FM Vs PCM modulation  
View related threads: (in this forum | in all forums)

Tower Hobbies
Enter up to 4 keywords or Tower stock numbers
Logged in as Guest



Users viewing this topic: none
  Printable Version
       

All Forums >> RC Helicopters >> RC Helicopter General Discussions >> FM Vs PCM modulation
Page: [1] 2   next >   >>  

Login
Message << Older Topic   Newer Topic >>
FM Vs PCM modulation - 11/19/2003 10:46:15 PM   
Razmo



Posts: 174
Joined: 10/27/2003
From: Chicago, IL, USA
Status: offline 		Whats the difference?
       Post #: 1

RE: FM Vs PCM modulation - 11/19/2003 11:27:09 PM   
Spaceman Spiff



Posts: 1652
Joined: 1/22/2003
From: Tucson, AZ,
Status: offline 		i am no expert, but i read that the difference is the signals are digitally encoded for PCM, the advantages are: it is less prone to interference, PCM is very popular for guys who run gass engines, because it does a better job of rejecting spark plug noise. Another advantage is the reciever is smart and can be set to a fail safe condition (engine shut down) if no valid signal is detected. I use FM and have had no problems, but PCM is the top shelf technology.

(in reply to Razmo)
       Post #: 2

RE: FM Vs PCM modulation - 11/20/2003 2:14:22 PM   
Gary.j.porter


 

Posts: 8
Joined: 11/18/2003
From: READING, MA,
Status: offline 		Also PCM provides 1024 servo positions and FM has 512, so in effect PCM has twice the resolution. Some FM RX's had firmware to provide a "Fail Safe".

(in reply to Spaceman Spiff)
       Post #: 3

RE: FM Vs PCM modulation - 11/20/2003 3:31:09 PM   
Razmo



Posts: 174
Joined: 10/27/2003
From: Chicago, IL, USA
Status: offline 		May be an ignorant question but I've never heard of PCM providing 1024 servo positions versus FMs 512. What does this mean?

(in reply to Gary.j.porter)
       Post #: 4

RE: FM Vs PCM modulation - 11/20/2003 5:12:08 PM   
Gary.j.porter


 

Posts: 8
Joined: 11/18/2003
From: READING, MA,
Status: offline 		PCM provides 1024 degrees of resolution on a 60 degee plane, meaning that in a servo arm at 90 degrees that travels from 60 to 120 degrees has 1024 points to stop in between in a PCM system. Older 1st generation radios PCM radios had a resoultion of 512. 1024,512 is all binary Eg: 1,2,4,8,16,32,64,128,256,512,1024,2048 on and on. A PCM radio has 10 bits of word and a few extra for error checking Eg: 0000000001 , 0000000011, 0000000111, 1001001011. I hope I explained that good enough :-). FM radios are no different in a sense than AM radios. They vary the amplitude and magnitude of the signal to tell the servo where to be. Harmonics, RF interference and a number of other things can interfear with the signal. Since PCM sends out a parity (the extra 2 bits with the 10) the reciver will "Lock" the servo(S)vif the data signal does not match the parity bit, thus by providing a "Fail Safe"

Hope that helps



P.S. "Fail Safe":"Servo Memory" will crash your vehicle the same as and old AM radio, PCM is much more "Clear"

(in reply to Razmo)
       Post #: 5

RE: FM Vs PCM modulation - 11/20/2003 5:40:48 PM   
MHawker



Posts: 2581
Joined: 3/1/2002
From: San Diego, CA, USA
Status: offline 		Just food for thought. They are both FM signals. It is PPM v.s. PCM.

_____________________________

Mike

Anyone know any good blonde jokes? I'll start- "There was this blonde customer service rep..... "

(in reply to Gary.j.porter)
       Post #: 6

RE: FM Vs PCM modulation - 11/20/2003 5:48:21 PM   
Razmo



Posts: 174
Joined: 10/27/2003
From: Chicago, IL, USA
Status: offline 		I think so...So total servo travel points on PCM is 2048 versus PPMs (thnks MHawker 1024??

(in reply to Gary.j.porter)
       Post #: 7

RE: FM Vs PCM modulation - 11/20/2003 6:35:30 PM   
Gary.j.porter


 

Posts: 8
Joined: 11/18/2003
From: READING, MA,
Status: offline 		No actually, don't confuse him

NO SUCH thing as 2048!! ..... YET!

PPM is Pulse-Position-Modulation, basically a FM radio. PPM is what the name is, Quasi-Digital. The signal is either high or low, on or off and sends data within that.


PCM is a completely different modulation and is a binary FULLY digital signal as described earlier.
FM or PPM still has amplitude and modulation and although it pulses, its not true digital with NO REDUNDANCY
AM is, well AM and SUX

Follow these links, If you thing I am full of bull :


http://www.futaba-rc.com/faq/faq-pcm1024.html A good explanation of PCM

http://www.mh.ttu.ee/risto/rc/electronics/radio/signal.htm Very in depth comparison, theory and difference

(in reply to Razmo)
       Post #: 8

RE: FM Vs PCM modulation - 11/20/2003 6:41:02 PM   
Razmo



Posts: 174
Joined: 10/27/2003
From: Chicago, IL, USA
Status: offline 		I don't think you are full of bull at all. I was thanking Mhawker for clarifying FM as PPM. I was directing the questions to you. I appreciate your input.

I'm still a little confused. How many degrees are there in typical servo travel? (from end to end)

(in reply to Gary.j.porter)
       Post #: 9

RE: FM Vs PCM modulation - 11/20/2003 6:51:22 PM   
Gary.j.porter


 

Posts: 8
Joined: 11/18/2003
From: READING, MA,
Status: offline 		60 degrees of travel, follow the 1st link to Futaba.

This is from thier website

What does it mean when you list a radio or receiver as PCM 1024?
The original PCM radio systems had a servo resolution that was 512 points on the 60 degree rotation. The newer equipment offers 1024 points, or a resolution two times better than the previous 512. Depending upon what model of PCM transmitter you own, it will be able to use EITHER 512 OR 1024, not both. So you need to know which type your radio uses. To make this easy, Futaba labels the face of the 1024-type transmitters and receivers with "1024".

http://www.futaba-rc.com/faq/product-faq.html#q16

(in reply to Razmo)
       Post #: 10

RE: FM Vs PCM modulation - 11/20/2003 9:13:52 PM   
Razmo



Posts: 174
Joined: 10/27/2003
From: Chicago, IL, USA
Status: offline 		Gary,

Your input has been very helpful. I don't own a heli radio yet. I grew up flying airplanes (age 14 thru 17). Then found more interest in the babes I know helis have come a long way since then and learning something new everyday. I've always been a precision flyer and looking to get as deep as I can get out of the helicopter as I can now that I'm 27. I've spent about 20 hours on G2 over the last year. I've got the oriention down, just about completed inverted orentaion as well. I'm simply trying to learn as much as possible so I can put all the appropriate pieces together. I'm in the middle of reading Ray's authoritave heli manual. Moving to Chapter 4. Any feedback is appreciated. Thanks

(in reply to Gary.j.porter)
       Post #: 11

RE: FM Vs PCM modulation - 11/21/2003 12:13:37 AM   
captain1


 

Posts: 11
Joined: 9/9/2003
From: St.IvesNSW, AUSTRALIA
Status: offline 		Dont let the marketing mumbo jumbo get to you.

PPM is an analogue signal. That means that it has a higher resolution that 1024 PCM. PPM would probably be a 10240000000000000000 PCM equavilent. Since it is analogue.

That said, being analogue PPM cannto distinguish between noise and signal, so singnal to noise ratio that is too high completly destroys the incoming data and your servo dont go where you want them to go.

PCM on the other hand is digital. Which means the actual servo position is more or less where you want it to go. The 60 degrees of travel is divided (quantised is teh correct term) into 512 or 1024 positions and then that position is sent as a number. So the resolution (or precision if you prefer) is either 0.117 or 0.0585 degrees in PCM. You would think that that makes it inferior to PPM and on those grounds it does. However the fact that it has been quantized and then modulated into a digital signal means that the actual signal is more easily differentiated from noise then our analogue PPM equvialent. So then when the signal starts to deteriorate the PPM servo's go just about anywhere while the PCM ones go close enought to where you want them not to make a difference.

The PPM vs PCM argument is the exact same as the vinyl vs CD one. Vinyl reproduces the music more accurate, but can your ears really tell the difference however your ears sure can hear those minute scratches on the vinyl!

I hope that makes sense - I find it hard to explain what I took four years in university to learn!

< Message edited by captain1 -- 11/21/2003 12:14:00 AM >

(in reply to Razmo)
       Post #: 12

RE: FM Vs PCM modulation - 11/21/2003 2:50:45 AM   
Gary.j.porter


 

Posts: 8
Joined: 11/18/2003
From: READING, MA,
Status: offline 		Your welcome Ramzo, the people on these boards are very helpful so if any more ?'s arise just ask

captain1- good CD-Vinyl analogy. But 4 years to learn what I learned today for Ramzo(and myself) on the net

For anyone else reading this thread and you want to read up these 3 links and their links help allot

http://www.torreypinesgulls.org/Radios.htm (very good, also uses the analogy CD-Vinyl)

http://www.futaba-rc.com/faq/faq-pcm1024.html A good explanation of PCM

http://www.mh.ttu.ee/risto/rc/electronics/radio/signal.htm Very in depth comparison, theory and difference


Happy flying

(in reply to Razmo)
       Post #: 13

RE: FM Vs PCM modulation - 11/21/2003 5:09:21 PM   
Razmo



Posts: 174
Joined: 10/27/2003
From: Chicago, IL, USA
Status: offline

(in reply to Gary.j.porter)
       Post #: 14

RE: FM Vs PCM modulation - 11/21/2003 6:56:13 PM   
Razmo



Posts: 174
Joined: 10/27/2003
From: Chicago, IL, USA
Status: offline 		With all that said, does PCM (at 1024) allow for better precision of controllability when flying OR simply better precision when the RF begins to deteriorate?

(in reply to captain1)