Talk:Pulse-position modulation

pulse position modulation receiver

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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?

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)

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)

The meaning of M

In the notation M-ary PPM (or FSK)(M={2,4,8,16...}) M generally refers to the number of slots rather than the number of bits per sequence. In general M=2^k where k is the number of bits sent per symbol. M takes the place of k in the intro. While the introduction is correct, the notation is somewhat different than what is often seen when M-ary PPM is discussed.

This is further confused when the notation of finite fields is considered, in this case M is generally the exponent, eg GF(2^M), which corresponds to the notation used in the intro.

See "Laser communication transmitter and receiver design" Caplan.

Use in IR Remote controls

I am a bit surprised that nobody mentions their use in IR remote controls like in https://4donline.ihs.com/images/VipMasterIC/IC/SHRP/SHRPD008/SHRPD008-661.pdf?hkey=EF798316E3902B6ED9A73243A3159BB0 — Preceding unsigned comment added by 79.214.255.97 (talk) 20:04, 22 September 2016 (UTC)