How Apple keeps 1080p file sizes in check

Apple last week introduced a revamped version of the Apple TV. In addition to a completely new interface, the new Apple TV finally includes support for 1080p videos.

But is the Apple TV, with in 1080p in tow, even that useful? After all, the enhanced video quality is great but if the file sizes for TV shows and movies is excruciatingly large, 1080p will be more trouble than it’s worth.

But not to worry, Chris Foresman of Ars Technica points out that even though Full HD video includes 2.5 as many pixels, the corresponding file sizes are only about 1.5 times as big as its 720p brethren. Some examples highlighted by Foresman include the movie Hugo. The 720p version of Hugo checks in at 3.99GB while the 1080p version checks in at 4.84GB. Not too bad. And in case bandwidth an issue, note that the standard def version comes in at 1.74 GB.

So how does Apple achieve Full HD quality without upping the ante on file sizes to the realm of impracticality?

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 1920×1080 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 1280×720 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.

Edible Apple

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