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-   -   What does PCM mean on a transmitter? (http://www.rcuniverse.com/forum/beginners-85/5108557-what-does-pcm-mean-transmitter.html)

chevy43 12-12-2006 08:07 PM

What does PCM mean on a transmitter?
 
What does PCM mean? I'm looking at new radios and need to know.

bkdavy 12-12-2006 08:18 PM

RE: What does PCM mean on a transmitter?
 
PCM - Pulse Code Modulation

PPM - pulse position modulation

Essentially, it is a different method of modulating the signal between the transmitter and the reciever. PCM is a digital signal that is supposedly more resistant to interference from non-RC sources. There's and excellent discussion here[link]http://www.aerodesign.de/peter/2000/PCM/PCM_PPM_eng.html[/link]

Brad

chevy43 12-12-2006 08:43 PM

RE: What does PCM mean on a transmitter?
 
So when I look at radio specs I see FM, PCM and PPM and I am suposed to decide which one.

I have no clue which to pick.

chevy43 12-12-2006 09:01 PM

RE: What does PCM mean on a transmitter?
 
Looks like I can get it with the synthesiesed chanel modual for $330. Not bad. I don't think I will ever use all the bells and whistles but still that isn't bad.

Flying freak 12-12-2006 09:06 PM

RE: What does PCM mean on a transmitter?
 
Quote:

ORIGINAL: chevy43

Looks like I can get it with the synthesiesed chanel modual for $330. Not bad. I don't think I will ever use all the bells and whistles but still that isn't bad.
Because i take it your a begining, please be carefull with a synthesiesed tx, make sure you ALWAYS know what channel you are on it happens somtimes were you assum you are channel 50 (your normal channel) but forget that the last time you came the channel was taken so you went a 13.... you turn you radio on thinking your on 50.. and well you may just have bought yourself a very expensif piece of junk..

chevy43 12-12-2006 09:11 PM

RE: What does PCM mean on a transmitter?
 
Yes, thank you.

I have been flying for about 8 months now but I have a lot to learn about radios.

bruce88123 12-12-2006 09:39 PM

RE: What does PCM mean on a transmitter?
 
If you are thinking about changing TX modulation types and are thinking in the $330 area don't leave out the new Spectrum DX7 from your consideration. Just a few dollars more and NO frequency conflicts EVER.
http://www.horizonhobby.com/Articles...ArticleID=1626

chevy43 12-12-2006 09:46 PM

RE: What does PCM mean on a transmitter?
 
I would consider that exept that I would have to replace all of my recievers and buy theirs. That just isn't ok right now.

Charlie P. 12-12-2006 10:18 PM

RE: What does PCM mean on a transmitter?
 
PPM and PCM are both FM carrier wave.

PCM allows you to have presets in the event of loss of signal so you have some control over how you crash. :D (i.e.: I want to be making a right turn at low throttle when I smack the earth or a tree).

I have heard that PCM is more resistant to interference, but it "masks" small amounts that a PPM (FM) would show and allow you to land knowing there was a small problem before it became a fatal problem. Probably hearsay spread by cheapos like me who cling to FM/PPM.

[link=http://www.natew.com/rcheli/frame_main.cgi/html.PCMvsPPM?PRINT=TRUE]PCM vs. PPM/FM[/link]

[link=http://rcsource.hobbypeople.net/hsjack/rcaradio.asp]AM vs. FM vs. PPM vs. PCM[/link]

My opinion is that if you use digital servos you will benefit from PCM and see a difference in performance. If you use analog (I guess that is the other style) servos you are just as well off with PPM.

Campgems 12-12-2006 11:59 PM

RE: What does PCM mean on a transmitter?
 
Go with the PCM radios. The Futaba 9CAP will work in either mode, but the 9CAF only works PPM. It gives you a broader choice of receivers.

By the way, the 9C is a great radio. It is amazing what you can do with mixing and such. The only complaint I have is the speaker for the timer is very weak. I've learned to live with it, and am getting good at dead sticks. Well not really, but I'm getting practice. Guess it's time to get the egg timer back out.

Don

bruce88123 12-13-2006 08:36 AM

RE: What does PCM mean on a transmitter?
 

Quote:

ORIGINAL: Campgems

Go with the PCM radios. The Futaba 9CAP will work in either mode, but the 9CAF only works PPM. It gives you a broader choice of receivers.

By the way, the 9C is a great radio. It is amazing what you can do with mixing and such. The only complaint I have is the speaker for the timer is very weak. I've learned to live with it, and am getting good at dead sticks. Well not really, but I'm getting practice. Guess it's time to get the egg timer back out.

Don
Not true. The transmitters included in the 9CAPS and the 9CAFS systems are identical with the exception that one DEFAULTS to FM when setting up new models and the other DEFAULTS to PCM during setup. Either one can be over-ridden during setup however.
The major difference is that the 9CAPS system is shipped with a PCM RX and the 9CAFS is shipped with a FM RX. I believe there may be a servo difference as well.
See pg 9 of the manual
http://manuals.hobbico.com/fut/9c-su...ies-manual.pdf

Montague 12-13-2006 09:26 AM

RE: What does PCM mean on a transmitter?
 
Btw, the "failsafe" stuff Charlie was talking about is a function of the receiver, not the transmitter. In the past, if you wanted a receiver to throw out bad data and eventually go to a default if it lost signal, you needed PCM. However, now there are PPM receivers that have all the same features. Some are smart enough to recognize bad frames and hide momentary interference as well as PCM receivers and have programable fail safe settings. The programming is different, since it's done through the RX instead of through the TX, but the result is the same.

With PPM, you can use multiple brands of receivers, since it's an industry standard (just watch out for the "shift", or use auto-shift-sensing receivers).

With PCM, every brand transmitter has it's own way of encoding, so you have to use that brand receiver.

Basic 1024 PCM is pretty much the same as PPM as far as data transmission rates are concerned, and the signal between the receiver and servos is always the same regardless of PCM vs PPM, Digital vs non-Digital servos. If it wasn't the same, the gear wouldn't work in mix-and-match setups as it does now.

However, there ARE some transmitters that have modes that have special faster frame reates in PCM mode.

PPM transmitters/receivers are also limited to 9 channels. If you want more than 9, you have to be in PCM mode. And up until recently there were very few 9 channel PPM receivers out there, they mostly stopped at 8. So if you are doing a lot of mixing, or have a complicated scale bird that uses a ton of channels, you'll be forced to PCM for that reason alone.

This site has some great details about how our radios work. It's worth a read:

http://www.torreypinesgulls.org/Radios.htm

bruce88123 12-13-2006 09:54 AM

RE: What does PCM mean on a transmitter?
 
One example of a 9 channel FM RX with fail safe
http://www2.towerhobbies.com/cgi-bin...?&I=LXFJZ5&P=0

and a 12 channel FM synthesized with failsafe
http://www2.towerhobbies.com/cgi-bin...p?&I=LXEXW5&P=

Note: Not a recommendation of either as I am not familiar with them.

Montague 12-13-2006 10:33 AM

RE: What does PCM mean on a transmitter?
 
Fwiw, I believe that the Multiplex 12-channel RX will only do 12 channels with a Multiplex TX in a special mode that uses a non-standard, higher frame rate. If you read the article posted above, you can see why it's not possible to have a PPM receiver with more than 9 channels with the current "standard" framerate. To get more channels, you have to change the timing between the TX and RX, and that means doing something that won't work with other systems.

I just flipped through the manual for the 12 channel RX, and it confirms that that Multiplex transmitters can use a special PPM mode with a faster frame rate to do 12 channels. The 12 channel RX looks for a signal, then decides if the signal is a "standard" PPM signal or their own mode that they call "PPM 12".

Really, in theory, you could do as many channels as you want in any mode, you just need to design your data encoding for it. It's just the "standard" PPM encoding used right now that maxes out at 9.

bruce88123 12-13-2006 11:49 AM

RE: What does PCM mean on a transmitter?
 
For those who want to read the manual themselves:
http://www.multiplexusa.com/Support/...H%20Manual.pdf
Interesting concepts.

chevy43 12-13-2006 06:26 PM

RE: What does PCM mean on a transmitter?
 
My head is spinning!! If this all wasnt' confusing enough it apears that FM and PPM are the same?

I'm just going to get a Hitec Optic 6 and I think it is FM or PPM and that should work with my futaba etc recivers.

8 or more channels? What are you guys flying?!?!

bruce88123 12-13-2006 09:36 PM

RE: What does PCM mean on a transmitter?
 

Quote:

ORIGINAL: chevy43

My head is spinning!! If this all wasnt' confusing enough it apears that FM and PPM are the same?

I'm just going to get a Hitec Optic 6 and I think it is FM or PPM and that should work with my futaba etc recivers.

8 or more channels? What are you guys flying?!?!
Got some MORE confusion for you. PPM and PCM are both "types of modulation" used on FM transmitters. PPM is also used on AM transmitters that some people still use. PPM and PCM are the way the signal is "coded" on the basic FM carrier wave.
"OLD STYLE" Futabas encode PCM at 1024 bits/sec and the "NEW STYLE", like the 14MZ, encode at 2048 bits/sec.
As for what I fly? Right now nothing that needs more than 6 channels.

B.L.E. 12-13-2006 11:26 PM

RE: What does PCM mean on a transmitter?
 

Quote:

ORIGINAL: bruce88123


Quote:

ORIGINAL: chevy43

My head is spinning!! If this all wasnt' confusing enough it apears that FM and PPM are the same?

I'm just going to get a Hitec Optic 6 and I think it is FM or PPM and that should work with my futaba etc recivers.

8 or more channels? What are you guys flying?!?!
Got some MORE confusion for you. PPM and PCM are both "types of modulation" used on FM transmitters. PPM is also used on AM transmitters that some people still use. PPM and PCM are the way the signal is "coded" on the basic FM carrier wave.
"OLD STYLE" Futabas encode PCM at 1024 bits/sec and the "NEW STYLE", like the 14MZ, encode at 2048 bits/sec.
As for what I fly? Right now nothing that needs more than 6 channels.
I believe that "1024" and "2048" is the resolution, not the refresh rate. 1024 = 2^10 = the number of possible combinations of a 10 digit long binary string of 1's and 0's. 2048= 2^11.

bruce88123 12-13-2006 11:35 PM

RE: What does PCM mean on a transmitter?
 
You're correct. I stand refreshed in my memory.:D

Montague 12-14-2006 10:28 AM

RE: What does PCM mean on a transmitter?
 
And to top it off, there was at least one AM/PCM system made years ago. In many ways AM/PCM could be made more interference-proof than our FM and FM/PCM systems, but AM got a bad name years ago due to the wideband AM systems of the time. In theory, you can make an AM set more narrow band and more selective than you can with FM.

Oh, and "FM" is actually a slight misnomer for our sets as well. There's another term that is more correct (that I've forgotten, it might be FSK?). But basically our FM transmitters don't actually modulate the frequency the way FM music radio stations do. Instead, they only use 2 frequencies, and hop back and forth between them. It's the amount of time spent at the center frequency compared to the amount of time spent at the shifted frequency (5khz off, if I recall correctly) that encodes the servo position. So, if you're transmitting on channel 12, which is 72.030Mhz, and you have a positive shift FM TX, then you're actually broadcasting on 72.030Mhz part of the time, and 72.035 part of the time.

chevy43 12-14-2006 01:01 PM

RE: What does PCM mean on a transmitter?
 
http://www.torreypinesgulls.org/Radios.htm

Thanks for this site Kirk,

It really does exlplain this stuff pretty well. I probably understood about 85% of it. I recomend this reading to anyone.


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