In the comments on yesterday's AppleTV story, many people doubted whether 1080p support in the third-generation AppleTV will be very useful. That question becomes even more relevant when observing the file sizes of 720p vs 1080p versions of the same movies and TV shows in the iTunes Store: despite the fact that 1920x1080 means 2.25 times as many pixels as 1280x720, file sizes often differ by a factor of 1.5 or less. So what's going on—is the 1080p content pretty much indistinguishable from the 720p version, or did Apple succeed in magically upping the compression?

A little of both, it seems.

In the iTunes 10.6 preferences under Store, you can select whether HD downloads come in 720p or 1080p resolution. In the iTunes Store, movies conveniently list the file size for the selected format. A couple of examples: Hugo is 1.74GB in SD, 3.99GB in 720p, and 4.84GB in 1080p. Monty Python and the Holy Grail is 1.00, 2.91, and 3.65 gigabytes, respectively. However, to determine the quality difference, I looked at two (currently free) TV episodes: episode 1 of season 5 of The Big Bang Theory, and the first episode of Awake.

(Note that you can always download different versions of movie trailers from the US iTunes Store even if you don't have an iTunes account.)

Getting both 720p and 1080p versions is pretty simple. If you have already downloaded the 720p version of a TV show, you can change your Store setting to 1080p, and then re-download in 1080p by navigating back to the episode. The new HD version doesn't overwrite the old one—you'll simply end up with three versions of the same episode: SD, 720p, 1080p. You can select which one you want to play in iTunes using the right mouse button contextual menu.

Time to watch some TV

I first looked at The Big Bang Theory and admit I was a little disappointed. In most scenes, you really can't tell the difference between the 720p and 1080p versions, even when jumping back and forth between two QuickTime Player windows cued up to the same frame. I even suspected that the 1080p version was really upconverted 720p. But at the 4:02 mark, a juice bottle does appear noticeably sharper in the 1080p version.

If you already have this show in 720p, I personally wouldn't waste time re-downloading at 1080p, even if the file size for this episode isn't much larger—856MB versus 743MB for the 720p version. Also, the 720p version will sync to the iPhone 4 as well as the older iPads and it will play on the second generation AppleTV. The 1080p version only plays on third-generation iPad and AppleTV, and the iPhone 4S. On the other hand, if you also have an SD or 720p version, the lower resolution version will be synced to older devices.

I also had a look at the first episode of the new show Awake. Even in dark scenes there is always some extra sharpness in the 1080p version, but in brighter scenes the higher resolution version is significantly better. I certainly recommend the 1.75GB 1080p version over the 1.45GB 720p one. These two screenshots were taken at the 5:36 mark from the top left corner of the image playing back at 1080p (so the 720p version was zoomed, as it would be on a 1080p flat panel). This is a small detail, and the 720p version isn't that blurry, but this kind of improvement is visible throughout the scene.

The reason that the 1080p versions of the iTunes Store videos can be a good deal better without doubling the file size—or worse—can be found in the tech specs of the new AppleTV and the new iPad. The AppleTV now supports H.264 compression for 1920x1080 resolution video at 30 frames per second using High or Main Profile up to level 4.0, the iPad and the iPhone 4S the same up to level 4.1. The profile indicates what kind of decompression algorithms the H.264 decoder has on board—the "High" profile obviously has some tricks up its sleeve that the "Main" or "Baseline" profiles known to previous devices don't support. The level value indicates how many blocks or bits per second a device can handle.

The A4 SoC that Apple used in the iPhone 4 and the second generation AppleTV as well as the A5 in the iPad 2 can handle Main Profile level 3.1, which is good enough for 1280x720 video at 30 frames per second. The original AppleTV can only handle 720p at 24 frames, and earlier devices are limited to SD video. This means that the increased 1080p resolution breaks compatibility with everything older than the iPhone 4S anyway, so Apple is free to use the high profile, resulting in better compression for a given quality level. The files are larger, but not that much larger. Whether the increased resolution comes with extra visual fidelity, however—and thus worth the extra download time—will vary from movie to movie and show to show.