Streaming Media - August/September 2007 - (Page 74)

webcasting with windows media WHY WINDOWS MEDIA FOR WEBCASTING Reach—Windows Media is the most broadly distributed technology for watching webcasts. It’s preinstalled on most of the world’s computers and available for other systems, like Mac OS X and a variety of mobile devices. And the forthcoming Silverlight will increase the reach of Windows Media playback and open up new opportunities for richer integration of other media with the video and audio. Performance—Windows Media has the fastest encoders in the business. We have lots of optimization and can split encoding over four processor cores at the same time. And when even that isn’t enough, the Tarari card is available to offer much faster acceleration, supporting even HD encoding in very high quality. Efficiency—The flip side of good performance is efficient encoding. Since webcasting has to be in real time, a faster codec means it can get more done on each frame of video, enabling it to deliver better quality in fewer bits. Our Windows Media Video 9 implementation of the VC-1 codec is also extremely efficient for low bit rate Scaling—Video sources are typically 720x480 or 720x576, but most web streaming is at lower frame sizes—you need north of 2Mbps to get value out of more than 640x480. So the video has to be scaled down. Scaling does use some CPU power, but you can get some more juice for compression by using a capture card with built-in hardware scaling. Another important aspect of scaling is to get the aspect ratio right. Scaling isn’t just a matter of reducing frame size by a fixed value. For instance, 720x480 never gets scaled to 360x240. Instead, the output resolution should match the aspect ratio of the source. With a 4:3 source, 320x240 is a good choice. With 16:9, 432x240 will leave circles looking like circles. Deinterlacing—Traditional analog video is interlaced, where the even and odd lines are captured half a frame’s duration apart. In interlaced video, thin horizontal lines are seen wherever there’s motion. These look bad in their own right, and they’re hard to encode, leaving a lot of artifacts. Ideally, the webcasting source will simply be shot progressive (supported by all modern cameras), but Vertical and horizontal blanking in analog sources can often leave some black edges. A VHS source will sometimes have “tearing” at the bottom of the screen with dramatic distortion of the image. You should also crop a letterboxed or windowboxed source. Those big black rectangles use some bits and, more importantly, a lot of CPU time as the codec looks for something in them to encode. Better to crop them out and let all the horsepower apply to the parts of the content that matters. encoding. On the audio side, Windows Media Audio 9 is the most efficient of the commonly used webcasting codecs, and the new low bit rate modes of Windows Media Audio Professional lead the industry in quality delivered below 128Kbps. Settings To get the best results when webcasting with Windows Media, or any other format for that matter, you need to pick the optimal settings. Here’s a look at those—some Windows Media-specific, some applicable to any format. VIDEO PROCESSING Video processing is everything that happens between the source video frames and the codec itself. 74 STREAMING MEDIA August/September 2007 when that’s not possible, the video will have to be deinterlaced. Again, this uses some CPU power, and also degrades detail when outputting more than half as many lines as in the source (like more than 320x240 with interlaced 720x480 source). [For more on this, see Jan Ozer’s “Shooting for Streaming Part 4: Progressive vs. Interlaced,” June/July 2007, pp. 42–50.] Cropping—You only want to include video that you actually want to be seen. You want to leave out anything around the edges of the frame that isn’t image. This is common in analog sources, where vertical and horizontal blanking can often leave some black edges. A VHS source will often have “tearing” at the bottom of the screen with dramatic distortion of the image. The live encode should be cropped in to exclude that non-image content. http://www.nxtbook.com/nxtbooks/streamingmedia/0607/index.php?startpage=44

Table of Contents Feed for the Digital Edition of Streaming Media - August/September 2007

Table of Contents Sweet Sixteen Letters to the Editor Technology and Business Trends IBM Takes Webcasting to the Desktop ROI in the EDU Bank Serves Espresso, Enterprise Video Online Video Advertising: Hit or Miss MLB’s Foul Bawl A Crash Course in Flash Video Newspapers in Multimedia Metamorphosis Webcasting Large Entertainment Events: Seeing Through the Hype Webcasting With Windows Media Can’t Start A Fire . . . A WiMAX Primer Mobile Video at Last Telestream Episode Pro Grass Valley ProCoder 3 Silverlight Is No Flash Killer

Streaming Media - August/September 2007

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