in "tuning project," why only "software mercury engine" ?
graphic card : amd radion hd 7700
Because you card in not a certified card.
CS5.5 only had certain nvidia cards on their list.
Search for 'hack' on this forum to take care of this 'problem'.
I don't think "the hack" will help you here, or at least with CS5.5. I believe that only nVidia video cards allow for hardware accelerated MPE (Mercury playback engine) for this version.
With the latest CC version (7.0.1), Adobe is supporting quite a few ATI cards for OpenGL, so trying that version could be a possibility.
You are correct. The hack is only for nvidia, was on another planet for a moment.