Crop video without re-encoding it?

Hi everyone,
I used quicktime X to screen record a skype video chat and im trying to crop it so that theres just the skype window and not my desktop. I used imovie and did this but the video quality is terrible when i export it (shadows are like flcikers of grays). Im wondering if theres a way to do this to keep te exact video quality ( when importing to imovie i clicked keep original quality but still turned out terrible after i exported it) or to simply crop the video i need out of the movie. thanks!

 

hey sorry, i noticed i had posted in the hardware section first and all i do is open quicktime x and go to screen recording. for imovie i create a new project and then import video file (clicked keep original quality even though theres an error flag saying the playback quality might be bad...dont know why this is) and then i crop it, it looks great in the preview window of imovie. then i go export-> movie and after its done it looks crappy. Im sorry if that doenst make sense iv never done anything video related with my mac.