A bit of advice to anybody that wants it. This took me a long time to figure out even with the help of a Futaba Engineer. I Have a Robbe / Futaba FC16 transmitter running PCM. I'm building a 1:50 Walrus Class submarine (with X-rudders). The first test (above water) I used the Elevon mixer in the transmitter to mix channel 1 and 2 for the rudders. As the boat evolves the APC + depth controller were installed and I needed a x-mixer to mix the seperate depth and direction signals after the APC and Depth controller.
I was very surpised to find sluggish response from the servo's and very much stuttering. The problem turned out to be an incompatibilty between the x-mixer and the receiver.
I have an 8 channel receiver (PCM) which gives 100 pulses per servo per second in stead of the normal 50 for PPM. First I thought is was the refresh rate from the receiver that was the problem so I came up with a schematic that eliminated every second pulse to the servo's. (50 pulses per second) But the problem was still there.