Difference between FM and PCM
 

I am new to RC and don't know th diference betwen the two systems.
Thanks fore the help

RE: Difference between FM and PCM
 

If you are truly new to RC the biggest difference you will find is the PCM receiver costs twice the FM(PPM) receiver. Is it worth it? In my opinion,no.
Do a search on PCM and you will find many other opinions. The subject has been beat to death.

RE: Difference between FM and PCM
 

PCM is FM, so is most PPM. You will find all the major manufacturers higher end systems are FM whether they are PCM or PPM. There are still a few AM PPM systems available and, many years ago you could also get PCM using AM. To better give you an explanation of the various systems, check out www.torreypinesgulls.org/Radios.htm

RE: Difference between FM and PCM
 

Thanks to both dirtybird and Rodney for helping me.

RE: Difference between FM and PCM
 

I just want to point out that digital servos for some systems work better for pcm. That is because of the pulse rate. For airplanes I usually have Fm because of the cost and PCM in my helicopters. In the helicopter it does make a difference since it has metal parts which cause interferance with fm.

RE: Difference between FM and PCM
 

so you use AM receivers in your heli??? brave soul...

RE: Difference between FM and PCM
 

The RC industry has been real sloppy in their use of modulation and encoding terminology which has lead to a real confusing jumble. As previously mentioned above, nearly all the latest "serious" systems use FM modulation on the carrier signal. This is not your "normal" FM as in your auto radio, but a form known as Phase Shift Keying ( PSK ). The actual control info which is sent by the carrier can be Pulse Position Modulation ( PPM ) where the width of pulses in a train of pulses controls the postion of each servo channel when the Rx sorts out the proper pulse and sends it to that servo channel. Or it can be Pulse Coded Modulation ( PCM ) where the Tx sends binary numbers, one for each servo channel, and the Rx decodes this into a pulse width sent to each servo.

Thoroughly confused? Recap: Most systems are FM; some are FM/PPM and some are FM/PCM.

RE: Difference between FM and PCM
 

I think all modern FM systems use FSK, do you know of one that uses actual direct digital modulation like PSK?

RE: Difference between FM and PCM
 

This is understandable because for a long time all RC gear was AM.

Then, as the necessary components became readily available and cheap, some manufacturers switched to FM, which offered superior performance.

For a long time RC users had the choice of AM or FM.

Then one smart manufacturer decided to use the power of microprocessors to convert the transmitter stick positions into binary numbers and transmit those numbers instead of a set of simple pulses. This was PCM.

So, the common conception was that really old gear was AM, contemporary gear was FM, and the really new "state of the art" stuff was PCM.

Now that AM has largely disappeared, people still describe the remaining two systems as FM and PCM -- even though both actually use frequency modulation.

I realise that most of us oldies know this already -- but that was for the benefit of newbies who might still be a little confused.

RE: Difference between FM and PCM
 

That's nonsense. "Digital" servos refers to the higher frequency pulsing sent to the motor, and has nothing to do with digital or analogue data. ALL PCM systems, without exception, convert the digital PCM code back into analogue PPM for the servos. ALL receiver outputs are analogue PPM. PPM refreshes its data faster than PCM.

H

RE: Difference between FM and PCM
 

Xjet, do you realize you just said "a real confusing jumble" is "understandable"? Like my wiring inside my planes, an understandable jumble, Hah!

Matt, you're right, I said Phase Shift Keying and should have said Frequency Shift Keying. I don't know of any hobby R/C systems using PSK and doubt they exist due to the standard (cheap) components for FSK/FM that work so well. The main thing is it's not music type FM. Maybe some of the UAV systems use PSK?

Harry, you probably just shot down daMan who just spent a whole lot of dough on digital servoes. Reminds me of an explanation of how music CDs work that I saw in a newspaper: "The music is encoded as tiny microscopic holes on the surface of the CD and where there are holes, there's music, and where there are no holes, there's no music." (Get it, binary music! )

RE: Difference between FM and PCM
 

As I said in post #2 this subject has been beat to death. Now we just flogged it a bit more.

RE: Difference between FM and PCM
 

Maybe so...but there are always new people who don't know all the answers yet......;)


Me being one of them

RE: Difference between FM and PCM
 

Huh? Did you mean lower latency? Cause as far as I know, most PPM systems update the servos at the same rate.

RE: Difference between FM and PCM
 

A PPM system can refresh the data every 20 milliseconds or less, depending on the number of channels being transmitted and the time of the pulses. My system in 9 channel mode with all channels at centre 1.5ms is refreshing every 18 - 19 ms or thereabouts. In 12 channel mode it refreshes approx every 20ms if all channels are at centre. Compare that to the Futaba 9Z which is 8 proportional channels and a 1-bit channel, at 28.5ms. It is only the new 14MZ that gets PCM down into PPM times.

H