I'm not an expert but I know a little bit about radio so here is some information that may help someone:

FM is frequency modulation, it's how the radio puts servo information inside of a radio transmission. There are two ways to modulate servo data used today, PPM and PCM. PPM simply takes the servo information and stores it in the pulse width, while PCM takes the servo information and stores in a binary format using a protocol.

In a nutshell it's basically the difference between an analog and digital cell phone. With the analog phone you get static, but sometimes you can talk though it, where digital phones you don't get static, instead you don't get anything if enough data is lost to make the protocol stop working.

I have never used a PCM system so I am speaking without experience, but my next system will be PCM simply because I would rather have my equipment use my last good known commands until the signal is completely lost, in which case do some programmed action, instead of do whatever random thing the interference is transmitting.