Just made a quick search on wiki, and found no good definition of our RC PPM modulation.
Some short definition:
The modulation can be positive or negative.
The frame synch(ronization) signal exceeds the duration of any channel (typically 2.5 millisec (ms) or more).
The channels (their number vary from 2 to 12), are transmitted in sequence, starting with channel 1.
Each channel is represented by a channel synch (of approx 300 ms) plus a variable time. The channel position is given by the sum of synch PLUS var time. Full left position of the servo is 1000 microsec (us), full right is 2000 us. Some system can exceed, giving 800-2200. The servo travels from -45° to + 45° approx. The servo signal is always positive, and its length is the length of the ppm synch+var.time of a channel.
The full frame will be repeated approx. 50 times per second. Less (45 ? times) for 12 channels. For systems with few channels, the frame synch will be quite long, to wait for the approx. 20 ms of the 50 s-1 system.
This view gives the relation with the servo signals: ('voie' is the french for channel)
Mr. Thobois 'site is a good RC electronics site.
here a 6-channel ppm signal
from
You can find other infos on PPM and diagnostic tools on (in french)
Phil.
rc radio control modulation protocol ppm whatis signal scale model