Streaming Media - June/July 2008 - (Page 64) “Size of Coded Video Sequence.” Most producers use a number between two and four for this value. As seen in the Episode Pro screen shown in Figure 1, since B-frames aren’t an option when using the Baseline Profile, this option is only available when using the Main Profile or higher. “Reference frames” is the number of frames that B- and P-frames can search for prediction. In general, increasing the number improves quality, though values beyond three tend to increase encoding time with little quality boost. Beyond these are a number of highly technical options that improve quality, usually either at the expense of encoding time or required decompression horsepower. For example, using “adaptive B-frame placement,” a 16x16 “Search shape” (as opposed to 8x8), and the “Hadamard transformation” will improve quality but slow compression speed while “Reference B-Pictures” and “Allow pyramid B-frame coding” may also boost quality at the expense of increased playback requirements. Interestingly, the newest version of Sorenson Squeeze uses the same MainConcept codec but exposes only the controls shown in Figure 6. Though “AVC Profile,” “BFrames,” and “CABAC Coding” are familiar, “Encoding Effort” is obviously an amalgam of some of the quality versus encoding time controls shown in Figure 5, though Sorenson doesn’t identify which. Slices is a term that Sorenson also doesn’t define in their manual. At this point, I’m sure you understand that even programs that use technology from the same vendor will expose different controls and make different decisions under the cracking the h.264 codec Figure 6. H.264 controls from Sorenson Squeeze H.264 files with two interfaces: the QuickTime Format, which produces MOV files, and the H.264 Format, which produces MP4 files. Both of these options can be selected using the Format list box in the Adobe Media Encoder. When encoding using the QuickTime Format, your sole H.264-related option is “Frame Reordering” (Figure 8). When producing using the H.264 Format, you can choose the Profile and Level directly, but you can’t directly enable or disable B-frames. When inserting your target frame width and height, note that Premiere Pro will display an error message if you exceed the parameters of the then-selected level. Wikipedia defines the resolutions and data rates supported by all levels at http://en.wikipedia.org/wiki/H264. Choose the Level necessary to support your target resolution, and you should be able to avoid this error message. If you refer back to Figure 1, you can see Episode Pro’s encoding screen, which now should be old hat. Figure 7. The H.264 encoding parameters in QuickTime Pro (and Compressor) Figure 8. The H.264 encoding parameters in the Adobe Media Encoder encoding to the QuickTime Format hood for their users. With this as background, let’s review some other configuration option screens from other popular programs, starting with QuickTime Pro, which is also the H.264 interface to Apple Compressor (Figure 7). Several points here—one H.264-related, most general. First, you choose CBR or VBR using the “Optimized for” box. (Select “Streaming” for CBR and “Download” for VBR.) For “Key Frames,” choose “Automatic,” which tells QuickTime Pro to insert key frames at scene changes. Regarding data rate, once you click the “Restrict to” option and insert a data rate, the “Quality” slider grays out. “Multi-pass” versus “Single-pass” is self-explanatory; generally choose the former unless you’re in a real hurry. “Frame Reordering” is the sole H.264-related configuration option. Check this box to enable B-frames and select the Main Profile, which I recommend for SD but not HD videos. The only option not shown is a “Deblocking” filter available elsewhere in the interface that should always be enabled (and probably is enabled automatically in most other encoding programs). Summary I’ve summarized the key encoding parameters in Table 6. Again, when producing video at SD resolutions, optimize for quality, since most of your target audience should be able to play the files at or close to full frame rate. With HD source video, the playback requirements are so significant that I recommend using the parameters that require the least decompression horsepower. Jan Ozer (jozer@doceo.com) is a frequent contributor to industry magazines and websites on digital-video-related topics and is the co-author of Hands-On Guide to Flash Video (Focal Press). Comments? Email us at letters@streamingmedia.com or check the masthead for other ways to contact us. Premiere Pro With the current version of Premiere Pro, you can produce 64 STREAMING MEDIA June/July 2008 http://en.wikipedia.org/wiki/H264
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