720p & 1080i

As I mentioned when talking about standard definition video, traditionally TV pictures have been delivered in an interlaced manner. This means that each full frame of video is split up into two halves made up of odd and even lines. However, a better way to deliver video is to display each frame as a whole, therefore avoiding the frame mismatch that can occur with an interlaced signal – this method of video delivery is called progressive.

With this in mind, there’s a fair amount of debate over which gives better image quality, 720p or 1080i – the argument being that even though 720p has fewer lines, the fact that they are delivered as a whole frame potentially makes for a better image. However there is one very important factor that discounts that argument – both LCD and plasma panels are progressive by default. In fact since the vast, and I mean vast majority (in Europe at least) of high definition TVs are either LCD or plasma based, you can’t help but get a progressive picture.



Let me elaborate on this a little. As already mentioned, a CRT television draws an image in two passes by scanning from the top left corner of the screen to the bottom right, but both LCD and plasma TVs are fixed pixel devices, so there is no scanning involved. This means that even if you wanted your LCD or plasma high definition TV to display an interlaced image, it couldn’t.

What about if you feed it a 1080i signal? That’s a good question, but one that’s easily answered. If you pump a 1080i signal into an LCD or plasma TV, the television de-interlaces the signal before displaying it on the panel. So, even though the signal being fed to the TV is interlaced, the actual footage displayed on the panel will be progressive. Therefore a 1,920 x 1,080 panel showing 1080i content should look superior to a 1,366 x 768 panel showing 720p content, while pumping a 720p signal to a 1,920 x 1,080 panel should also look less impressive than when that same screen displays 1080i content.



Now, before all those readers who have bought 1,366 x 768 HDTVs start to cry, there’s more to a great high definition picture than resolution alone. Some of the best quality high definition pictures right now can be seen on TVs with only 768 lines, because the image processing in those sets is so good. In fact it’s worth remembering that you could look at two HDTVs from different manufacturers but using the same panel, and the resulting pictures could be entirely different due to the proprietary image processing that each manufacturer implements.

One thing’s for sure though, traditionally an HDTV with a 1,920 x 1,080 panel would cost you significantly more than a similarly sized 1,366 x 768 model. But that changed recently with the launch of the Toshiba Regza 42WLT66, which offered a Full HD 1,920 x 1,080 panel at a stunning price point. Let’s hope that this is a sign of things to come.

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