Giz Explains: Why HD Video Downloads Aren't Very High Def
Sep 10 2008
By matt buchanan

Yesterday Apple introduced HD TV downloads to the iTunes store,
meaning you can watch Peter be super emo on Heroes at a crispy 720p
resolution. That's a higher resolution than DVD, and technically, yup, that's HD.
There's a catch though. Like every other video download service touting HD
videos, it's all actually lower quality than DVD.

It's all about bitrate: How much data is packed into a file, described as
bits per second. Generally speaking, a higher bitrate translates into higher
quality audio and video, though quality can also be affected by codec—the
encoding and compression technique that was used to make and read the
file—so bitrate is not an absolute mark of quality, but it's still a very good indicator.

You're probably most familiar with this bitrate business when it comes
to ripping your CDs. When you shove a CD into your computer, your ripping
program will ask what format you want and what bitrate you want. A song
ripped at a higher bitrate will sound better, with more presence and detail,
but it does take up more space.

The same principle applies with video, though it's actually a bigger deal,
because it's easier to see quality differences in video than it is to hear
differences in audio. The bits make a huge difference when you get into
fast moving stuff like sports or action movies—to be frank, they'll look
like splattered, smeared shit in highly compressed low-bitrate vids.
This chart below, expertly crafted by George Ou at ZDNet provides a solid
starting point for comparison, with average bitrates of most digital video available.