Keep in mind that a PPM stream decoding function requires two different process to happen, frame start/stop detection and the individual channel number's width information . First before it can accurately process the individual channel data it has to 'sync' up with the PPM framing rate so that it will knows which pulse represents which channel consistently. After the last channel there will always be a longer zero period that fills out to the end of the 20-25msec frame period. Only after your decoder's algorithm has detected the end of the frame will it know that the next 1 to 2msec wide pulse represents the channel 1 information, and each in order after that until the end of the PPM frame.
That make sense?
Lefty