HD1080i De-mystify HDTV 1080i ::: know why before you buy

Saturday, April 21, 2007

1080i 1080p Myth Busting

We a lot of seemingly authoritative bad and old writing out there, it is time for some basic...


1080i is worse than 1080p.

Not really. Broadcasts are in HD1080i and properly managed at both ends, is very good. Since Flat panel displays do not scan but instead assemble each frame in video memory, it is always a 1080p screen result. Nearly all the 1080p displays available in 2007 do a great job of de-interlacing, although i have a preference for DCDi ( Directional Correlation De-interlacing ) as done by Faroudja.

1080p in HD-DVD and Blu-ray is better mostly from the reduced compression and not the interlace (i) or progressive (p) delivery. In Fact some 1080p 24 at 24 frames per second from film productions is more "filmic" ( a word i have heard used often ) at a lesser frame rate and more motion blur than the usual 30 or 60 frames persecond that a good 1080p LCD HD display can show you. Odd - even arrival of interlaced data is alternate rows of pixels requiring processing to assemble an image. Over and above that is the 16 x 16 pixel block used in compression codecs.

-- So really where is the problem for 1080 video? It is not Interlacing.

MPEG and Compression

The usual problem for digital video in any form is not in the progressive or interlaced delivery of the image frames, but in the over-compression of the digital processing. Current broadcast standards are delivery of TS or Transport Streams in MPEG2. This is required because the MPEG compression de-compression process must happen at the both send and recieve ends. So the Broadcaster uses MPE2 to compress and your Cablebox firmware uses a built-in standard MPEG2 chipset to de-compress. Its old technology, from back in the DVD days. Direct-TV and others are migrating to MPEG4 variants which are somewhat better, but blocking errors are still the outcome when this technology is mis-used.

Worst Case Scenario: BIG BAD MACROBLOCK

The problem is in bad data or over-compression used to save bandwidth, and visible evidence of this is square chunks of the image looking out of place and time. That artifact is known as a MacroBlock... if there is any detriment to digital works, it is abuse of the mpeg2 compression codecs that reveals blocking in the frame. All your digital video is made up of these blocks and when its all good, you probably do not see them or know that they are there. This is different from interlacing which is a very fine comb-like edge artifact.

Basically a cheap broadcaster running out of Bandwidth will smash and squeeze the video stream right up to the point of customer complaint. Then you see macro blocking.

Now that you know what all this is, complain when you see it. Click to see below -- a very extreme case of compression recovery failure in macroblocks. Click Play and then move the slider to see and select frames for view. ( there is no audio ) A variety of error cases is visible but they happen fast so the slider helps see this in detail.

You should never see anything this bad, but it serves to expose what your digital video is really doing, in that you are always seeing blocks of data. Extreme cases like this make it much more obvious. This example was sent to me on a DVD from a friend who thought his system was broken. It was recorded direct from his cablebox. It is important to note, that this processing problem is not your new flatpanel display, but happens before that at the Cablebox/Converter, and can be from line noise in the cable, a problem at the source, or bandwidth overload, among other things.


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