recievers or does it program in with the other airplane functions. I guess if I need to buy all PCM then so be it but woiuld like to use my FM still. Any help and radio selection advice would help me in my purchase.......

nate1778, Either the Cap9C or 9Z will transmit to both PCM and FM receiver. It's easy to set up the programming for FM, you do it when you first start programming that model. The radio remembers whither it is PCM or FM and will remind you if you have it wrong. The only time it would be wrong is if you were flying a model with PCM or FM and changed models to fly (you can memorize up to 14) and the new selected model had the oppisite resolution then your radio would blink at you and telll you, you need to change the resolution. The way you change it is to cycle the power to the radio. It's that easy.
Thanks.
Barry

Hey guys, I just bought my first PCM rx and was wondering how to program failsafe. It is a seperate unit i have to buy, I know i can prgrm my Hitec Digitals, but not sure about the rest. Thanks

What transmitter do you have? That is how you program the failsafe--see transmitter instructions for programming PCM failsafe, program it then test it by shutting off you transmitter and making sure the servos go where you want them.

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This is trivial but I like this about PCM--f you turn on the plane before the transmitter your servos don't go crazy.

I also have a Berg6 with failsafe and like that receiver as well--not PCM, but does have some type of signal recognition and lock and failsafe does work when I test it.

I have a 9cap. So all my failsafe programming will be done there?

Yes look in the manual. Mine is under Failsafe and Battery failsafe, page 43.

Tom Fawcett posted this the other day in the Wildhare support forum so credit goes to him for the explanation. This is also about the best and most concise explanation of the pro vs. cons of PCM vs. PPM(FM).

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In cases where the radio link is less than perfect, PCM can cause as many problems as it solves.

A regular FM receiver just takes the signal coming in, splits it into 8 streams (for an 8 channel radio) and sends the individual streams out to the 8 different servo channels.

If there is noise or an invalid servo signal, that also gets passed straight out to the servo. This is why, with a PPM (FM) radio you see the surfaces jump around when noise creeps in, they are just following the signal which sometimes is not valid.

With PCM there is processing involved at both ends, Tx and Rx. In the Tx, the 8 (or whatever) signals are combined into a frame where each servo position is represented digitally by a binary number. This is where the "PCM 1024" comes from, they use 10 bits to represent the servo's position so there are 1024 possible positions.

The values for all the channels are then packed into a frame (now nominally 80 bits of data for 8 x 10 bit channels) with a header and trailer, and at the end of the fram a check sum is appended that is calculated as the bits are transmitted.

When the receiver gets the frame, its internal processor recalculates the checksum of the received frame and compares it to what was received. IF they are equal the new values for each servo are transformed into the signal which is sent to each the servo.

But if the checksums do not match, the received frame is discarded and the values from the most recent good frame are retained. This is what is called going into a "hold", the receiver "holds" the last known commanded position that was without errors.

It gets more complicated with fail safe, but the basic strory here is that PCM covers up small errors that will show up with PPM. If you are having problems, you may not see them with PCM resulting in a mistaken idea that your radio is working great when it's not.

If you EVER have an insufficient range check, do not fly, and I recommend installing a PPM/FM receiver (temporarily) to help diagnose the problem. It's sort of like turning on a light, suddenly you can see all the problems that were hidden by the PCM Rx.

Once you get an acceptable range check with a PPM/FM receiver and with the engine running, now you can go back to the PCM and recheck the range.

It's worth the $60 (Hitec Supreme 8 channel FM) to keep one of these around just for checking out you plane, even if you never fly with it.

It most certainly is digital. The information is encoded in a pulse train of on/off (digital) current where the pulse position or width determines, as you said, how far the servo moves. There is no analog voltage or current ramp in the signal anywhere. PCM sends the on/off (digital) information coded in a different way.

Ok, the signal coding is digital as it is just high/low but what I meant was the pulse length is analogue.

Any engineers in the house? Maybe one of them can solve the digital question.

I use an FMA failsafe FM receiver in place of the standard Futaba FM receiver. With staight FM, had at least on glitch per flight, usually at the same location on the field, but non with the FMA failsafe. An LED on the FMA radio blinks a code to tell me how many times it was hit in a flight. Have not had one glitch in several hundred fights on several FMA receivers. The FMA has digital signal recognition which, apparently, does a good job. I can program each channel for "last good signal" or "failsafe position". To test, I had a buddy turn on my backup transmitter - on same frequency. Radio went from in-control to failsafe several times but remaind level and controlable.

PCM receivers have about 10db better noise rejection overall according to empirical tests, not just theory; that's enough reason to go with them if you have an expensive toy in the air. They also have better adjacent frequency rejection which is very important on those days where the flying field is crowded.

The fancier FM receivers are narrowing this performance gap, but I think PCM is still superior.

Clark