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)
