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I have heard information to the contrary: SD interlacing includes a vertical lowpass filter, while HD interlacing should not. I do not claim to know this for a fact.
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That's interesting. I must admit that I noticed there was a lowpassing when I took a 720p50 content and tried to turn it into a 576i50 one. It was unwatchable without (strong) lowpassing, thus I concluded that lowpassing was necessary for interlacing, and extrapolated the same was happening in HD. I was confirmed in that opinion when I read that
document, that explains how a 2160p video was transformed into a 1080i one (a 540p field is created by averaging four lines), but perhaps that document is incorrect.
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I remember reading a Philips paper in which they compared 25p, 50i and 50p when encoding as bitrate-constrained MPEG2. The conclusion was that MPEG2 + 50i + HQ deinterlacing had best rate-distortion characteristics. Perhaps because MPEG2 lacked deblocking?
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There have been a lot of studies regarding 1080ixx vs 1080pxx. AFAIK, all of them but one (EBU's) concluded 1080i was better. I don't really know what to make of that, and I would have liked to see 720pxx thrown in the lot too.