Streaming Media - August/September 2008 - (Page 58)

Figure 4. MainConcept (bluish-green line) and Dicas peaked higher than Compressor (black line), but had lower lows to compensate. others. Encoding a prescaled, predeinterlaced clip took these variances off the table, so any qualitative differences should be almost totally codec-related. I produced all SD test files at 640x480 with a frame rate of 30 fps and a data rate of 468Kbps for video and 32Kbps for audio. I produced using multipass CBR encoding using the High Profile and CABAC entropy coding with tools that enabled this selection. I used the High Profile setting because this is supported in both the Flash Player and the most recent version of the QuickTime Player. I set the keyframe interval at 300 and enabled “natural” keyframes or the equivalent in all encoding tools, which should produce keyframes at all scene changes. Examining each file within Semaphore, I noticed that while the average data rate was very close among the three files, the Episode (Dicas codec) and Carbon Coder (MainConcept codec) files strayed further from the average than the file produced by Apple Compressor (Apple codec). Specifically, the peak data rate for Apple was about 920Kbps, while the Carbon Coder strayed up to 1.21Mbps and Episode up to 1.28Mbps despite all three files being compressed in CBR mode. Comparative results are ranked in Table 1. These data spikes create a risk that when streaming over constrained-bandwidth connections, the Carbon Coder and Episode clips could be interrupted at the peaks, which is always cause for concern, though much less so in the broadband world where most connections are several times faster than 500Kbps. To try to create an equivalent encoding pattern in Compressor, I tried encoding in VBR mode, but I got the same peak bit rate. I used a variety of techniques to try to reduce the peak data rate in Episode and Carbon Coder, but I never got closer than the results reported earlier. Figure 4 illustrates this point in Semaphore, which allows you to load two files at once to compare the respective data rates. The black line is Compressor, using the encoding parameters shown in Figure 1, the green line is MainConcept, using the CBR parameters shown in Figure 3. Though you would expect a CBR stream to be perfectly flat, in practice, they never are, and different encoding tools deliver different degrees of conformance to the target data rate. As you can see in Figure 4, there are several points where MainConcept is much higher than Compressor (MainConcept peaks), but there are also several troughs where MainConcept is lower (MainConcept troughs). Had I run the comparison with Dicas, the results would have been similar. How much did the difference in data rate affect the results shown in Table 1? Theoretically, Apple’s quality should have been lower at the peaks and higher at the troughs, essentially balancing out any advantage that MainConcept/Dicas might have had. Not surprisingly, even though Dicas and MainConcept outperformed Apple in the most high-motion sequence, Apple’s quality did often exceed Dicas in low-motion sequences, although MainConcept won nearly every comparison. Anyone using the three encoding tools in the field would experience the same results, so they do represent reality, even if it is less of an apples-to-apples comparison than I would have liked. Let’s look at some still-image examples. Figure 5 shows the highest-motion sequence in the test file, and the Apple frame is noticeably more degraded than Dicas, which clearly trails MainConcept. While this isn’t totally obvious in the screen shots in the magazine, you should be able to view higher-resolution images on StreamingMedia.com in the web version of this article. The next image (Figure 6) is a relatively low-motion clip with a hard-to-compress beige background from one of the data rate troughs in both the MainConcept and Dicas clips. As you can see, the Dicas clip shows a so many codecs, so little time Figure 5. MainConcept clearly delivers the best quality in this image. 58 STREAMING MEDIA August/September 2008 http://StreamingMedia.com

Table of Contents Feed for the Digital Edition of Streaming Media - August/September 2008

Streaming Media - August/September 2008 Table of Contents No Doubt - Editor's Note Mo’ Videos, Mo’ Money: Solving the Problem of Network Cost Why Good Design Matters More Than Ever: Just. Press. Play. Technology and Business Trends by the Numbers High-Touch Encoding With Microsoft Expression Encoder 2 The State of the Content Delivery Market, 2008 Get a Life: New Tools Put Lifecasting Within Anyone’s Reach So Many Codecs, So Little Time: Comparing H.264 Files Using Codecs From Apple, Dicas, and MainConcept Writing an RFP for Maximum ROI: Things to Consider and Questions to Ask When Writing a Request for Proposal for a Corporate Webcasting System The Patent Wars Producing Screencams in Camtasia Studio Analyze That Using Video to Communicate Change for Business Success Intellectuals’ Property Rights

Streaming Media - August/September 2008

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