Look at it this way. PCM is sending a string of numbers. 1s and 0's. If a voltage is there its on and its a 1, if its not there, then its off and its a 0.

PPM is sending a value. Somewhere between all the way off and all the way on. Its easier for a reciever getting a crappy signal to identify if something is off or on, than to discern where between off and on that it is. Hence PCM's better ability to pick out information from an other wise problematic signal.

1024 pcm means that a sticks total travel is divided by 1024 increments, each stick position having an absolute value. 1024 by the way is exactly 10 bits of binary data. 512 was the old version that used 9 bits. The technology allowed them to compress the signal and send more data so now we 1024 resolution, I want to say the 14mz uses 2048 which would be 11 bits of information. The timing of the signal tells the reciever which stick position update (ie elevator then aileron) its getting in a string of numbers.