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The Moving Picture: Compressing Video for the iPod
Posted Apr 6, 2007 Print Version     Page 1of 1
  

Many videographers are creating iPod-compatible files for clients or for posting as demo files on their websites. You would think that with a gazillion iPods sold, the process of getting video onto an iPod would be very straightforward.


Well, there's straightforward, and then there's straightforward. If you're a soccer mom showing off vacation videos, your audience will be impressed that you got the videos on the dang thing in the first place. On the other hand, if you're a bride showing off the four-minute montage that you paid top-dollar for, you expect perfection, and you might get a bit miffed if you see interlacing artifacts or if your mother-in-law looks a bit squeezed. In this column, I'll identify some of the fundamental issues involved in converting video to iPod format, and discuss how to produce optimal quality results.

Let's start by looking at the iPod's video-related specifications, which have changed since the first video-capable iPod was introduced in 2005. Before starting, check which version you (or your client) has and customize your settings accordingly. Note that the iPod itself has only a 320x240 resolution screen, so if you're producing for viewing solely on the iPod, there's no reason to encode at a larger resolution. On the other hand, the iPod does have an NTSC-output port for displaying videos on a TV. If you plan to use this, or think your client might, consider encoding at 640x480 resolution.

The iPod supports two codecs, H.264 and MPEG-4. The former has is higher quality, but takes longer to encode and requires more resources to decode. Each has more variations than a Chinese buffet, with obscure options like Low-Complexity Baseline Profile, or Simple Profile. These, in my humble opinion, are compelling evidence that you need to find an effective tool with a template so you don't have to think about these things. As you'll see, if only things were so simple.

Anyway, you can find all the technical specifications on Apple's website, and you should print these and become familiar with them. If you exceed the bandwidth limits when encoding the file, or use the wrong codecs, the video won't play on the iPod, though it will play perfectly fine on your computer. Note that not all tools support these audio and video codecs; for example, Adobe Premiere Pro can't output AAC audio (though Premiere Elements can). This means you'll have to find another tool to produce your final output.

If you have the right tool, like Apple's Compressor, Techspansion's iSquint, Sorenson Squeeze, or ArcSoft MediaConverter 2, powered by the ADS Tech InstantVideo To-Go iPod encoding accelerator, you can just choose the iPod preset and encode. All four tools do everything necessary to produce a high-quality file. Most other consumer convert-to-iPod tools work well for the soccer mom, but forget one critical element that's essential for professional work, rooted in the ways that streaming video differs from what you watch on television.

Here goes. The first consideration is aspect ratio. You'll note that the 320x240 video file we'll be producing has an aspect ratio of 4:3, while at 720x480, the original DV file has an aspect ratio of 4.5:3. I could spend pages going into these differences and how to deal with them, but the short answer is that you want to change the aspect ratio or 4:3, not preserve or maintain, the aspect ratio. If you end up having to adjust settings manually to get the proper results, be on the lookout for these controls, or options like "square pixel" output, which is what you want.

Experiment with some short clips. Those with a 4:3 aspect ratio should fill the screen, with no distortion. Usually, if the aspect ratio is hosed, you'll either see letterboxing on the top and bottom, or folks stretched about 10% (or skinnier by 10% if working with PAL, which most folks seem to find more acceptable). In 16:9 videos, aspect ratio issues are a bit tougher to detect since the video will be letterboxed top and bottom, but watch for the stretching or shrinking.

The bigger problem with most iPod encoding tools is deinterlacing, or more specifically, the lack thereof, which is necessary when starting with interlaced source. Interlaced source videos (such as 480i60 DV and 1080i60 HDV) have two fields for every frame, while streaming video is frame-based. In high-motion videos, simply combining the two fields into a frame produces a noticeable artifact that looks like little slices in your video.

To avoid this, the aforementioned programs—Compressor, iSquint, and Squeeze—all deinterlace during encoding, preserving video quality. On the other hand, a host of other tools that I tested, including QuickTime Pro, InterVideo iVideoToGo, and ArcSoft MediaConverter 2, running with the hellaciously fast InstantVideo To-Go iPod encoding accelerator, don't deinterlace, so higher-motion segments can look awful. On the other hand, these tools have brain dead-easy templates that make it incredibly simple to produce iPod video, and don't cost $500 like Sorenson Squeeze.

What to do? Output a deinterlaced, progressive video file from your video editor at the target resolution of your iPod file (320x240 in our case) and at 29.97 frames per second. I typically use the QuickTime animation codec for my intermediate files (Millions of Colors), with uncompressed audio, which preserves quality for the final encoding stage. Then load the intermediate file into any consumer iPod video tool and you should get great results.

Jan Ozer (www.doceo.com) is a frequent contributor to industry magazines on digital video-related topics and the author of Adobe Digital Video How-Tos: 100 Essential Techniques with Adobe Production Studio, published by Peachpit Press.



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