Discussions

For beginning of the story see here :

http://diydrones.com/forum/topics/ppm-encoder-problem-loosing?commentId=705844%3AComment%3A462572

I've finally received a 2.4 Ghz receiver and transmitter system.

So i did measure PWM outputs on channels 1 and 2 for my 72 MHz receiver, then for my 2.4 Ghz receiver.

Here are the results for the 72 Mhz receiver :

We can see clearly that the rising front of channel 2 begin before falling front of channel 1.

Now let's see the pwm outputs for the 2.4 Ghz receiver :

Here we can see that channel 2 rising front start just after falling edge of ch1.

For information, the total frame period is not very stable for 2.4 Ghz. There is about 2 ms jitter. But this does not seem to cause problems to the ppm encoder.

I think that this explain why i have problems with the ppm encoder and the 72 Mhz receiver. The ppm encoder is simply not able to reliabily decode the signal.

So i would recommand to all users, to stay away from receivers with interleaved pwm outputs (possibly FM receivers). I can't be sure, but it seems that 2.4 Ghz receivers are ok. This needs to be verified nevertheless because i've only tested an Hitec optima 9 receiver.

Here are the receivers i've tested :

Hitec optima 9 (2.4 Ghz) : compatible

Hitec Fusion 9 (72 Mhz)  : not compatible with actual ppm encoder code

Hitec Freedom 9S (72 Mhz) : not compatible with actual ppm encoder code

Do not forget that if you are targeted by the ppm encoder problem, you have great chances to crash because of it after a couple minutes. So it's better to check before to fly if you have a compatible receiver.

I think that futur revision of the ppm encoder code will be fully reliable, but in the meantime, be carefull.

I'm still waiting for an ISP programmer, so i can try to modify the ppm encoder code to solve this problem.