Apple iPad
Dec 1, 2010 12:00 PM, By Jan Ozer
Find out your best option when encoding for the iPad.
Figure 1. A 720p video played on an Apple iPad is scaled down to 1024 x 576 with letterboxes on top and bottom.
There are three ways of transferring video to the iPad: the physical cable, Wi-Fi and 3G. If high-quality video is your goal, the best scenario is transferring the video to the iPad via direct cable, since bandwidth restrictions don't apply. This would be the case if you're encoding your own demo materials for a pitch meeting or encoding HD content for upload to iTunes. In these instances, I'll assume that the video is targeted for full-screen playback on the iPad, as opposed to video in a window. That being the case, what encoding parameters should you use to produce your video?
Starting with resolution, if you're producing for double-duty viewing on both the iPad and general-purpose computers, 720p (1280 × 720) is a good choice. It plays well on the iPad and is the overwhelming choice of producers distributing HD TV shows on iTunes. On the other hand, if you're primarily producing podcasts for mobile viewing, consider at least three other options: two high resolution and one low resolution.
Remember that the iPad's screen is 1024 × 768. When playing 720p video, the iPad scales it down to 1024 × 576 and displays black bars on the top and bottom, so any horizontal pixels beyond the 1024 are essentially a waste. This is shown in Figure 1, a 720p podcast from the hit show “Glee.”
So the first option is encoding your videos at 1024 × 576, which has 36 percent fewer pixels than 720p. This should translate to identical quality at 64 percent of the file size, and 64 percent of the download time for files downloaded from iTunes. The second option is 960 × 540, which is the resolution that iTunes uses if you choose a video and click Advanced > Create iPad or AppleTV version.
The third option is 640 × 360, which came into the picture courtesy of an e-mail I received from a buddy who went to the 2010 NAB Show. A few days earlier, I had asked his advice about encoding for the iPad. While at NAB, he met with Apple's technical marketing manager, Mac OS X - QuickTime, who pointed him to a new technical note that he just published on encoding content for distribution to the iPhone and iPad. He said the iPad scaler is so good that sending content at higher res than 640 × 360 just isn't worth the bandwidth. In addition to saving space and download time, a 640 × 360 video will also play in the iPhone and iPod touch, as well as many older iPods, as long as you encode using the Baseline profile.
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, 960 × 540 was used by about 10 percent of the videos that I checked. I found no videos produced at 1024 × 576; this is not shocking since the iPad is so new, but clearly not a mandate. There were a several videos produced at 640 × 360, though obviously none were the HD section that I was checking.
Figure 2. Shown here is video encoded at 1024 x 576, 960 x 540 and 640 x 360, and then played on the iPad.
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. This is definitely not the most sophisticated of tests, but the results were illuminating.
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, as you can see in Figure 2 on page 52.
Figure 3. Shown here is the file iTunes rendered when the author clicked Advanced > Create iPad or Apple TV version.
Select figure to enlarge.
As expected, the biggest difference is between the 640 × 360 video and the other two. If your videos contain significant fine detail, you need to encode at one of the two HD resolutions. The 1024 × 576 version is slightly more crisp than the 960 × 540 — again expected because the larger version is displayed at its native resolution, while the 960 × 540 version is scaled slightly. The iPad's scaling capabilities may be awesome, but scaling always degrades quality to some degree.
So, 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 1024 × 576, though the more conventional path is 960 × 540.
If your source video is a talking head or similar footage with very little detail, you might try producing at 640 × 360 and comparing that quality to either of the HD encoding resolutions; you'll be surprised at how little difference you'll notice, you'll cut your encoding and administrative load, and you'll save download time and disc space for your viewers. On the other hand, if there is a lot of text or other fine detail, it's going to look pretty mangled if you encode at 640 × 360 and display at 1024 × 576 display on the iPad.
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