PappyFarland typed his response faster than I did, but here is mine just in case you feel like reading a little more....


JackStar07:
Although I have never used a CPD4 I have an idea of how it works. The CPD4 gets connected between the receiver and the servos. The CPD4 is a sensor that corrects for level attitude. This feature makes it RF band independent but If I understand correctly, it will not handle CCPM helicopters.

The FS8 is a system that incorporates a receiver. Since it is a receiver, it can handle CCPM applications and the commands from the sensor are routed thru the receiver to the servos. The receiver knows which servos to move in order to "level" the aircraft. The receiver has a lot of interesting features like failsafe and mixing. It stores all this information as you set-up the system on your aircraft.

So there you have it. The basic difference between the CPD4 and the FS8. Both do the same thing in principle, but they do it thru different approaches.

Rafael