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Question about PCM and FM

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Old 12-09-2004, 09:36 PM
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skid2964
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Default Question about PCM and FM

Im not real familiar with PCM or why I would need it, but I've considered getting a Futaba PCM 7 channel radio.... if I do, will this run my FM recievers also? All FM recievers(all neg shift)? or just Futaba?
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Old 12-09-2004, 09:52 PM
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Default RE: Question about PCM and FM

PCM is a nice feature if you're trying to protect an expensive plane. I don't know the exact fundamentals of PCM vs. FM, but I do know that first was AM, then FM, and now PCM. PCM receivers are supposed to be very fast in receiving the signal from the tx, and you can program "Fail Safe" to help you get home if you ever lose one of your plane's functions or if there's interference.

If you're just plugging it into a a little .40 or .60 size plane, I wouldn't really worry about it. Now, if you've got a $1000 invested in a plane, then I'd probably do it. I hope this helps you.
Clinton
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Old 12-09-2004, 09:55 PM
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Default RE: Question about PCM and FM

I'm pretty sure that Airtronics and JR are POSITIVE shift, while Futaba, Hitec are negative shift. I hope someone else responds to this, otherwise, you'll have to do further research. But if you're going with a computerized radio by Futaba, you may only be able to run negative shift receivers. Now, on my Hitec Eclipse 7, you can run both negative and positive shift receivers. My guess is that if you can find a manual for your chosen radio, it'll be in there. That's where my information on the Eclipse 7 radio was. Good luck!
Clinton
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Old 12-09-2004, 09:58 PM
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Default RE: Question about PCM and FM

NOOOOOOOOOO!!! not again!!! pcm and fm have nothing to do with each other! they aren't opposites, they really aren't related directly at all. FM and AM are, PCM and PPM are, and positive and negative shift are. Pick one from the first group, pick one from the second group, and finally one from the third. you can have a receiver from any one of those combinations.
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Old 12-09-2004, 11:03 PM
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Default RE: Question about PCM and FM

I know they're not related. I'm trying to point out that AM came out first, then FM, then PCM or PPM. Obviously, FM will work just fine, but if he's got an expensive airplane, then PCM will, in my opinion, be the better choice since you can get "FAIL SAFE." Thanks,
Clint
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Old 12-09-2004, 11:12 PM
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Default RE: Question about PCM and FM

Im just trying to find out if I buy a PCM transmitter, can I ALSO use it for my planes with FM recievers. Does the PCM Tx also have some sort of "FM Mode" ?
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Old 12-09-2004, 11:23 PM
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Default RE: Question about PCM and FM

Yep, that should work for ya.
Clint
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Old 12-10-2004, 05:06 AM
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Default RE: Question about PCM and FM

ORIGINAL: Extra_230-RCU
PCM receivers are supposed to be very fast in receiving the signal from the tx, and you can program "Fail Safe" to help you get home if you ever lose one of your plane's functions or if there's interference

I know they're not related. I'm trying to point out that AM came out first, then FM, then PCM or PPM. Obviously, FM will work just fine, but if he's got an expensive airplane, then PCM will, in my opinion, be the better choice since you can get "FAIL SAFE." Thanks,
Clint
You have not understood FM and PPM/PCM or failsafes at all.
PCM receivers are not faster at picking up the transmission. The transmission rate is dictated by the Tx, the rx has nothing to with it. PCM is transmitted slower, not faster, than PPM.

Failsafe does not get your plane home in the event of loss of control. All it does is change the location of the crash.

PCM and PPM did not come after FM. They have absolutely nothing to do with AM or FM. The data is coded as PPM or as PCM. Neither of those have anything to do with radio whatsoever. They are data codes. To transmit them by radio they have to then be encoded again, as AM or as FM. Your model radio is AM or FM, and it is also PPM or PCM. But it is impossible to choose between FM or PCM because they are completely different things. PCM is sent on standard FM. PCM can not be better than FM because the phrase is meaningless.

H
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Old 12-10-2004, 08:10 AM
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Default RE: Question about PCM and FM

Hi!
This is a neverending topic for discussion.
First of all a distinction (Harry is of course right):

PPM and PCM define the manner in which a desired servo position is encoded into an electrical signal that is transmitted to the receiver using one out of two modulation techniques, AM (Amplitude Modulation) and FM (Frequency Modulation).

to answer skid2964's original question: As far as I know all "PCM transmitters" can also be switched to use PPM encoding.

The merits of these systems have been discussed at length. In the end the results of the discussions are usually inconclusive.
IMHO one of the best attempts to explain how these systems function can be find here: [link=http://www.torreypinesgulls.org/Radios.htm]http://www.torreypinesgulls.org/Radios.htm[/link].

There is a development of the PPM technique that is becoming more and more widespread which utilise digital signal processing in the receiver in order to improve the performance of the signal link. Multiplex, which was one of the first manufacturers to offer PCM radios have dropped these in favour of the new improved PPM technique. To put it simple: The digital signal processing allow the receiver to recognise the signal from a certain transmitter (your own) and discriminate against interference. It also allow the reciever to apply different actions in case signal is lost or corrupted, e.g. servo hold or programmable fail-safe.
The advantage is of course that this technique works for any PPM transmitter.

Given the new breed of PPM receivers, IMHO the only reason to buy a PCM transmitter today is that you get a lot of bells and whistles in the form of programmable mixers etcetera that may not be available in PPM transmitters. Of course this is not dependent on the encoding technique used, but rather the fact that most PPM transmitters up to now has mainly been aimed at the budget end of the market. This will probably change over time, see for example the Multiplex Royal Evo.

/Red B.
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Old 12-10-2004, 12:00 PM
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Rodney
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Default RE: Question about PCM and FM

Both Red B. and Harry gave you some good info. As a matter aside, the first PCM for radio control was an AM system, if I remember correctly put out by Futaba back in the late 50's or 60's. Now, if the various companies would come up with a standard for PCM encoding, we would all benefit. Right now they are not even compatable within each company, at least two versions from Futaba and two from Hitec, don't know about the other companies.
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Old 12-10-2004, 07:04 PM
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Default RE: Question about PCM and FM

In short

The Futaba will work with

1. Futaba Pcm Recievers
2. Futaba Ppm recievers ( FM to you)
3 Hitec Recievers for Futaba shift not JR
4. Fma recievers
there is probably some other obscure ones.

Pcm is on the radio, if you want it, so is ppm, cycle the power after changing it in the menu to switch between modes.
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Old 12-10-2004, 11:20 PM
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Reg Hinnant
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Default RE: Question about PCM and FM

Hi Gents,
Pardon my dumb question.
I have an old JR Century 7 FM system. How can I find out if it is Neg or Pos shift?
Thanks
Reggie
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Old 12-10-2004, 11:28 PM
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Default RE: Question about PCM and FM

ORIGINAL: Reg Hinnant
Pardon my dumb question.
I have an old JR Century 7 FM system. How can I find out if it is Neg or Pos shift?
If you only need 5-6 channels, just buy a Berg receiver and you won't have to worry about the shift -- it'll automatically adjust to whatever shift your transmitter uses.

This also means that if/when you update your transmitter, you won't be limited by the need for compatibility with your new receivers so you'll have the very widest choice.

The Berg receivers also offer many of the benefits of PCM even though they are PPM units.

Their smart computer-based decoders automatically reject invalid signals (such as interference) and the 6-channel version even has failsafe so you can pre-program the positions the servos should go to in the event of a total loss of signal or overwhelming interference.

The best thing is that these receivers are not really any more expensive than the equivalent "dumb" FM/PPM receiver yet those dumb receivers can't do any of this cool stuff.
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Old 12-10-2004, 11:55 PM
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Default RE: Question about PCM and FM

ORIGINAL: HarryC


PCM and PPM did not come after FM. They have absolutely nothing to do with AM or FM. The data is coded as PPM or as PCM. Neither of those have anything to do with radio whatsoever. They are data codes. To transmit them by radio they have to then be encoded again, as AM or as FM. Your model radio is AM or FM, and it is also PPM or PCM. But it is impossible to choose between FM or PCM because they are completely different things. PCM is sent on standard FM. PCM can not be better than FM because the phrase is meaningless.

I think many of these arguments will be invalidated by tthe new Futaba 14MZ. I do not think it is FM at all.
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Old 12-11-2004, 03:45 AM
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Default RE: Question about PCM and FM

ORIGINAL: mr_matt
I think many of these arguments will be invalidated by tthe new Futaba 14MZ. I do not think it is FM at all.
I've seen nothing in the published material to indicate that it's using anything other than FM modulation with PCM (or some variant) encoding.

All the bells and whistles are in the logic circuitry and user-interface.
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Old 12-11-2004, 03:49 AM
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Default RE: Question about PCM and FM

ORIGINAL: Reg Hinnant

Hi Gents,
Pardon my dumb question.
I have an old JR Century 7 FM system. How can I find out if it is Neg or Pos shift?
Thanks
Reggie
Hitec also makes a shift selectable rx. Their 7 channel rx is shift-select. They are $45 at servocity.com
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Old 12-11-2004, 04:00 AM
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Default RE: Question about PCM and FM

ORIGINAL: skid2964

Im just trying to find out if I buy a PCM transmitter, can I ALSO use it for my planes with FM recievers. Does the PCM Tx also have some sort of "FM Mode" ?
Yes, If you buy a pcm tx, you will be able to change it to ppm mode. Check in your tx manual for how to do it
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Old 12-11-2004, 05:51 AM
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Default RE: Question about PCM and FM

mr_matt wrote:
I think many of these arguments will be invalidated by tthe new Futaba 14MZ. I do not think it is FM at all.
Most probably it is transmitting using FM modulation. As for the encoding of the signal its PCM or PPM as usual. However Futaba have upped the reolution in PCM mode to 11 bits (2048 positions) in addition to 10 bit resolution (1024 postions). Apparently the have also increased the frame rate. Good news but nothing sensational.
The main selling point of the MZ14 Tx will probably be the user interface which has a lot of bells and whistles.

/Red B.
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Old 12-11-2004, 06:29 AM
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Default RE: Question about PCM and FM

ORIGINAL: mr_matt

ORIGINAL: HarryC


PCM and PPM did not come after FM. They have absolutely nothing to do with AM or FM. The data is coded as PPM or as PCM. Neither of those have anything to do with radio whatsoever. They are data codes. To transmit them by radio they have to then be encoded again, as AM or as FM. Your model radio is AM or FM, and it is also PPM or PCM. But it is impossible to choose between FM or PCM because they are completely different things. PCM is sent on standard FM. PCM can not be better than FM because the phrase is meaningless.

I think many of these arguments will be invalidated by tthe new Futaba 14MZ. I do not think it is FM at all.
The 14MZ is very much FM. If you know of a way to transmit a radio signal without modulation, please share.
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Old 12-11-2004, 08:44 AM
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Default RE: Question about PCM and FM

ORIGINAL: Pilotsmoe

ORIGINAL: skid2964

Im just trying to find out if I buy a PCM transmitter, can I ALSO use it for my planes with FM recievers. Does the PCM Tx also have some sort of "FM Mode" ?
Yes, If you buy a pcm tx, you will be able to change it to ppm mode. Check in your tx manual for how to do it
I skimmed through the manual for the Futaba 7C, is lists a "FM version" and a "PCM version" transmitter, but does not speak of switching modes ... has got me thinking I should get the FM Tx and be done with it ... I may call them and confirm the PCM unit will do FM (PPM) before I make a decision.
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Old 12-11-2004, 10:49 AM
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Default RE: Question about PCM and FM

I have had the 7 and 8 and now the 9 and they can all be switched in the programming. Just checked, and I still have the FP- T7UAP . Little line right after the name says PCM- PPM. Hope this helps. I have about a dozen transmitters kicking around going back to single channel. Try flying a plane with one button Thats all we had back then.
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Old 12-11-2004, 11:34 AM
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Default RE: Question about PCM and FM

Thanks for the responses.
I need to clarify my reason for the question. I'm flying the small stuff in a gym. I have a M5 receiver that works (excellent receiver for indoors or outdors) and the RFFS receivers work Ok too.
I am interested in trying Falcon's new narrow band, FM, 4 channel receiver but they have this warning.
" Compatable with Futaba, JR and modern transmitters with negative shift (USA customers - NB: JR sets in the USA tend to be positive shift - so please check)."
So, thats why I'm trying to find out how I can tell what shift the JR set I have is.
http://www.falconmodels.uk.com/acata..._C_Flying.html
Any help will be greatly appreciated!
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Old 12-11-2004, 12:11 PM
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Default RE: Question about PCM and FM

Reg :
Looks like your sentence above tells it. (Fut, JR and other modern trans on negative shift.)
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Old 12-11-2004, 01:33 PM
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Default RE: Question about PCM and FM

ORIGINAL: skid2964

ORIGINAL: Pilotsmoe

ORIGINAL: skid2964

Im just trying to find out if I buy a PCM transmitter, can I ALSO use it for my planes with FM recievers. Does the PCM Tx also have some sort of "FM Mode" ?
Yes, If you buy a pcm tx, you will be able to change it to ppm mode. Check in your tx manual for how to do it
I skimmed through the manual for the Futaba 7C, is lists a "FM version" and a "PCM version" transmitter, but does not speak of switching modes ... has got me thinking I should get the FM Tx and be done with it ... I may call them and confirm the PCM unit will do FM (PPM) before I make a decision.
Yes, they will do both modes, but as has been discussed, PPM and PCM are both FM on this radio. For example, my 40 sized prop planes are using PPM receivers and the radio (formerly a 9CA) model programs for each use the PPM mode of the radio, but my jets use PCM recievers (for the extra channels and failsafe) and are programmed accordingly. When switching to a model using a different mode, the power needs to be cycled on and then off again.
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Old 12-11-2004, 03:22 PM
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Default RE: Question about PCM and FM

ORIGINAL: Lt. Dan
ORIGINAL: mr_matt
I think many of these arguments will be invalidated by tthe new Futaba 14MZ. I do not think it is FM at all.
The 14MZ is very much FM. If you know of a way to transmit a radio signal without modulation, please share.
Well you can't transmit information without modulation but FM is just one of *many* possible modulation methods.

Remember the old AM RC equipment -- they worked just fine :-)

Ideally the 14MZ should have been spread spectrum so that frequency conflicts were no longer an issue -- but they took the conservative (aka: safe) option of sticking with their existing RF technoogy for this product and focusing on adding as many bells and whistles to the user-interface as they could.

Whether that's a good or a bad thing is something we could argue about for a long time :-)
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