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PCM or PPM

Old 04-04-2007, 05:07 AM
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skaap2k
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Default PCM or PPM

Hi!

I was wondering what advantages / disadvantages using a PCM receiver over a PPM receiver gives you? from what I can tell with my Tx, it has a built in failsafe function of some description while using PCM ... could someone shed more light on this for me please?

Thanks,
Rob

Old 04-04-2007, 06:10 AM
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Flying freak
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Default RE: PCM or PPM

pcm-pulse code modulation
ppm-Fm

pcm is a coded signal it is the norm in giant scale, jets and thigns like that the the reason for this is because it is a coded modulation if you get ssmestray signals from another source the RX in THEORY wont pick them up skipping over those till it gets theroec you wont know where fm would pick it up...
Old 04-04-2007, 06:17 AM
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Default RE: PCM or PPM

Basically, they both use the same sort of transmission medium, that is, FM modulation. The difference is sort of like a regular record player versus a CD player. They both use a disk, that spins, and the end result is the same, music plays from your speakers or headphones. But if you bump the record player, it skips, and you loose your music untill the needle finds the groove again and starts playing. A CD player will skip as well, but you really have to give it a good hard bump, and when it does skip, the music will start exactally where it left off.


Simple, eh?
Old 04-04-2007, 07:14 AM
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skaap2k
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Default RE: PCM or PPM

So,

PPM = Record player
PCM = CD player ?

are there any disadvantages to using it?
Old 04-04-2007, 07:27 AM
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Default RE: PCM or PPM

Costs more. Fail-safe doesn't protect you from a crash but it MIGHT help.
Old 04-04-2007, 10:47 AM
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Default RE: PCM or PPM

fail safe controls need tobe set by you.when the reciever does not get the right code within the signal it goes to fail safe which moves servos to a preset cfondition.usually everthing to neutral and power off.at least this can prevent a flyaway .
Old 04-04-2007, 01:47 PM
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Default RE: PCM or PPM

PPM is an analog signal, noise on the signal will get passed to the servos as position information and you get whats seen as a "glitch"

PCM is digital, the reciever checks the frame against a precomputed value, if its good its sent to the servos, if its bad its discarded and the next frame is read. You could drop several frames and never know it. If enough bad frames in a row are recieved the system goes into lockout where your servos goto failsafe, either last position they held or neutral with throttle at idle or cut. As soon as it picks up good frames you get control back.

That makes the plane/heli less dangerous in a crash and as aerowoof said, its not going to fly off into the sunset.

Disadvantage is cost and the checksum computation takes some processing cycles that would other wise be used for sending position information. The latency is small and probably most of us would never notice.
Old 04-04-2007, 03:14 PM
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Default RE: PCM or PPM

PPM is NOT analog. It is a digital pulse signal where the position of the pulse is varied slightly.

http://www.answers.com/topic/pulse-position-modulation

http://cnyack.homestead.com/files/modulation/modppm.htm

Narrowband RF (Radio Frequency) channels with low power and long wavelengths (i.e., low frequency) are affected primarily by flat fading, and PPM is better suited than M-FSK to be used in these scenarios. One common application with these channel characteristics is the radio control of model aircraft, boats and cars. PPM is employed in these systems, with the position of each pulse representing the angular position of an analogue control on the transmitter, or possible states of a binary switch. The number of pulses per frame gives the number of controllable channels available. The advantage of using PPM for this type of application is that the electronics required to decode the signal are extremely simple, which leads to small, light-weight receiver/decoder units. (Model aircraft require parts that are as lightweight as possible). Servos made for model radio control include some of the electronics required to convert the pulse to the motor position - the receiver is merely required to demultiplex the separate channels and feed the pulses to each servo.

More sophisticated R/C systems are now often based on pulse-code modulation, which is more complex but offers greater flexibility and reliability.
Old 04-04-2007, 05:32 PM
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Default RE: PCM or PPM

Hi there all

What is that frequency used in Spektrum Radio which is 2.4Ghz? is it PPM or PCM, and as we use 72.810 or 72.830 either PPM or PCM, what these frequencies have to do with 2.4 Ghz, can anyone explain please.

Thanks

Mody
Old 04-04-2007, 05:47 PM
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ag4ever
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Default RE: PCM or PPM

ORIGINAL: armody

Hi there all

What is that frequency used in Spektrum Radio which is 2.4Ghz? is it PPM or PCM, and as we use 72.810 or 72.830 either PPM or PCM, what these frequencies have to do with 2.4 Ghz, can anyone explain please.

Thanks

Mody
Neither, different technology.

The frequency is 2.4 Ghz, but I am not sure of what type of modulation is being used.

In the very basic form, you have three ways to transmit data on a radio wave. My changing the amplitude or strength, by changing the frequency or what Hz it is slightly, or by Phase modualtion of changing the timing of when the wave peaks and dips. All other ways are sub forms of these three basic types of modulation.
Old 04-04-2007, 05:47 PM
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Default RE: PCM or PPM

2.4GHz is a completely different frequency, think of it as 2400MHz as apposed to your 72MHz (in the Americas) or 35MHz (in Europe).
this range of frequencies is commonly known as the ISM (Industrial, Scientific, Medical) band.

From what I've read, this will avoid channel conflicts in the future, I'm not sure how the Tx+Rx negotiate an actual frequency, or in the case where it might use frequency hopping (changing frequencies often to avoid interference), some sort of pairing may be used.
It will probably only work using PCM as it will be more advanced in signal detection.

To generate a 2.4GHz frequency is also a lot more power efficient, however the antenna's are a lot more specialised, from my own experience in the wireless world (Data Communication) using 2.4GHz for RC planes could limit range depending on the power output from the Tx (in Europe, the max allowed output is 1watt for 2.4GHz), other types of interference could be encountered too, any water which gets into the antenna will prove disasterous as water absorbs signals on this frequency (your microwave at home operates at 2.4GHz too!!)

I'm interested to see other people using 2.4GHz Tx+Rx's before I even think about trying it!!

RN
Old 04-04-2007, 06:16 PM
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Default RE: PCM or PPM

Another disadvantage of PCM is that rx is pretty much radio specific. A futaba 1024 PCM tx wom't be read by a JR or HiTec PCM rx... or even another futaba rx. With PPM you can grab a JR rx and use a Futaba tx (most of the time) and it'll work fine.
Old 04-04-2007, 06:38 PM
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Default RE: PCM or PPM

Thanks for the replies, ag4ever and skaap2k.

Well I was just curious about it, and I upto some extent understood it.

Mody

Old 04-04-2007, 07:43 PM
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Default RE: PCM or PPM


ORIGINAL: armody

Hi there all

What is that frequency used in Spektrum Radio which is 2.4Ghz? is it PPM or PCM, and as we use 72.810 or 72.830 either PPM or PCM, what these frequencies have to do with 2.4 Ghz, can anyone explain please.

Thanks

Mody
2.4 GHz is the BAND and not the specific frequency. Just like 72.810 and 72.830 are part of the 72 MHz band.
Old 04-05-2007, 07:16 AM
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Default RE: PCM or PPM


ORIGINAL: broke_n_bummin

Another disadvantage of PCM is that rx is pretty much radio specific.
Not radio specific but BRAND specific.

A futaba 1024 PCM tx wom't be read by a JR or HiTec PCM rx... or even another futaba rx.
A Futaba 1024 will work with ANY Futaba 1024 RX as long as it's on the same frequency.
It will also work with ANY Futaba FM RX on the same freq as long as you switch the TX to PPM/FM mode. ALL Futaba PCM TX's are capable of this.

With PPM you can grab a JR rx and use a Futaba tx (most of the time) and it'll work fine.
A JR RX will NOT work with a Futaba TX (in the U.S.) unless one or the other is shift selectable. JR has shift select RX's available and the 14MZ Futaba TX is also shift selectable.
Old 04-05-2007, 07:37 AM
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Default RE: PCM or PPM

Bruce while it may not be analog in the true sense, there's no error checking and any noise in the signal is passed to the servos, where a PCM signal is protected against that by use of a check sum.
Old 04-05-2007, 08:44 AM
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Default RE: PCM or PPM


ORIGINAL: BarracudaHockey

Bruce while it may not be analog in the true sense, there's no error checking and any noise in the signal is passed to the servos, where a PCM signal is protected against that by use of a check sum.
While I agree there is no error checking and that PCM verifies via check sum, the passing of noise depends on the quality of the RX and applied filtering.

Nobody is arguing that PCM isn't better in most respects.

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