Whats the difference?

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.

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".

May be an ignorant question but I've never heard of PCM providing 1024 servo positions versus FMs 512. What does this mean?

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"

Just food for thought. They are both FM signals. It is PPM v.s. PCM.

I think so...So total servo travel points on PCM is 2048 versus PPMs (thnks MHawker 1024??

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[8D]

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/electr...dio/signal.htm Very in depth comparison, theory and difference

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)

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

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

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!

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[X(]

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/electr...dio/signal.htm Very in depth comparison, theory and difference


Happy flying

[sm=thumbup.gif]

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?

Better repeatability would be a better definition (position #576 is always position #576).

better signal to noise ratio := greater range given the same noisy environment.

Simply put:

FM - frequency modulation. This is the method of "preparing" signal to transmit is to the receiver
PPM - Pulse Position Modulation. This is actual "code" of the position of the control stick.

Here is the "picture" of what happened when you move the stick:

the position of the stick is converted to PPM signal. PPM signal is modulating a "carrying" frequency. The FM signal gets transmitted to the receiver, where FM signal is converted back to PPM “image�, and then into analog (level) signal which is actually moves a servo arm.

PCM is just another way of “encode� position of the control stick.

Here is the link where you can see both modulation methods side by side:

http://freespace.virgin.net/emhc.site/ppmpcm.html

Another bad opinion "failsafe function useless". Where does he get off? Fail-safe is for saving people and property, not aircraft.[:@]

Thats what I thought!!!! Out of the years of flying seems as if the fail safe is worthless unless it worked for the airplane, like counter acting the wind and gauging ground clearence to a save landing SOMEWHERE

"Another bad opinion "failsafe function useless". Where does he get off? Fail-safe is for saving people and property, not aircraft"

Easy, Heli's are not an Airplane that when strait and level COULD! go 2 mile before crashing into a human or animal at full throttle at 60 MPH. So you think a heli at 75 feet in full forward flight that looses control and the blades will slow down enough not to kill someone because the throttle was cut, hmmm 75 feet 30 mph fall like a rock to the ground in 3 seconds, yep those blades are sill going hot. Also , no it will not travel 2 miles. To me "failsafe" on a heli is moot[:-]


Flame all U want I have no more to say[:@]

Wow quite some nonsense here!

Both PCM and PPM are FSK modulated signals (so not really 'FM'). Basically it transmits on TWO frequencies, one a little bit below the frequency you picked (the '0') and one a little above it (a '1').

In PPM your stick position is transmitted as the LENGTH of a pulse, so a low pulse of 1ms (milli seconds) means the stick full in one corner and a pulse of 2ms means the stick in the other corner. This does NOT give you infinite precision though, as switching on or off is never instantaneous, because radio waves are waves - you have only a limited number of waves per millisecond, and you need usually a few 'waves' to detect that something is transmitted. I read that the actual resolution of PPM is LESS than PCM in most practical cases.

In PCM your transmitter position is send as a 10 bit binary code. Midstick is 512 = 100000000, low end is 0 and high end is 1023=111111111. Both a 0 and 1 have fixed transmission time of 150 microseconds so PCM can transmit about 13 data bits in the time that PPM sends one servo pulse. However PCM is usually somewhat slower than PPM because alot more bits are sent. Most gory details are in a publication on my website:
http://graphics.tudelft.nl/~wouter

I thought this topic might flush the academics out of the woodwork.

So it transmits on two frequencies. Would you say then that it is using different frequencies to "modulate" the input signal. Sorry but where I come from that is commonly called frequency modulation. Yes frequency shift keying is a specialist form of FM but it is still FM. To say that it is not FM would be like saying someone is not human because they are dutch!


I disagree here. Ok so you say we need a few "waves" to detect a signal. How many are we talking here?? 10?? 20?? Lets say 20. So now in a 72MHz radio we are getting 72 million waves per second. Thats 72000 in one millisecond, our worst case scenario. So to compare it to PCM it would be like having a 3600 bit PCM receiver. Seems a lot more acurate than 1024.
[/quote]

In the long run however the precision on both systems is more than what is needed. Assuming full servo deflection is 60 degrees then even 512PCM has descrete servo positions 0.117 degrees apart. This gives a maximum error of 0.058 degrees. Even if the servo mechanics are that precise, the remaining helicopter mechanics would not be affected by such a small change (unless of course your servo horns are in the region of 30cm long). And if you manufactured a helicopter with links this precise I am sure the aerodynamics would not allow such a small change to be seen in flight preformance. If it were you would have a very uncontrollable heli in your hands.

As I said before, the main advantage with PCM is it's error detection abilities. A good analogy of that is having a construction worker receiving verbal commands as opposed to verbal+written. The the verbal only scenario (PPM) the command (signal) might not have been heard correctly over the construction noise (poor signal to noise ratio) and the worker might do the wrong thing. On the second scenario (PCM) the command is heard and then the written command is received (redundant error detection bits) and if they match then the command is performed else it is ignored).

What, protecting people is worthless?[sm=confused.gif]

Flaming not required, are you unwilling to discuss?

You think there is no value in maintaining a predictable flightpath while eliminating power to the head?
You think headspeed will not decay?

The thing is, before fail-safe is engaged, a significant amount of time without signal must pass. Significant enough that it's unlikely you'll be getting control back and manage a controlled landing.

fail-safe: it's not perfect but it's better than the alternative ... an un-controlled heli over a crowd.[X(]

----------------------
my fail-safe settings

heli:
Throttle to idle, rest of channels to hold (best bet to decay headspeed while maintaining heading)

aircraft:
Throttle to idle, sticks to the corner (results in snap-spin {least distance over ground})

What, protecting people is worthless?[sm=confused.gif]
[/quote]

Who said protecting people is worthless?? Based on the principal if heli flight, a heli is always out of control unless corrected. Unlike a fixed wing. Besides out of all the heliflights that happen daily how many people have been killed from a helicopter because it lost RF. I'll bet it's nearly ZERO. That is all I meant.