RC Radios, Transmitters, Receivers, Servos, gyros Discussion all about rc radios, transmitters, receivers, servos, etc.

Difference between FM and PCM

Reply

Old 12-18-2004, 09:06 AM
  #1  
gowens
Junior Member
Thread Starter
My Feedback: (1)
 
Join Date: Dec 2004
Location: Williamsburg, VA
Posts: 10
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Difference between FM and PCM

I am new to RC and don't know th diference betwen the two systems.
Thanks fore the help
gowens is offline  
Reply With Quote
Old 12-18-2004, 09:40 AM
  #2  
dirtybird
My Feedback: (5)
 
Join Date: Jan 2002
Location: San Tan Valley, AZ
Posts: 5,768
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
dirtybird is offline  
Reply With Quote
Old 12-18-2004, 11:18 AM
  #3  
Rodney
Senior Member
 
Join Date: Dec 2001
Location: FL
Posts: 7,769
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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
Rodney is offline  
Reply With Quote
Old 12-18-2004, 04:57 PM
  #4  
gowens
Junior Member
Thread Starter
My Feedback: (1)
 
Join Date: Dec 2004
Location: Williamsburg, VA
Posts: 10
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Difference between FM and PCM

Thanks to both dirtybird and Rodney for helping me.
gowens is offline  
Reply With Quote
Old 12-18-2004, 08:54 PM
  #5  
da_man
My Feedback: (30)
 
Join Date: Nov 2002
Location: flemington, NJ
Posts: 493
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
da_man is offline  
Reply With Quote
Old 12-18-2004, 11:00 PM
  #6  
smokingcrater
Senior Member
 
Join Date: Sep 2002
Location: Up north, ND
Posts: 2,353
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Difference between FM and PCM

In the helicopter it does make a difference since it has metal parts which cause interferance with fm.
so you use AM receivers in your heli??? brave soul...
smokingcrater is offline  
Reply With Quote
Old 12-19-2004, 12:31 AM
  #7  
hebertjj
Senior Member
 
Join Date: Jan 2002
Location: Knoxville, TN
Posts: 324
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
hebertjj is offline  
Reply With Quote
Old 12-19-2004, 01:27 AM
  #8  
mr_matt
My Feedback: (10)
 
mr_matt's Avatar
 
Join Date: Dec 2001
Location: Oak Park, CA,
Posts: 10,325
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Difference between FM and PCM

ORIGINAL: hebertjj

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 ).
I think all modern FM systems use FSK, do you know of one that uses actual direct digital modulation like PSK?
mr_matt is offline  
Reply With Quote
Old 12-19-2004, 01:54 AM
  #9  
XJet
Banned
 
Join Date: Mar 2003
Location: Tokoroa, , NEW ZEALAND
Posts: 3,848
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Difference between FM and PCM

ORIGINAL: hebertjj

The RC industry has been real sloppy in their use of modulation and encoding terminology which has lead to a real confusing jumble.
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.
XJet is offline  
Reply With Quote
Old 12-19-2004, 05:20 AM
  #10  
HarryC
My Feedback: (1)
 
Join Date: Feb 2002
Location: private, UNITED KINGDOM
Posts: 3,560
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Difference between FM and PCM

ORIGINAL: da_man
I just want to point out that digital servos for some systems work better for pcm. That is because of the pulse rate.
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
HarryC is offline  
Reply With Quote
Old 12-19-2004, 09:10 AM
  #11  
hebertjj
Senior Member
 
Join Date: Jan 2002
Location: Knoxville, TN
Posts: 324
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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! )
hebertjj is offline  
Reply With Quote
Old 12-19-2004, 01:01 PM
  #12  
dirtybird
My Feedback: (5)
 
Join Date: Jan 2002
Location: San Tan Valley, AZ
Posts: 5,768
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
dirtybird is offline  
Reply With Quote
Old 12-27-2004, 02:27 PM
  #13  
exeter_acres
Senior Member
My Feedback: (2)
 
exeter_acres's Avatar
 
Join Date: Sep 2004
Location: Johns Creek, GA
Posts: 7,457
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Difference between FM and PCM

ORIGINAL: dirtybird

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


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


Me being one of them
exeter_acres is offline  
Reply With Quote
Old 12-27-2004, 02:39 PM
  #14  
mr_matt
My Feedback: (10)
 
mr_matt's Avatar
 
Join Date: Dec 2001
Location: Oak Park, CA,
Posts: 10,325
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Difference between FM and PCM

ORIGINAL: HarryC

PPM refreshes its data faster than PCM.
Huh? Did you mean lower latency? Cause as far as I know, most PPM systems update the servos at the same rate.
mr_matt is offline  
Reply With Quote
Old 12-27-2004, 03:12 PM
  #15  
HarryC
My Feedback: (1)
 
Join Date: Feb 2002
Location: private, UNITED KINGDOM
Posts: 3,560
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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
HarryC is offline  
Reply With Quote

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service