Apple employing improved compression for new iTunes 1080p videos
Apple announced on Wednesday that it was adding high-definition 1080p television content to the iTunes Store. The upgraded videos will be supported by the new Apple TV and the third-generation iPad, which packs more pixels than a 1080p television.
After comparing relative file sizes and video quality of the 720p and 1080p versions of iTunes content, ArsTechnica reported on Thursday that 1080p versions (1,920 x 1,080 pixels) were on average just 1.5 times larger than their 720p counterparts (1,280 x 720 pixels), despite having 2.25 times the number of pixels.
The report noted that the limited increase in file size is likely due to Apple's support support for the High profile for compression of H.264 videos. New High profile decompression algorithms on Apple's devices, including the Apple TV, iPad and iPhone, represent an improvement over the Main or Baseline profiles utilized in older devices.
Based on the tech specs for recent devices, Apple appears to have introduced the High profile for H.264 video via its A5 chip. The third-generation iPad, iPhone 4S, and the iPad 2 all support a decoder level of 4.1, which sets a maximum bitrate of 62.5Mbps for the High profile, while the new Apple TV, which sports a single-core A5 processor, has a maximum level of 4.0 (25Mbps). By comparison, the A4-equipped iPhone 4 and Apple TV support H.264 video with a Main profile at level 3.1 (14Mbps).
Report author Iljitsch van Beijnum went on to note that, in some cases, the 1080p version of iTunes TV shows was "pretty much indistinguishable" from the 720p version, though some shows and scenes did show noticeable improvement with the higher resolution. For instance, van Beijnum found that an episode of The Big Bang Theory didn't always show improvements with the 1080p version, though a side-by-side comparison of one particular frame did reveal improved clarity and detail. The 1080p file size for the episode was 856MB, compared to 743MB for the 720p version.
An episode of Awake, however, was described as always showing "some extra sharpness" in the 1080p version and "significantly better" quality in brighter scenes. As such, the author recommended the 1.75GB 1080p version over the 1.45GB 720p one. The below screenshots were taken from the top left corner of both versions of the file while playing back at 1080p (meaning the 720p version was zoomed as it would be on a 1080p flat panel).
Left: 720p Right: 1080p | Source: ArsTechnica