This has bothered me for a while, but a bunch of junk mail just came through the letterbox which prompted me to write this down…

Every new widescreen Plasma, LCD or CRT widescreen is marketed as ‘HD Ready’ and people snap them up with the idea that they’ll get a better, sharper picture. But it’s so often a lie. Many of these TVs (the ones that cost less than an arm and a leg) are not full-resolution HD, in fact many are lower resolution than standard definition PAL television.

Where I live we have PAL standard definition TV. It is essentially 768×576 pixels (it’s a little more complicated than that, but that’s where we’ll start). There is PAL 16:9 widescreen too, which is basically 1024×768px (again, more complicated, but that’s the number we’ll work with). All Television in NZ is PAL. There is no NTSC, and no HD broadcasting. However you can get HD DVDs and the newer game consoles support HD.

HD comes in two flavours, 1080 and 720, they are 1920×1080px and 1280×720px. This is the same all over the world. HD resolutions are standardised.

So, we’re working with these numbers, 576, 720 and 1080 - the frame heights of each format. An HD TV (as opposed to an ‘HD Ready’ TV) will be either 720 or 1080 pixels high (and the appropriate width for that height). It’s these extra pixels that give HD television the boost quality - a PAL image will store a maximum of 414,720 individual pixels from which to contstruct the image. HD 720 offers 921,600 pixels (more than 200% that of PAL) and HD1080 has just over 2 million pixels, nearly 5x more image data than PAL.

So where to the ‘HD Ready’ televisions fit in? Well they look pretty, but to save money they have fewer pixel in their screens. A lower resolution screen is cheaper to make. To cope with this, they resize the incomig pictures to fit their screen. There are basically two non-standard 16:9 screen resolutions in these TVs, they are: 1366×768 and 852×480.

Looking at the 768 pixel screen first, to display a PAL image, it must scale it up 133.33%, for a 720 image the the scaling is 106.7% and a 1080 image gets scaled 71.2%.

For the 480 screen it’s 83.3% for PAL (that’s right, lower resolution than standard definition PAL), 50% for 720 (a good number!) and 44.5% for 1080 line image.

What does it all mean? Well the problem is two-fold - first, these non-standard resolutions mean that the incoming video signal (whatever it is) has to be resized and in realtime, this means there has to be image processing circuits in the TV which take all the images, run smoothing and optimising routines over them and then resize the images so they can be displayed on the screen, this processing alone is messing with the picture. The second problem is the scaling factors. With the exception of 720 into 480, they are all very awkward factors. Mathematically it is very difficult to maintain fidelity when scaling 200 pixels down to 89 - lots of detail is lost, colours are ‘averaged’ and sharp edges are blurred.

This scaling is something that happens when viewing 720 HD on a 1080 monitor, but the factor (150%) is more even, and the other way around (66.6%) isn’t too bad either. But amounts like 106.7% and 71.2% are never going to scale very well. To some extent these problems also occur in scaling SD PAL into HD frames, but the conversions are still generally more even, and upscaling is more forgiving than downscaling.

What does it look like? Well it will depend on the TV in question and the methods it employs to do the scaling, but I’ve made some examples with Photoshop that demonstrate the factors with a 100×100px sample image.