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FM or PCM Whats the diff

Old 04-25-2002, 05:55 AM
  #101  
JohnW
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Default Flight Modes

HarryC... In all honestly I rarely use more than four flight modes, then again in all honesty I rarely use more than seven channels. Who really needs 12? Maybe sailplanes pilots but they are in thier own little world anyway. I was just giving you and the Multiplex clan a hard time. I primaraly fly IMAC but fiddle around with pattern and heli's. Planes typically get three flight modes and the heli typically gets five. I have enough voices in my head I don't meed my freaking transmitter talking to me too! By all.
Old 04-25-2002, 07:22 AM
  #102  
Nick Smith
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Default FM or PCM Whats the diff

We need to define 'Failsafe' and 'Hold' to understand what actually happens during a period of interferance or loss of signal.


'Fail safe' is a condition that exists on one or more of the channels when no recognized signal is being rx'd by the PCM part of the rx.. - It is programmable, and drives the selected channel(s) to a user define position - i.e. throttle low

'Hold' (sometimes wrongly referred to as 'lock out') is a condition that also exists on one or more channels, again when no recognized signal is being rx'd by the PCM part of the rx, the channel(s) hold their last good position.

What is important to note, is that as soon as a recognized signal is rx'd (i.e interference stops or reduces), these conditions no longer exists, and control normal is resumed. These conditions, do not 'trip in and stay in' - providing the interference ceases; or it's volume reduces so the PCM signal is dominant.

Also, it is also important to note, that most modern PCM systems, will allow a mix of 'fail safe' channels, and 'hold' channels.

If anyone is in doubt as to how their particular radio system will behave during a 'hit', set up a system on the work bench, and experiment with 'failsafe' and 'hold' and note the results; to simulate a 'hit' or loss of signal, turn off the tx or switch on another on the same freq. Obviously do this on the work bench at home - not whilst flying!!!

Also its worth noting and asking what are we are trying to achieve with our Radio gear? Well, safe reliable guidance of our model is the obvious answer, we could also, mention stuff such as confidence, cost, versatility, etc etc, but what we are not going to do (well not yet anyway) is to get a system that will protect us from someone else switching on on our chosen freq. If someone does it's probably curtains anyway whether it be PPM / PCM dual con / single con etc. No rc system will be immune from that , nor indeed are today's current systems designed to do that (although I believe Multiplex are about to 'break the mould' with their new 12, yes 12 channel 'intelligent tx' system) - to stop 'shoot downs' we put our faith in a humble 20 pence peg, and some good discipline!!

Hope this adds to the debate!!

See ya

;-)
Old 04-25-2002, 08:45 AM
  #103  
HarryC
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Default FM or PCM Whats the diff

Originally posted by Nick Smith

but what we are not going to do (well not yet anyway) is to get a system that will protect us from someone else switching on on our chosen freq. No rc system will be immune from that , nor indeed are today's current systems designed to do that (although I believe Multiplex are about to 'break the mould' with their new 12, yes 12 channel 'intelligent tx' system) - to stop 'shoot downs' we put our faith in a humble 20 pence peg, and some good discipline!!
;-)
The Mpx Channel Check has been available on their Tx for about 3 years now. It doesn't protect you from being shot down but protects you from shooting down someone else, so if everyone had it..........................!

Harry
Old 04-25-2002, 09:23 AM
  #104  
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Default Re: PCM is better than PPM

Originally posted by Cley
Here's another link to the site that the Multi-Plex proponents (salesmen?) have referred us to...
http://www.torreypinesgulls.org/index.html
Notice who the Host Sponsor is??
As I said before Multi-Plex isn't even competetive in todays market IMO, they don't even carry PCM!
Time will tell eh?
That sponsor refers to the hlg contest and the torrey pines site has been going for a long time before that banner appeared. Anyway according to torrey pines, radios stop at 9 channels and neither Mpx or Graupner who make 12 channel radios would sponsor that!

Ok Cley, Mpx is not competitive. I will go and buy a Futaba now if you will help me to choose which one:
Which Futaba Tx has 9 proportional channels?
Please name all the Futaba Tx that will not transmit if someone else is already on frequency?
Which Futaba Tx should I get that has stacks of memories on-board and also connects to my PC for back-up and software downloads?
Which Futaba Tx allows me to fit the switches, push buttons etc that I want where I want and not what the designer dictated I get?
Please name the Futaba Tx that gives me all this with servo speed control, channel assigning, control assigning etc for less than half the price of a 9Z.

Pardon Cley?! Are you seriously telling me that Futaba doesn't even make a 9 proportional channel radio! And Futaba allow you to switch on and shoot-down others on your frequency! No come on Cley, you are kidding. Aren't you? What, £1000 for a Futaba 8+1 channel radio with few of those features, when I can get a true 9 channel radio with all that and more for for about £350! Now I know you are just playing devil's advocate to get a debate going. You nearly had me there Cley, I almost believed you and went back to the Futaba (a make I have used since the mid 1970s) but which I sold in order to buy Multiplex! Drat, I will have to keep my Mpx for now.

Yep, Mpx is so uncompetitive it is used almost exclusively in European and UK gliding competitions just to handicap the competitors.

Come on Cley, we get the message that Mpx is not your cup of tea, and Mpx seems not to care about selling much in America whereas Japanese are always desperate to sell their chromed hedgehog Txs in America, but that does not make Mpx uncompetitive!

Harry
Old 04-25-2002, 07:08 PM
  #105  
JohnW
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Default Oh Please...

Come on Harry... Give me a break. Do you really need nine fully proportional channels? By you own admition in a previous post you can't even handle more than two conditions. There is a down side to nine fully proportional channels... speed. More channels means less efficient transmission... especially if you don't use all of them. You are just transmitting worthless garbage.

As for your Futaba Questions...

If you don't know how to check the Frequency board you are unsafe to fly in the first place, but I will have to admit that is a nice feature, BUT IT IS NOT AVAILABLE IN THE USA... check the multiplex web site.

You can backup a Futaba ZAP to flash memory.

Moving switches is a minor point IMO. The ZAP has enough switches on it and all are assignable to any function. Hence, the ZAP can "move" its switches via the firmware.

Useful speed controls such as servo time delays between switching conditions are in the ZAP.

Here in the States the Multiplexs are not cheap IMO. A ZAP WC2 is about $850 in the States or about 600 pounds.


SO... Here is a questions for you... I only need one with follow up.

Which TOC or Masters pilots uses a Multiplex?

Follow Up: For the pilots who don't use Multiplex, how can they possibly be competitive using inferior radios?
Old 04-26-2002, 12:28 AM
  #106  
Nick Smith
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Default FM or PCM Whats the diff

Hey look guys, what ever system one chooses is down to personal preference. There are many pro's and con's to all systems, so at the end of the day what suits someone might not suit someone else - it's down to opinion
Old 04-26-2002, 03:06 AM
  #107  
Dave Bowles
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Default Radios

You Know, I never really ever have gave a rats tail about what radios TOC pilots are using, I would suspect that its not the TX that makes anybody a good pilot. And in R/C , Just because somthing is Popular dosen't make it the best. All the buttons and beeps are Nice but are not required to be a good Pilot. There are plenty of products used in other countries that are not popular in the U.S. , dosen't make it inferior.
Old 04-26-2002, 05:32 AM
  #108  
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Default Finally....

Thanks Dave... Finally someone has gotten the point I've been trying to make. There are a lot of first rate radios available, but the most important part of any flight is the pilot. I'm happy with my ZAP and PCM, Harrys happy with his multiplex and IPD.
Old 04-26-2002, 08:27 AM
  #109  
HarryC
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Default FM or PCM Whats the diff

Monkeyboy, I was responding to Cley's statement that Mpx is "uncompetitive". I was not claiming that the 9Z is uncompetitive but having to belabour the point for Cley that the Mpx sets are very well featured and contain things that Futaba have yet to incorporate. With Mpx features and UK prices, I would be getting less features for more than twice the price by buying a 9Z, and that doesn't seem to make Mpx uncompetitive to me. Cley always seems to have a thing against Mpx for some reason, I have nothing against Futaba as I have used it for more than 2 decades! In the last 2 years my LHS has sold more Mpx computers than it has sold Futaba and JR computers added together, those sales were all to existing Futaba/JR users and that is not the indication of an “uncompetitive” radio.

The same thinking applies across Europe and different flying disciplines. You can point to lots of 9Zs at the TOC but we can point to Mpx dominance in certain European competitions. If some Mpx features are not available in the USA then it seems clear that Mpx really doesn't care too much yet about selling in the USA. Come to my club on a Sunday morning and it looks like an exclusively Mpx club, and all are very experienced fliers who have given up their Futabas and JRs in preference for Mpx.

The Mpx is not slowed down by its 9 channels unless all are used, since it only bothers with those channels which have been assigned. At the moment I use 8 channels, the ninth is unused but one day I might need it say for an in-flight needle adjustment and it would seem to be a backward step to give up a proportional channel for 2 position only channel. Since there are 3 (maybe 4?) European radios with 12 channels, there is obviously enough of a market for large numbers of channels, and that market is increasing with the way that jets and some gliders guzzle channels.

The 9Z has to be a great radio otherwise it would not appear at top competition level, but so is the PCM10 too. The 9C, 8U, 3810 etc are good radios. Mpx also make great radios. They are not uncompetitive just because they are not aimed at the American market in the way that the Japanese radios are.
Old 12-29-2002, 06:29 PM
  #110  
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Default FM or PCM Whats the diff

I didn't know the difference between FM and PCM so I did a Google search and came across this thread. Very informative but I'm still undecided as to what to get. I'm a newbie to RC planes and after that thread I'm leaning towards getting a PPM setup with a failsafe option.
Old 12-29-2002, 09:30 PM
  #111  
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Default points

first off, i have not read all the replies so if i repeat something sorry. someone asked for the make of receiver with the built in glitch counter? Schulze make it. there may be others. pcm v ppm. pcm does mask/ filter out interference. don't ask me how, but it helps. BUT you should not cure an aircraft with a dodgy signal using a ppm receiver by putting in a pcm receiver instead. this may cure the symptoms, but it does not cure the fault. it is good policy to get your aircraft glitch free on ppm with a good range check. THEN fit a pcm receiver if you wish for belt & braces insurance so to speak. using a glitch counting receiver is a big help as it picks up the glitche's you don't notice. just a point here, i'm in the UK on 35mhz, i don't know if schulze make receiver's on 72mhz?
Old 10-03-2003, 09:53 AM
  #112  
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Default RE: FM or PCM Whats the diff

Harry, I have read many of your comments and threads. I have yet to hear any information or comparisons to the Hitec Q-pcm. Does anyone have any knowledge of how this compares? I am convinced that IPD may be slightly better than PCM, but how does this Q-pcm play into the game? Is this just another option to put on the market that locks people into needing the Q-pcm Rx?
Old 10-03-2003, 10:18 AM
  #113  
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Default RE: FM or PCM Whats the diff

I have no idea what Hitec’s Q-PCM is but since all brands of PCM are specific to that manufacturer then by definition it will lock you in to Hitec Q-PCM Rx.

I am dubious about saying that Multiplex’s IPD is better than PCM. It is different. It has some advantages, but some disadvantages. What is clear is that IPD is a whole pile better than straight PPM with no failsafe. It gives PPM users a chance to have many of the PCM advantages, with the advantage that it works with absolutely any brand non-computer Tx, allowing for shift compatibility in North America. Mpx claim various things for IPD that are better than PCM but some of them may be fine in the test lab but of limited use or occurrence in the real world.

If I had PCM I would be tempted to stick with it, I don’t think there is anything much to be gained by using IPD instead of PCM. An IPD is cheaper, it may a tiny bit smaller and lighter than PCM Rx.

H
Old 10-03-2003, 10:46 AM
  #114  
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Default RE: FM or PCM Whats the diff

The difference is this:

FM - is an analog signal from transmitter to receiver AND analog representation of positions of the control sticks.

PPM (and PCM) is an analog signal from transmitter to receiver AND digital representation of positions of the control sticks.

The rest in the price of the radio is add-ons (e.g. computer, memory, additional signal processing, etc.). Other things being equal PPM and PCM will allow more precise control of the model (and PCM is more precise of these two). Does this matter for you – you decide.

In my humble opinion – for beginner good computerized radio is more than enough. As you will get dipper into the hobby, especially if you go into aerobatics – you want to consider more sophisticated radio.

Just remember – they are just like cars: next year newer mode, more options, etc…

Hope this helps.
Old 10-03-2003, 11:49 AM
  #115  
HarryC
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Default RE: FM or PCM Whats the diff

FM - is an analog signal from transmitter to receiver AND analog representation of positions of the control sticks.
PPM (and PCM) is an analog signal from transmitter to receiver AND digital representation of positions of the control sticks.
Other things being equal PPM and PCM will allow more precise control of the model (and PCM is more precise of these two).
Hope this helps.
It doesn't help at all, because it is utterly and fundamentally wrong in every respect.

The radio modulation we use is FSK, a subset of FM. It is a binary, not an analogue signal and in that respect it behaves as if it is digital. It does nothing to represent the stick positions, the data it contains does that. The data can be analogue binary PPM, or digital binary PCM.

PCM is not an analogue signal. PPM is analogue, PCM is digital i.e the data has been quantized to base 2. PPM and PCM are data signals, not radio signals and must not be confused with FM though PPM often is.

"PPM and PCM will allow more precise control" More precise than what?

"PCM is more precise of these two". Wrong, PPM has greater precision. PCM systems have either 512 or 1024 psoitions that they can operate to. In theory analogue PPM has an infinite number, in practice it is limited by the sensitivity of the pots and the operation of the encoders. In a previous thread some months ago I quoted the numbers being generated by my analogue Tx through the Rx to a signal counter and showed that in PPM I was generating a precision of greater than 1200 steps, in other words around 20% better precision than PCM.

Harry
Old 10-04-2003, 12:45 PM
  #116  
Rodney
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Default RE: FM or PCM Whats the diff

Both PPM and PCM use frequency shift keying which is in effect a form of FM. Both are sending a signal which is either one frequency or another, i.e. both digital in nature. The difference is in the manner in which this signal frequency change is handled versus time. There are many more transitions in a frame of PCM than in PPM but both are of a digital nature. The PCM signal is much more complex and requires more circuitry to properly decode. In either case, the signal to the servos are identical, a variable pulse width.
Old 12-19-2003, 06:30 AM
  #117  
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Default RE: FM or PCM Whats the diff

this is a great thread

What about gas engines. I overheard my hobby shop dealer telling a customer that PCM is not ideal in gas engines due to strong interference from the spark ignition system. Which is better to use in gas applications, PCM or PPM

regards
Blixsim
Old 12-20-2003, 01:37 PM
  #118  
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Default RE: Re: How it works

I have tested several RX. At the point where you can see the servos twitch at low signals on a range check the servos are drawing a lot of current. This for PPM. I found that you had to increase the signal by 20dB to get each servo current below 20mA! With the PCM RX this does not happen they keep producing a clean servo pulse right down to the point where they drop out at about 2 microvolts. The best PPMs needed 35 microvolt to get the servo current down below 20 mA each!
Old 12-23-2003, 06:55 AM
  #119  
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Default RE: Re: How it works

If PCM is digital does this mean I can use digital servos on the PCM setting on my JR X8310

blixsim
Old 12-23-2003, 07:35 AM
  #120  
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Default RE: Re: How it works

It makes no difference to a digital servo whether you use PCM or PPM. All servos are PPM, PCM receivers have to convert the PCM data into PPM before placing it on the servo sockets. The "digital" in servo refers to the way the servo provides power to the motor and has nothing to do with the position data.

H
Old 12-23-2003, 07:41 AM
  #121  
HarryC
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Default RE: Re: How it works

ORIGINAL: dave-g4nju
I have tested several RX. At the point where you can see the servos twitch at low signals on a range check the servos are drawing a lot of current. This for PPM. I found that you had to increase the signal by 20dB to get each servo current below 20mA! With the PCM RX this does not happen they keep producing a clean servo pulse right down to the point where they drop out at about 2 microvolts. The best PPMs needed 35 microvolt to get the servo current down below 20 mA each!
I suspect that what you are seeing is the difference between failsafe and non-failsafe rather than a difference between PPM and PCM. Have you done the test with a Multiplex IPD PPM failsafe receiver or FMA DSR PPM failsafe receiver? I would like to see the numbers from them I suspect they will be very close to PCM.

H
Old 12-23-2003, 01:27 PM
  #122  
famousdave
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Default RE: Re: How it works

The bottom line here.. if there is one. What works at one field may not work at another. What works on one plane, may not work on another.

I fly a JR 10X radio (what a work of art).

On FM - I have many JR FM (I really like the R700) receivers. I have used them on gas, glow and electric. At my field, they have all run flawless. When traveling, I have had hits, but no lost aircraft.

On PCM - I have several PCM receivers. The new JR 7 channel is awesome (under $100) as are the 9-channels. I have used them only on Gas and Helis. At my field they have been flawless. While traveling, I have had hits.


PCM and failsafe is overrated. The only time failsafe ever did any good for me is when someone "switched on" to my channel by accident. My plane lost signal but gave me enough time to scream at the offender and he shut his tx off. In that case failsafe saved my plane. I have had minor hits on PPM and PCM.

There is one case I would not use PPM for. HELIS. I would not fly a heli without PCM, and have not seen a heli fly very well with PPM. I have seen several crash and all had PPM receivers. All the machinery somehow interferes with PPM more than it appears to interfere with PCM.


Conclusion: If you have good equipment and maintain it (i.e. get it checked once a year, and replace crystals if crashed etc) you should not ever have lockouts with either system. I don't find either one better than the other except my observation with helis... in addition to the apparent noise immunity helis really need the added precision of digital servos and PCM, at least the way this pilot flies!

DP
Old 06-25-2006, 12:58 PM
  #123  
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Default RE: Flight Modes

I am glad to see most of the misinformation early in this thread was later corrected. PPM and PCM are both digital from transmitter to servo. PCM sends packets of information including a checksum bit that can be checked for validity on the receiving end where as PPM send a continious signal of repeating digital pulse width information. The packet idea is not new as it has been used in computer communications for many--many years. The analog part is in the control sticks and sliders of your transmitter which is only as good as the pots in the transmitter. There are digital encoders that could replace the pots; however, the cost is at least 20X that of the pots. PPM and PCM both have advantages and disadvantages; the design of the receiver RF section is the more important factor for signal integrity. I use both PPM and PCM and the biggest drawback to PCM is the packet scheme is propriatory (remember VHS verses Batamax.) Most of the advantages of PCM could be had with PPM if manufactures were so inclined. As long is there is no STANDARD and PCM has compatibility problems between not only different manufactures but many are not back wards compatible with their own radios I vote for PPM receivers with good front end false signal rejection. I am using a Futaba 12Z and love the bells and whistles I get with the G3 receiver but would be happier if it was PPM compatible. The technology is every where; don't let the adds fool you; the proprietary nature of PCM is not for your benefit there are public domain data formats that may even be better for data packet tranfer. Companies have tried this marketing scheme before and it is always good for a few and bad for many. Any body have to throw away a really expensive Sony VCR because it was Batamax format.

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