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Encoding for the Apple iPad, Part 1

Apr 20, 2010 12:00 PM, By Jan Ozer

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Figure 2. Video encoded at 1024x576, 960x540, and 640x360, then played on the iPad.

Figure 2. Video encoded at 1024x576, 960x540, and 640x360, then played on the iPad.

Grading the options

So what resolution makes the most sense? My first thought was to check iTunes and see what resolution most producers were using. I quickly saw that while 720p predominated, 960x540 was used by about 10 percent of the videos that I checked. I found no videos produced at 1024x576; not shocking since the iPad is so new, but clearly not a mandate. There were a gazillion videos produced at 640x360, though obviously none were the HD section that I was checking.

Next I decided to run some visual tests: first with a general-purpose test video, and second with a news video that included scanned newspaper text. Specifically, starting with the same source video, I produced three iPad versions at the three resolutions, encoding at a data rate sufficient to ensure the lack of encoding artifacts in all of them. Then I played the videos back on the iPad, paused the video on the same frame, and shot a picture of the video with a digital camera. Definitely not the most sophisticated of tests, but the results were illuminating.

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With the first video, I found that the Apple marketing manager was right: The lower-resolution video scaled to full-screen playback was almost indistinguishable from the high-resolution video. On the other hand, the newspaper text was a different story (har, har), as you can see in Figure 2.

As you would expect, you see the biggest difference is between the 640x360 video and the other two. Clearly, if your videos contain significant fine detail, you need to encode at one of the two HD resolutions. If you click the photo and view it at full resolution, you'll see that the 1024x576 version is slightly more crisp than the 960x540—again expected, because the larger version is displayed at its native resolution, while the 960x540 version is scaled slightly. The iPad's scaling capabilities may be awesome, but scaling always degrades quality to some degree.

To summarize, if you're producing for joint playback on the iPad and computer, encode at 720p. Going forward, when I'm producing podcasts or video bound for playback on the iPad, I'll produce at 1024x576, though the more conventional path is 960x540, and I wouldn't blame you for choosing that route.

If your source video is talking head or similar footage with very little detail, you might try producing at 640x360 and comparing that quality to either of the HD encoding resolutions; you'll be surprised how little difference you'll notice, and you'll cut your encoding and administrative load and save download time and disc space for your viewers. On the other hand, if there's lots of text or other fine detail, it's going to look pretty mangled if you encode at 640x360 and display at 1024x576 display on the iPad.

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