How to wreck HDTV

Last night I watched ‘House‘ which I had recorded earlier on a HDTV PVR. Naturally I recorded the high definition version from my local station, Southern Cross 10 in Canberra, which broadcasts HD at 1,920 by 1,080 pixels. As I was watching, I noticed from time to time a little bit of a shudder in on-screen motion. Weird. I fiddled with the deinterlacing settings on the DVDO VP50Pro. I had it set to ‘Film Bias’ mode as it turned 1080i into 1080p. I put it into full auto mode, and the shudder went away.

Intrigued, I transferred a couple of minutes of the show onto my computer for a closer look. The picture here is the same detail (100 pixels wide by 255 pixels tall) from six sequential frames of the show, as broadcast. You will notice the ‘combing’ caused by the use of interlaced video.

Six frames from House

But why is it interlaced? ‘House’ is not captured on video. It is filmed in 35mm. All the frames should be progressive, not interlaced. Indeed, this is the first major US TV show delivered on HDTV here that I’ve found to be interlaced.

Let us look a little closer at the interlacing. The subject of these details is moving fairly evenly, but there is significantly more combing on the first, second, fourth and sixth frames than there are on the third and fifth. Why would this be? This uneven combing is characteristic of a quick and dirty conversion from 60 hertz video to 50 hertz. In fact, if you look very carefully at the fifth frame, you will notice that the combing to the left of the head is still there, and still the same size. It’s just that it’s very faint.

There are three ways that you can create 50 fields per second video suitable for Australian (and European) TV systems from film. The obvious and best way is to create the fields from the 24 frames per second of the film. Each frame becomes two fields, and because in our 50 hertz part of the world we show fields at fifty times per second, the whole thing runs slightly fast. This is what normally happens, both with HDTV and SDTV.

Second, you can convert the 60 frames per second US version to 50 frames per second by reversing the 3:2 pull down performed on the film frames in the first place to create 60 fields per second video, reconstituting the original frames, and then proceed as with the first option. This is, obviously, unnecessarily complicated.

The third way is use the conversion system used with video-sourced 60 hertz material: six interlaced frames are turned into five. This involves most of the frames being some kind of average of original frames either side of them, thus some have hardly any interlacing, while others have quite a lot.

This system is most commonly seen with shows like ‘The Simpsons’, which originate with 60 fields per second video. For material originally shot on film, about the only place it is seen are the old Rank movies shown at 2am on ABC TV. These are movies usually made in the 1930s and 40s, and have obviously been converted from NTSC TV versions.

It is rather sad that a new show, such as ‘House’, should be treated in this cavalier way. I shall check again next week to see if, maybe, this was just some kind of a stuff-up.

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