Streaming Media - October 2008 - (Page 25)

Figure 4. Parameters for audio Figure 5. B-frames are especially powerful in screencams because of the pixel-perfect redundancy. The second wrinkle is that all Adobe tools offer only single-pass encoding, almost guaranteeing that you won’t get optimum quality. Even at three times the data rate of the Flix Pro file, which was the gold standard, Premiere Pro couldn’t match the files produced by the other encoders. Otherwise, Sorenson Squeeze and Episode Pro were indistinguishable from the file produced by Flix Pro and offered a full range of multiplepass CBR/VBR encoding with both VP6-E and VP6-S techniques. Fortunately, there aren’t a lot of codec-specific variables to consider when producing VP6 files, though each encoding tool will throw some unique controls at you. When producing screencams with VP6, follow the general rules I’ve previously defined, and use VP6-E unless your testing indicates that the files won’t play smoothly on your target computers. If this is a problem, try VP6-S. H.264 Encoding As I mentioned, the four H.264 encoding tools are powered by three different H.264 codecs: Premiere Pro and Squeeze use the MainConcept codec, Compressor uses Apple’s own codec, and Episode Pro uses the Dicas codec. In general, I encoded at the highest possible quality that the encoder supported, using the High Profile when available and setting all other quality-related options to the max. Probably the most important configurable item in the H.264 (and VC-1) world is the B-frame interval. Briefly, B-frames can borrow redundancies from the previous and/or next I- or P-frame. In real-world video, they’re helpful, but there’s so much motion (and random noise) between frames that after moving two or three frames from an I- or P-frame, little redundancy exists. In a screencam, however, true pixel-perfect redundancy abounds, which means that B-frames can be wonderfully efficient almost irrespective of distance from a P- or I-frame. That’s the theory, anyway. To test this, I encoded the test file twice in Sorenson Squeeze, once with no B-frames, next with a B-frame setting of 3, Squeeze’s maximum. The quality disparity was most apparent after dramatic scene changes, as you can see in Figure 5. Note that the difference wasn’t this significant throughout the respective videos, just after dramatic scene changes. Accordingly, in configuring all of the encoding tools, I used the highest B-frame setting available, which was usually 4 or 5. These configuration options aside, I did notice a significant quality disparity among the codecs when it came to screencam encoding. For example, one glance at the Compressor file convinced me that the Apple codec wasn’t the best choice, as quality was degraded at 30% above the target data rate. To verify that it was the codec and not the encoding tool, I encoded the test file in Squeeze using the Apple codec with similar results. Note that Sorenson has switched to the MainConcept codec, but if the program is installed over a pre-version 5 installation, the Apple codec is an option as well, though clearly not one you’d like to use for screencams. If delivery data rate isn’t an issue, you can get very good quality at 500Kbps, about 2.5 times the target for my lo w data rate tests. In an attempt to diagnose Compressor’s poor performance, I loaded the Compressor-encoded file into Inlet Semaphore and noticed that the data rate was almost completely flat with no variability for harder-tocompress areas. I verified that I had the right settings enabled and compressed again with identical results. In comparison, the 500Kbps file had all the normal peaks and valleys. The only conclusion to draw is that at low bitrates, the Apple codec simply isn’t optimized for screencam recordings. While probably not codec-related, Premiere Pro also posted unacceptable results. Strangely, when producing at 15 fps, Premiere Pro dropped nearly a third of the frames in the file. I tried at 30 fps, and Premiere Pro dropped no frames, but while the still frame quality was WWW.STREAMINGMEDIA.COM 25 http://WWW.STREAMINGMEDIA.COM

Table of Contents Feed for the Digital Edition of Streaming Media - October 2008

Streaming Media - October 2008 Table of Contents Standard Time Technology and Business Trends by the Numbers Encoding for Screencams Meeting the Enterprise Video Distribution Challenge Pulling Back the Curtain on Level 3 The 2008 Streaming Media Readers’ Choice Award Winners The Ultimate Guide to Creating Online Video Content That Works, Part 1 Brave New Platforms Video From the Democratic National Convention—Both Official and Unofficial—Made Clear That It’s No Longer Politics as Usual Creative Suite Turns 4 The Future of Internet Radio Decoding the Truth About Hi-Def Video Production Educating the Next Generation of Online Media Makers ViewCast Niagara GoStream SURF The Past and Future of Online Video: I’m Not Dead Yet!

Streaming Media - October 2008

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