RC PPM signals are not standardized and often differ between RC manufacturers.
i can confirm that through conjecture. the protocol maybe standardized, but the timing is not. my FlySky 8 channels RC PPM is 50Hz = 20ms frequency, all 8 PPMs are in that time frame, each PPM is 2ms pulse max making 8 channels requires 16ms to generate, still fit in the 20ms period with 4ms relaxed time for the receiver... thinking about how 12 channels is produced, it mus'nt be 20ms period... yeah, take that in discretes for the challenge,not to mention that within the 5-10% eqv PWM pulse you must have a decent resolution for your servo to work smoothly...