Ah, HD-DVD... The DVD format is built around a resolution of 480p -- adding lines requires more storage space, and the DVD format can only hold so much. There are currently two competing formats vying for the title of "HD-DVD". There's AOD (Advanced Optical Disc), which is supported by Toshiba and the DVD forum and therefore gets to actually wear the name "HD-DVD", and there's Blu-Ray, supported by most everybody else. Both use a blue laser to store more data on the same size disc -- instead of the 4.7 GB in a single layer on a DVD, they can store 15GB (AOD/HD-DVD) or 25GB (Blu-Ray). Even with more advanced compression formats, it takes that much space to store any significant amount of 720p or 1080i video.

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