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Old Nov 03, 2002, 09:49 AM
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Minneapolis, MN
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What Does PCM and PPM mean?

Hello,
I have read about radios that are PCM, or PPM. What does this mean? -Patrick
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Old Nov 03, 2002, 09:58 AM
(aka Cliff Lawson)
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United Kingdom, Essex
Joined Oct 2001
1,897 Posts
Hi,

PCM = pulse code modulation
PPM = pulse position modulation

Effectively PPM is the old "analog" type of signal and PCM is the more modern "digital" type of signal.

It's not unlike the way mobile cellphones have switched from the original analog systems a few years ago to mainly digital these days.

Or the way that television transmission started out analog but in the last few years digital transmission is starting to take over.

PPM just takes the position of the stick and adjust the width of a pulse transmitted over the air accordingly. In PCM a little computer chip does an A/d (analog to digital) conversion abd converts the stick position to a number (0..255 say) and then transmits that.

As such PCM is far more reliable and less suceptible to interefence but, on the whole, the equipment is more expensive.

At the present time almost all the transmitters/receivers in use by all but the "professional" r/c pilots (and those flying large/heavy - 7Kg+) are still PPM as things like dual conversion receivers are usually enough to deliver a fairly reliable link between Tx and Rx anyway.

I don't know if there's really any resaon for us mere mortals to switch to PCM but things might go that way over time as everything seeks to benefit from the use of "digital". On the other hand there's thousands of pilots with thousands of dollars invested in PPM Tx/Rx and I kind of doubt whether everyones going to dump all that overnight.

Most "computer" Tx's are switchable between PPM and PCM anyway so, when you get rich, you can buy PCM receivers and then choose to switch the Tx (but this is yet another thing where I guess it could be easy to leave the thing switched to the "wrong" state and disaster might ensue!)

Cliff
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Old Nov 03, 2002, 10:31 AM
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Berks, England
Joined Jun 2001
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On the subject of PCM/PPM there are also new receivers, that while technically PPM offer the main advantage of PCM - glitch filtering; eg the Multiplex IPD ones and the Schulzes.

Since these cost half the price of a PCM receiver I can't really see the point of PCM, especially as some of the PCM systems have a reputation for being a bit enthusiastic with the failsafe.

Stuart
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Old Nov 03, 2002, 11:07 AM
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Thanx Patrick
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