Wednesday, February 27, 2008

1080i and 1080p on Blu-ray

Many Blu-ray players boast the ability to output movies at 1080p/24, exactly as they are recorded on Blu-ray disc. They can output other, video-based content at either 1080i/60 or 1080p/60. The latest models of flat-panel HDTVs are typically able to receive and make use of 1080p/24, 1080i/60, or 1080p/60 input over an HDMI version 1.3 connection. But video-based high-definition content is recorded on disc as 1080i/60 and has to be converted to 1080p/60 for flat-panel display. This conversion, called "deinterlacing," can be done by the player or by the TV.

Here is an explanation of this confusing subject.

Movies shot at 24 frames per second are recorded on Blu-ray disc as (you guessed it) 24 complete video frames for each second of film. These 1080p/24 frames have 1,080 lines of 1,920 pixels each, and the lines are "progressively scanned" (the "p" stands for "progressive"). On an old-fashioned tube-type TV, the lines would be painted on the screen, all in their original order, by a moving electron beam. On a newfangled flat-panel TV, the lines are lit up all at the same time.

In "interlaced" scanning, first the odd-numbered lines are painted on the screen, then the even-numbered lines.

High-definition TV-based fare, as opposed to film-based material, might be progressively scanned at 60 frames per second, not 24. That would be 1080p/60 video. But to cut down on the amount of data, each frame could have exactly half its lines removed. The first frame would lose its even-numbered lines. The next would lose its odd-numbered lines. Then back to suppressing the even-numbered lines. And so on. That's 1080i/60 video. Its scan method, when presented on a tube-type TV, is interlaced: first the electron beam traces out the odd-numbered lines; then the beam traces out the even-numbered lines.

1080i/60 video won't work with a flat-panel HDTV, because it's interlaced. The display panel can represent only progressive video. So the flat-panel TV cannot display 1080i/60 video until it has been "deinterlaced."

Progressive 1080p/60 video is never recorded as such on Blu-ray disc, because it requires too much data. Interlaced 1080i/60 video can be recorded as such on Blu-ray disc — just as can progressive 1080p/24 video that has been derived from film.

When 1080i/60 non-film-based video is read in by a player from a Blu-ray disc and passed to a flat-panel 1080p HDTV without modification, it is input to the TV as 1080i/60, and it has to be deinterlaced by the TV. Its missing pixel rows have to be filled in somehow. There are several techniques for doing this, and some work better than others.

When 1080i/60 video is read in by the Blu-ray player and sent to the TV already converted to 1080p/60 — i.e., it is converted by the player itself from interlaced video to progressively scanned video — it is the player that does the deinterlacing.

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