Talk:Pulse-position modulation

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pulse position modulation receiver

The article currently claims "the radio control of model aircraft, boats and cars. PPM is employed in these systems"

I thought model airplane servo motors use (very narrow) Pulse-width modulation, not PPM.

Do servos really use PPM?

[edit] Good Question!

I wrote the majority of this article after completing a Doctorate thesis on a particular variant of PPM for optical communications. Before I edited, the article *only* mentioned PPM applications for RC (Remote Control) servos, and I left these alone. I honestly don't know whether PWM or PPM are used for these standard RC servos.

It seems to me that PWM is more likely for RC applications, because it is a totally asynchronous modulation, requiring no system clock whatsoever. The demodulation circuitry for PWM would be much simpler than PPM. Thus PWM would be a far more likely candidate for a simple demodulator circuit than PPM.

According to the Model Airplane FAQ, RC transmitters do indeed use analog PPM (Pulse Position Modulation), except for much more complicated high-end "digital PCM" systems. The reciever inside the model decodes the PPM it recieves into into a PWM (pluse width modulation) signal to drive the servo.
A quick Google seach leads me to some pictures of waveforms [1], [2], [3], [4].
which do indeed show fixed-width pulses seperated by variable amounts of spacing, pretty much the definition of PPM, and also show how simple the reciever electronics can be.
Perhaps counter-intuitively, the "pulse" is a brief gap where the RC transmitter stops (turns off) transmitting -- the RC transmitter transmits a constant tone the rest of the time.
(My understanding is that *optical* PPM systems typically turns *on* the light during the short pulse, and turns *off* the light the rest of the time -- exactly the opposite).
Should I re-draw some of these waveforms for inclusion in this article?
--70.177.117.132 02:11, 22 November 2006 (UTC)

[edit] M-FSK?

I assumed "M-FSK" in the article meant Multiple frequency-shift keying and linked accordingly. Please correct if incorrect. -- atropos235 ✄ (blah blah, my past) 03:01, 7 June 2007 (UTC)