Redwulf__34

I'm looking at buying a radio and from what I do understand I will want PCM if running gas engines. I do plan to run gas engines, so it seems I want to get PCM. What I don't understand is the relationship between FM and PCM. Is it one or the other or is it both.........

The radio I've settled on is the Futaba 7CAP. While I see the advantages of the 9CAP I just can't afford it right now.

I've been watching a 7CAP on E-bay but I didn't understand the PCM thing well enough to bid on it. It probably sold for $150 and was only 7 months old. Can anyone help me understand it in a reasonable way that I can make an educated buy. I'll probably just wind up supporting the local hobby store and buying it there and probably won't "screw" it up. But if in the mean time I run across a good deal on E-bay.....who knows. If I'm going to buy new I'll definately buy from the local guys. But it would be nice to if I could find almost new and get a deal.

Richard L.

My Feedback: (24)

There are several threads in the Radios forum. Here are some examples:

* [link=http://www.rcuniverse.com/forum/m_3318903/anchors_3318903/mpage_1/key_PCM/anchor/tm.htm#3318903]FM and PCM. whats the difference?[/link]
* [link=http://www.rcuniverse.com/forum/m_3592872/anchors_3592872/mpage_1/key_PCM/anchor/tm.htm#3592872]do i need a pcm receiver with gas?[/link]
* [link=http://www.rcuniverse.com/forum/m_3810936/anchors_3810936/mpage_1/key_PCM/anchor/tm.htm#3810936]PCM radios?[/link]
* [link=http://www.rcuniverse.com/forum/m_3926417/anchors_3926417/mpage_1/key_PCM/anchor/tm.htm#3926417]PCM vs FM[/link]

There are other threads on the same topic. The "search" function is one of your best friends. I only use PCM nowadays.

TLH101

My Feedback: (90)

I never use PCM in any of my gassers. Most on here will think you need it, but you don't.

Redwulf__34

I've read many of the threads but they are a bit confusing. I was hoping for something "real" basic.

The particular system I as looking at did not say PCM on the RX. I will assume then it was not.

TLH101

My Feedback: (90)

Both types are "FM", but they are called PPM and PCM. It has to do with the way the signal is encoded and sent to the receiver. PCM is supposed to reject "stray" signals and interference better than PPM. I don't know enough about the details to really speak more on the subject, without possibly putting out some eroneous info.
Like Richard mentioned, you will likely get a more accurate answer from the radio forum.
Try this, I have not read it, but may help:
http://www.aerodesign.de/peter/2000/...M_PPM_eng.html
Or this:
http://www.natew.com/rcheli/frame_ma.../html.PCMvsPPM

Redwulf__34

Thanks TLH, That's just what I wanted to know.

Peter_OZ

My Feedback: (1)

FM is in fact not "true" FM (Frequency Modulation) but is for our uses a form of frequency compression. True FM varies the carrier wave frequency.

AM on the other hand modulates the amplitude of the carrier RF wave.

For Analouge encoding PPM (pulse code modulation

It uses analouge encoding whereby a series of "pulses" are encoded onto the carrier signal. If you look at the pulses before they modulate the carrier RF wave what you will see is a "reference" pulse followed by a pulse for each channel. These channel pulses will move back and forth proportionaly with respect to time as you move your sticks.

For PCM Encoding:

This is a form of encoding where by the proportional varying control voltage from the "sticks" is converted into a digital signal in the form of a propietry code. If uses special encode / decode programs to achieve this. each manufactuer has their own code, hece JR will not work with Futaba and conversely.
The signal contains not just stick position but also checksums so that both TX and RX can have some level of error detection.

It is said the digital signal is more accurate but this is dependant on the sampling rate, i.e how many times per second the microprocessor can sample the stick position. In actual fact analouge being infinitely sampled is more accurate but is not digitised.

Where the interferrence improvement comesin is that with analog modulated signals either AM or FM that an interffering signal can cause for AM the amplitude to be glictched or spiked causing error or for FM the frequency compression is caused to jitter when the signal is interferred.

With PCM it does not care if the amplitude or freq compression is interferred with as long as there is small amount of signal present it can decode the orignal encoded data. this assumes the signal is not totally blocked.

So as for just the RF side there is no difference in signal quality or performance between systems using PPM or PCM.

Where the difference lays is in the encoding, AM relies on the amplitude of the signal on the carrier wave to generate the data, any little glitches can destroy this quite easily, FM is also succesptible to interfference cuasing unwanted freq compressions.

PCM can have interffernce in either amplitude or frequency but even a very low level signal can be decoded. It also has sample and store feature where if it is totally blocked or gibberish is decoded it will hold the last good pice of data it recieved.

Pretty high level but gives you a bit of an idea.

hope it helps your understanding. If i had a whiteboard you could view I could draw some diagrams!

cheers
Peter

Redwulf__34

Well explained Peter.

Tha't the most understandable it's been for me. I feel pretty good about understanding it now.

thanks!

Peter_OZ

My Feedback: (1)

glad to help out

cheers
Peter

BarracudaHockey

My Feedback: (11)

Just an FYI

The 7C (and 9c for that matter) will transmit either PCM or PPM modulation, and is selected PER MODEL, so if model 1 has a pcm reciever, you go into the proper menu and select pcm for that model, while model 5 has a ppm you select that and when you change models it will change transmitter modes, you just have to power off and on the transmitter when you change models.

The designation 7CxP or 7CxF (the x being A for airplane, H for heli) simply means thats the defualt setting for a new model, and the type of reciever that ships with the set, the transmitters are identical with some minor differences in switch position and throttle ratchet between the helicopter and aircraft version.

PJ_TankPilot

I have not seen mention of battery failsafe. Futaba PCM will throttle down if the receiver battery gets too low. I recently had a new 4 cell battery that one cell shorted out while flying. The receiver throttled down the engine letting me know I had a problem. Probably saved the aircraft.

Keep in mind that PPM is about 50 year old technology and PCM is recent. The only disadvantage I have ever found with PCM is the price. About 40-60 USD per receiver. Also, if you need more than 8 channels, you must use PCM.

The field I usually fly at has periodic interference on all RC channels. It will be bad for a day then go away for months. When it occurs, people flying PPM have fits with the glitches, those flying PCM have no problems. A bad PPM glitch at a bad time can give you a heart attack. [sm=what_smile.gif] Keep in mind that dual rates, expo and epa do not apply during a PPM glitch. A fast servo can move a lot before the signal gets good again.

8178

My Feedback: (17)

Or PCM control failsafe settings that I think are mandatory for large or fast aircraft.

BobH

My Feedback: (2)

PCM new....? Actually PCM was invented in 1937 by a British engineer.

8178

My Feedback: (17)

Actually, our AM radios do use not true amplitude modulation either. Our AM radios, transmit a train of pulses and the amplitude of the pulse signal is always the same. There is a sync pulse and then the length of the pulse for each channel changes depending on the stick positions. After all channels are sent it restarts with a sync pulse. It is really pulse width control.

8178

My Feedback: (17)

Yes, it has been used in the telecommunications world for decades.

Redwulf__34

BarracudaHockey
Do I understand you right that ALL 7CAP transmitters are PCM capable?

Here would be my ideal scenario,

I buy the 7 CAP now with the lesser expensive flight pack, (hence saving a few bucks) for my smaller aircraft. Then when I'm ready to build my first "big" bird I simply buy a PCM receiver and flight pack. Then I'm set? I just switch the transmitter depending on which plane I decide to fly. PCM or PPM..........That would be ideal for me if that's the case.

Rodney

As 8178 stated, AM is not really amplitude modulated, it is a series of on/off RF pulses with the information being transfered by the width of the time between the leading edge of each on pulse. Also, the so called FM is not really FM in its true sense, it is a form of frequency shift keying whether PPM or PCM, i.e. either one frequency or another. True FM allowes infinite varieation between the two end frequencies being transmitted and is not done in our typical RC transmitters.

BarracudaHockey

My Feedback: (11)

Redwulf

Yes, the 7CAF and 7CAP are identical with 2 exceptions, the default modulation type for a blank model and the reciever that ships with it if you buy a complete radio set. If you buy a 7cap and want to fly a PPM (what many mistakenly call FM) you simply go into the parameter menu for that model and change the modulation type. If you buy a 7CAF then that will be the default and you would have to change it for any model that you wish to fly a pcm receiver with.

Bax

My Feedback: (11)

Actually, they ARE "true" AM and FM, if you go by the circuitry involved in the various components. When you add the modulation to an "AM" transmitter's output section, you do it through the same kind of circuit that you would use if you were transmitting voice. In fact, if you just put a voice signal (or music, and so on) on the wire going to the output circuitry, you'd have a voice transmitter. If you put a small amplifier and speaker on the demoulator of an AM receiver, you'll be able to hear such a voice tranmission. We used to monitor a local pager at one time by doing simply that...it was a very powerful transmitter adjacent to an RC channel, and made that channel impossible to use, but we could hear its transmissions when we boosted the demodulated signal that came through the RC receiver.

Same with an FM system. The modulator in the transmitter and the demodulator in the receiver are the same basic circuits you'd use for voice or music transmission, and could be used for such if it was allowed in the frequency bands we use.

.....and for regulatory purposes, for example, an FM (PPM or PCM) transmitter has a transmission definition of "F1D", which means it's an FM carrier with pulsed information for telecommand purposes.

And maybe we're just bandying about the semantics of AM or FM.......

Rodney

What I mean by AM as used in RC not being true AM is that the signal is either on or off, no amplitude modulation, so it is more like a very highe speed CW ham signal than true AM where the amplitude of the RF signal is varied over a wide range to provide intelligent info. Also, while both PCM and PPM do have true FM modulation, it is also not varied infinately by the modulating signal, it is either one frequency or another with no "in between states" such as you get with typical FM signals designed to be heard by the human ear. You are correct when you say we are probably arguing symantics so I'll say no more on the subject.

pilotpete2

Bax,
Thanks for the clarification, one question though, on our 72 Mhz FM systems when we refer to neg and pos shift my impression was that the PPM output was inverted on one as compared to the other, does this result in the carrier (FM) being deviated in either a plus or minus direction?
Thanks,
pete

Bax

My Feedback: (11)

The PPM/PCM output from the encoder could be identical on two units. Just one unit's modulator applies the signal to shift the frequency one way, and the other unit's modulator shifts it the other way. Thus, one transmitter's carrier is deviated one way, and the other transmitter's carrier deviates the other. It depends upon the choices made by the designer.

akschu

My Feedback: (5)

Redwulf__34,

The biggest drawback to the 7cap is that it won't allow you to bind two channels together and have trim so you can do dual elevator servos. Since you mention you have gas airplanes you may want to save a little more and get a 9cap otherwise you may be stuck looking at another upgrade.

That said all of the 7ca and 9ca transmitters are the same and can transmit pcm of ppm, so the only difference in the package is which receiver you get.

Another thing to think about is that pcm transmits codes and when the reciever can't understand the codes due to interference it basically just cuts out and goes to the default fail safe stting. The ppm reciever sends everything it hears to the servos. Because of this, interference shows up as glitches. They both have advantages and disadvantages. On pcm, you get the fail safe, and you don't have to worry about a glitch on a low pass, but with ppm, sometimes it's possible to fly though the interferance.

The best example I can give is analog vs digital cell phones. One you talk though the static (analog) the other your voice gets cut off or garbled (digital).

You pick whichever works for you, but for me, I live in Alaska and don't have interference problems so I just run ppm recievers because they are cheaper.

schu

Redwulf__34

How big does the airplane have to be to need a dual servo elevator?

I have pretty much decided on the 7CAP. I've had the local hobby store order me one.

I was planning to build a 102" Meister Scale BF109. Am I going to have a problem?????

akschu

My Feedback: (5)

It's not a size thing as much as how it was designed. I just got a QQ 73" Yak this summer and it has a seperate elevator servo for each half I would have been stuck trying to sync servos with a y-cable if I didn't have a 9c.

schu