| Pulse Code Modulation (PCM) uses the FM carrier wave just like pulse position modulation does. The difference is that PCM actually transmits a digital word. This word is a series of binary bits. The information transmitted in the digital word contains the position of the controls along with checksum and failsafe information. The checksum adds redundancy into the system that is used to check whether or not correct information was received. If the information is corrupt then it is rejected. The failsafe information is used to decide what positions should be held by the servos when no accuarate information is being received. The digital words can vary in length, but most are 1024 bits long. If the checksum indicated that any bit in the word is incorrect then the entire word is rejected. The failsafe information is used to decide what positions should be held by the servos when no accuarate information is recevied for a fixed period of time, usually half a second. |
