Most plasma screens (except for some of the really most recent high-end ones, like less than 6 months old) have fewer than 1080 physical lines of resolution on the glass (usually either 1024x768 or 1366x768) so if you put a 1080i signal to them not only are you getting interlaced video instead of progressive video, but you're up-converting to 1080 lines of resolution and your TV is then down-converting that into 720 (or 768) lines of resolution.
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