High Definition uses MPEG 4 and the audio codec is AC3,not really sure which one you are looking for to be honest.I was wondering are you looking for a video codec or the sound codec,because it uses two different codecs.
i guess both because when i play it the video and sound dont come out good. btw what program can i play it in? thanks
MPC should play the HD files you find floating around. Many will also have AAC audio. http://www.free-codecs.com/download/Media_Player_Classic.htm Unfortunately the H264/x264 encoding requires a lot of horse power to play right. Once you get into the extended resolution stuff (1280X720 etc) a newer computer is a must.