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Old Feb 17, 2012, 08:48 AM
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Ppm & pcm

What is PPM and PCM, Exponential, swash AFR mixes, etc. when talking about tx and rx.
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Old Feb 17, 2012, 10:22 AM
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http://www.easyrc.com/index.html read away .
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Old Feb 17, 2012, 02:06 PM
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Originally Posted by miss5031 View Post
What is PPM and PCM, Exponential, swash AFR mixes, etc. when talking about tx and rx(
Question is so wide ranging, it is easier to refer to existing readily available information e.g.

. AM, FM, PCM & Spread Spectrum radios - FAQ re differences

. Beginners guide to RC - The original

. Beginners Guide to R/C Flight

. Videos - How to and DIY 101 Setup for EP & Heli

and much more under sub sections
"FAQ = Answers for Beginners and Advanced RC'ers"
"RC system FAQ Tutorials, how the systems work."
"Radio Systems, Accessories, Alterations and FAQ."
at:
Alan's Hobby, Model & RC FAQ Web Links

Alan T.
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Old Feb 17, 2012, 07:52 PM
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Thanx for the info. You guys really are helpfull. These links are great, very informative.
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Old Feb 17, 2012, 10:17 PM
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Originally Posted by miss5031 View Post
Thanx for the info. You guys really are helpfull. These links are great, very informative.

PPM and PCM are only two ways of sending information from the transmitter to receiver using RF wideband. using first one or second one should not be inportant to modeller, what its important is that the receiver actually can read the information well without problems (something that Futaba for example has develop for their products).
PPM is Pulse Position Modulation and PCM is pulse code modulation. PPM works changing pulse period of a signal for every channel, so if you see PPM signal from your RC radio, yo will see some square pulses, in which each one corresponds to every channel and if you move the sticks, the period for that "channel pulse" will change. receiver reads the variation of pulse width and changes by the same rate the servo of the channel.

PCM is quite diferent. every channel is represented by some bits depending on resolution of the radio (10 bits, 12 bits, 16 bits). so, every channel value will be represented by that amount of bits. if channel range is -100, 100 for example, you will have 200 "steps" of movement for that channel, so, if your radio is 12 bits, you will have 4096 posible steps, in the range, 200/4096 or 0.048 step/bit.
if stick value is 100, so data value will be 4095. ir the stick is on the middle data value will be 2047 and so. that number should be represented by binary codes ans send it trough the radio module.

which one is better?. depends on Radio Technology. most of 2.4Ghz radio modules works with PPM, and some FM modules works with PCM.

is the best way i found to explain PPM and PCM.

Exponentials, dual rates, etc, are bassically math functions applied to sticks to change behavior of receiver signals respect to stick movements. they're usefull to change flying characteristics of the model.

Regards;

Mauricio
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Old Feb 18, 2012, 07:27 AM
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Originally Posted by Mauricio2346 View Post
most of 2.4Ghz radio modules works with PPM
Only AM/FM can work with PPM. None of the commercial 2.4GHz modules use FM/AM, they are all digital. Therefore 2.4GHz transmission is some sort of PCM. Of course you can build an FM 2.4GHz tx, but there is no point except for the fun of it.

Of course most of the 2.4 modules accept PPM as input, but they don't directly modulate it like an FM/AM module, instead transform the PPM into digital code, transmit it, then in the rx the code is directly split into channels, no ppm there.
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Old Feb 18, 2012, 08:53 AM
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Only AM/FM can work with PPM. None of the commercial 2.4GHz modules use FM/AM, they are all digital. Therefore 2.4GHz transmission is some sort of PCM. Of course you can build an FM 2.4GHz tx, but there is no point except for the fun of it.

Of course most of the 2.4 modules accept PPM as input, but they don't directly modulate it like an FM/AM module, instead transform the PPM into digital code, transmit it, then in the rx the code is directly split into channels, no ppm there.
of course, 2.4Ghz module accepts PPM as input (i referred that point). modulation protocols depends on what manufacturers want to use for their products. FASST for Futaba (continous frecuency shifting), DSMX for JR, etc. all protocols based on PCM modulation for 2.4Ghz, and (i dont really know), FSK for FM modules.

what is a fact is that 2.4Ghz send acknowledgement bytes every sample period for all channels. receiver espects to receive that binary code (8 bytes, 16 bytes) and unique to each transmitter. that's why you need to bind the radio to each receiver to make a "link" to receiver.
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Old Feb 18, 2012, 10:45 AM
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Originally Posted by Mauricio2346 View Post
what is a fact is that 2.4Ghz send acknowledgement bytes every sample period for all channels.
No, 2.4g radios do not universally send acks. Most of them are essentially 1-way systems except perhaps during the bind process.

Andy
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Old Feb 18, 2012, 12:49 PM
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And binding is necessary so the receiver knows which transmitter to listen to.
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Old Feb 24, 2012, 02:16 AM
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No, 2.4g radios do not universally send acks. Most of them are essentially 1-way systems except perhaps during the bind process.

Andy
most of them are one way radios but transmitter always sends aditional information (transmitter name) to receiver. so, if dont, how receiver can recognize always the same transmitter?
binding process resets the receiver and store a new "transmitter name" until binding is done and binding plug is removed.
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