"At this point in the movie, Emperor Palpatine sends a radio message to contact Darth Vader, and Vader instructs Admiral Piett to move out of the asteroid field so they can get a 'clear signal'. So one could perhaps deduce that Palpatine's signal was kind of broken up and blotchy, similar to bad phone reception in a tunnel or something.
Only when they move out of the field and Vader takes the call, it's still messed up and dropping in and out. I guess that's science fiction for you. They can travel faster than light at the drop of a hat, but still use non-error-correcting low bandwidth analogue radio signals."
Omg why is Darths and Droids so good.