Streaming Media - June/July 2008 - (Page 56) COMING NEXT ISSUE: profile. For example, Figure 1 shows Telestream’s Episode Pro H.264 encoding interface. Since I’ve selected the Baseline Profile, Telestream disables the “Entropy coding” mode, forcing me to use CAVLC (contextadaptive variable-length coding) with no access to CABAC (context-adaptive binary arithmetic coding). The “Number of B-frames” option is also disabled, preventing me from creating a set of configuration options that might not work on a player that supports only the Baseline Profile. Note that I explain B-frames and entropy encoding below. Know Your Target If all players in a group (such as iPods) conformed to the same profile and all relevant players were very well-defined, then a compressionist’s job would be much simpler; you’d just choose a template and start encoding. However, in many instances, such as general-purpose playback on a computer, you have to consider the range of viewing platforms when making your configuration decisions. Let’s start with the simplest case—the iPod—and work toward more general configurations. iPod If you’re encoding for one or two known iPods, then you’re in good shape; check the specs for those models and encode away. If you’re encoding for general distribution to a variety of different iPods, then the problem is more challenging, as revealed in Table 1. If you want to produce one H.264 file suitable for both computer and iPod playback, it’s possible but even more complicated. That’s because the iPod has gotten more powerful over the years and can play back increasingly high-resolution and H.264 content with higher data rates. For general distribution to a range of iPods old and new, you need to encode for the lowest common denominator, which in this case would be the iPod Touch’s 480x320 screen. Note that the original iPod can play video encoded using the older MPEG-4 codec at 480x480 resolution, which is another option if you’d like to create one file that can play on all iPods yet has a resolution large enough for most computer desktops. Table 2 shows the same set of detailed parameters for Apple’s iPhone, which shares the same 480x320 screen size as the iPod touch. Obviously, when producing video for playback across both types of devices, you need to choose the lowest common demominator, or the specs for the least powerful device that you’d like to support. Had I written this article 8 months ago, I would next venture into producing for progressive download or streaming or for general desktop playback. But in the interim, of course, Adobe announced that both the Flash Media Server and Flash Player would support H.264, changing the landscape entirely. Content Delivery 2008 H.264 for Flash Sometimes timing is unfortunate, and that’s the case here as it relates to producing H.264 video for Adobe Flash. Specifically, Adobe was in the process of creating a white paper defining how to encode H.264 video for Flash while I was writing this article, but details weren’t made available to me. I pulled material for this article from a range of sources, as you’ll see below, but stay tuned for the white paper because it will likely be more definitive, at least concerning production for the Flash Media Server. According to the Adobe website, Flash supports the Baseline, Main, High, and High 10 H.264 profiles with no levels excluded. Theoretically, this means that the Flash Player should play, and the Flash Media Server should stream, H.264 video produced at a maximum resolution of 4096x2048 and a data rate of 720Mbps. Presumably, most implementations to date don’t stretch the envelope quite that much. Adobe does have 1920x1080i video streaming from its HD Gallery (www.adobe.com/products/ hdvideo/hdgallery) but doesn’t disclose the encoding parameters used on those files. ORDER AT: http://www.streaming media.com/magazine/ 56 STREAMING MEDIA June/July 2008 http://www.streamingmedia.com/magazine/ http://www.adobe.com/products/hdvideo/hdgallery
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