With PCM coding, you can't rely on 20 mS frames. That's for analog PPM systems. PCM coding has its own timing system. Unfortunately, the exact coding scheme for Futaba PCM systems is considered proprietary, and Futaba, Japan has not released it.

Your best bet would be to use the analog servo output signals from each channel and route them through your processor.

Such discussion on how to do this is well beyond the scope of this forum. How to implement such a device would best be discussed in the general radio forum.