Základní pojmy: PPM, CPPM, PWM, PCM a S.BUS

PCM = pulse code modulation
PPM = pulse position modulation
PAM = pulse amplitude modulation
PWM = pulse width modulation

Tyto zkratky označují způsob zakódování údajů o výchylkách v jednotlivých kanálech v signálu, přenášeném mezi vysílačem a přijímačem, případně mezi přijímačem a servy.

PPM and PCM, as you know, are different coding methods to get the same information out to the receiver. PCM receivers cost a bit more because the decoder is a bit more complex. The two kinds react differently to interfering input.

If a PPM receiver gets interference, the receiver will think the interference is part of the signal and send erratic information to the servos, which will cause them to act up. The model can have a little reaction as a slight 'bobble' or can have the servos jam over and the model wind up in the ground.

When a PCM receiver gets the interference, the processor sees it as bad information and just holds everything where they were the last time good information was received. If the interference lasts more than a second or so, then the receiver will move the controls to postions selected by the modeler. That's what is meant by "failsafe". Sometimes, though, the failsafe doesn't do much good because the model's in a bad place when it gets "hit" and the failsafe can't prevent a crash in that case.

Usually, though, most interference is momentary, and it makes little difference which receiver you have, since the model will stay aloft long enough for you to save it when the interference is over. With PCM, though, the model will usually not jump around, but the modeler will notice that he's locked out of controlling the model. When the interference ends all control is regained.