Streaming Media - June/July 2008 - (Page 62) Figure 3. I saw little quality difference between the tricked-out Main Profile and the Baseline Profile. Figure 4. Windows Media files are easier to play back but don’t offer the same quality as H.264. Overall, however, unless I was targeting only high-end computers for playback, I’d be more concerned with playback smoothness than quality at HD resolutions and would use the Baseline Profile. Interestingly, except on the Mac, the HD Windows Media file (here, I used the VC-1 codec rather than WMV-9) was clearly the easiest to decode, and on the MacPro, it was relatively close to the H.264 file produced with the Baseline Profile. If playback performance on lowpower systems is a priority and your HD codec decision isn’t yet set in stone, then you might consider WMV/VC-1. That said, at least when encoding with the VC-1 presets from Carbon Coder, VC-1 doesn’t deliver the same quality as the H.264 files encoded using the tricked-out Main Profile configuration. This is evident in Figure 4, or at least it would be if you could see the original files. To summarize, when producing H.264 at lower resolutions—say, 640x480 and below—it appears that optimizing your file for quality is the best decision. In contrast, with HD video, since playing back H.264 video is so processor-intensive on even moderately powerful computers, it makes the most sense to optimize for playback frame rate. What about encoding time? This varied by encoder. With Carbon Coder, the differential was minimal (on an eight-core HP xw8400) while on Episode Pro (on a Core 2 Duo-based MacBook Pro), the file produced using the tricked-out Main Profile took about twice as long. If you’re a high-volume shop and plan to produce using high-quality parameters, run a few quick tests on your encoding station to determine if you can afford the time to do so. default value or allows multiple controls to be manipulated by a single, more general control. For example, Figure 5 shows the advanced encoding parameters available in Rhozet’s Carbon Coder, which licensed its H.264 codec from MainConcept. (Adobe also licensed its codec from MainConcept, as shown in several figures below.) Let’s take a moment and review some of the most frequently exposed controls, starting with Entropy coding mode because we’ve already covered Profile and Level. Note that I summarize recommendations for optimizing quality or playback smoothness with these controls in Table 6 below. Briefly, with entropy coding, CAVLC is your only option with the Baseline Profile. CABAC is available with the Main Profile and beyond and produces better quality but creates a stream that’s harder to decode. The “Number of B-Pictures” is the number of B-frames used between P- or I-frames in the GOP which Rhozet calls , Optimizing H.264 Encoding Controls With this as background, all that’s left is to figure out how to optimize for quality and display rate. By way of background, let’s start with an overview of the H.264 encoding ecosystem. Specifically, understand that most developers of encoding software license their H.264 codecs from third parties such as Dicas and MainConcept. These H.264 vendors implement the encoding techniques they feel are most valuable, then supply code modules to the encoder vendors with all configuration options exposed. From there, the encoding software vendor essentially decides which encoding parameters to allow end users to configure. If a configuration option isn’t exposed, the encoding vendor either chooses a universal 62 STREAMING MEDIA June/July 2008 Figure 5. The H.264 encoding parameters in Rhozet Carbon Coder 3.0 Table 6 - Recommended Parameters When Optimizing for Quality or Display Rate Resolution Focus Profile Level Entropy Encoding B-Frames Reference frames Deblocking Filter SD High Quality Main Resolution dependent CABAC Yes—2–4 3 Yes HD Simple to Decode Baseline Resolution dependent CAVLC No 3 Yes
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