Shane Conder's Whateveritis of Nothing

There is a page floating around the net currently that appears to do a good job of comparing a 480p display to a 720p display (it's here, for reference). At first glance, it's great.  You mouse over and see the 720p while mousing away and seeing the 480p.  There's a very clear difference between the two.  The 720p view has much more detail than the 480p view.

Then I realized something.  This comparison is somewhat flawed.  What it's actually comparing is 720p to a 480p screen that's being scaled to 720p with an unknown algorithm from paint.net.  So really, it's comparing how a 480p picture would look on a 720p screen when scaled with that algorithm. 

I also got to thinking about this with respect to our TV.  Our TV is a 40" 1080p LCD screen.  That means, by definition, anything being displayed on it is being scaled to 1080p since you can't change the fact that it has 1920x1080 pixels on the screen.  When it's drawing a native 1920x1080 image, all is pixel perfect.  But when it's drawing a 1280x720 image full screen, this is actually a 720p image being scaled to 1080p even if the info says that it's 720p (the source is 720p and the screen mode is 720p, but the screen still has 1080p pixels being used). 

The same is true when it's drawing any of the other resolutions, such as 480i, 480p, and 1080i.  In the 480x cases, there are black bars on the side so the aspect ratio is kept correct so it's not actually lighting up all of the 1920 pixels across the screen.  However, since we know the aspect ratio is 4:3, we also then know that in the 480 modes what we're really looking at is 480x scaled to 1080p at 1440x1080 rather than the widescreen 1920x1080.

When the TV is doing the scaling, like it does with a component input, it uses it's algorithm to figure out how to draw and smooth the pixels on to the screen.  With VGA input, though, at 1920x1080, the source of the video must do the scaling (in my case, either a computer or the XBox 360). 

What this means is that when I compare the quality of a 480i digital channel to a 1080i or 720p HD channel it's all being compared with the TVs scaling to 1080p.  This is not necessarily how these channels would look on a 720p or 480i screen as our TV has more pixels. 

The same goes for the comparison of 480p to 720p where the comparison images are both 720p, but one has been scaled and scaled rather well, at that.  For a different comparison, you could do a non-smoothing scale that would basically similar 480p with larger pixels in the same size as a 720p screen, or put gaps simulating the same size pixels but with a greater pitch.  Either of these methods would try to simulate a a different number of pixels being used while normalizing to the viewers screen.

Anyway, just some random thoughts on the topic...  in the end, we decided on a TV that scaled better than most but not quite as well as the $1k more expensive Sony Bravia Pro series.  The point being that the quality of the scaling on an HD TV is quite important these days.  A good scaler can actually produce really good images.  Our TV has a good scaler, but the XBox 360 has a much better one.

Posted by Shane on January 25, 2007 7:47 AM | Permalink