Quote:
Originally Posted by
John Mason /forum/post/14284795
OP mentions 720p sources, and since 1080i/p has twice the format resolution of 720p, 1080i sources often look better, resolution-wise, than 720p sources. Members here have been observing that since HD broadcasts began.
720p sources, though, may often appear similar to 1080i programming. That's because 1080i
effective resolution (resolvable detail) often approximates the effective resolution from 720p. Much 720p programming is from 1080/24p master tapes; oversampling/downconversion from 1080 to 720p results in greater image sharpness. (
Fig. 1 in Adam Wilt's column illustrates why a modulation-transfer-function curve, comparing contrast versus resolution, can appear sharper because it covers more area--the result of oversampling/downconversion. Part 3 Fig. 5 charts in this Arri
draft paper also shows how the boosted contrast at coarser more-visible lower frequencies, such as 20 line pairs per mm, helps make a 4k film scan appear sharper than a 2k scan.)
By contrast, even though well produced 1080i programming often appears 'crisper' than 720p, a lot of 1080i/p's potential effective resolution is missing. Oversampling/downconversion with 4k cameras/recordings should theoretically change that, although MPEG-2 delivery sources might not handle such boosted HD details coupled with motion. An adequate codec such as MPEG-4 might provide full 1920X1080 effective resolution, matching the format resolution. Current 1080i is limited to ~1700-line effective horizontal resolution from sampled (74 MHz) images, although additional filtering reduces resolution still further. Vertical resolution varies with image motion; the Kell effect reduces 1080 lines to ~756 (scan lines X 0.7) and
early HD testing recorded 400 line vertical resolution with motion.
HD-delivered movies, whether 720p or 1080i, might appear worse resolution-wise compared to live other non-24p-based material, because movies are usually heavily filtered at the camera lens. One spectrum analysis
study indicates telecined maximum horizontal resolution (equivalent detail) is only 800--1100 lines. While Blu-ray discs with better codecs or higher bit rates and occasional 4k downconversions should deliver greater detail, it's not clear (without more spectrum analysis) whether 'better' discs (resolution-wise) are only providing detail at the higher end of that 800--1100 range. -- John